HSC Chemistry Notes

Module 4 – Industrial Chemistry – Option

Construct word and balanced formulae equations of chemical reactions as they are

encountered in this module:

- The Equilibrium Constant:

o For the basic reversible reaction, depicted as:

aA + bB cC + dD;

- Uses of Sulfuric Acid:

o Fertiliser Production:

Creating ammonium sulfate:

sulfuric acid + ammonia ammonium sulfate

H2SO4 (aq) + 2NH3 (aq) (NH4)2SO4 (aq)

o Dehydrating Agent:

Dehydration of ethanol:

ethanol ethylene + water

C2H5OH (l) C2H4 (g) + H2O (l)

Dehydration of sucrose:

sucrose carbon + water

C12H22O11 (s) 12C (s) + 11H2O (l)

o Oxidising Agent:

The oxidation of copper into copper ions:

FULL: Cu (s) + 2H2SO4 (aq) CuSO4 (aq) + SO2 (g) + 2H2O (l)

Oxidation: Cu (s) Cu2+ (aq) + 2e−

Reduction: H2SO4 (aq) + 2H+ (aq) + 2e− SO2 (g) + 2H2O (l)

- Sulfuric Acid Ionisation:

o Step ONE: H2SO4 (l) + H2O (l) HSO4−

(aq) + H3O (aq) ΔH = -90 kJ/mol

o Step TWO: HSO4−

(aq) + H2O (l) SO42−

(aq) + H3O (aq)

- Production of Sulfuric Acid (Contact Process):

o Sulfur Dioxide Production:

sulfur + oxygen sulfur dioxide

S (l) + O2 (g) SO2 (g)

o Sulfur Trioxide Conversion:

sulfur dioxide + oxygen sulfur trioxide

2SO2 (g) + O2 (g) 2SO3 (g)

o Oleum Absorption:

sulfur trioxide + sulfuric acid oleum

SO3 (g) + H2SO4 (l) H2S2O7 (l)

o Oleum Dilution:

oleum + water sulfuric acid

H2S2O7 (l) + H2O (l) 2H2SO4 (l)

- Electrolysis of Sodium Chloride:

o Molten NaCl:

Overall reaction:

2NaCl (l) 2Na (l) + Cl2 (g)

Half-equations:

2Cl− (l) Cl2 (g) + 2e−

Na+ (l) + e− Na (s)

o Concentrated NaCl:

Half-equations:

2Cl− (aq) Cl2 (g) + 2e−

2H2O (l) + 2e− H2 (g) + 2OH− (aq)

o Dilute NaCl:

Half-equations:

2H2O (l) O2 (g) + 4H+ (aq) + 4e−

2H2O (l) + 2e− H2 (g) + 2OH− (aq)

o Overall reaction:

a2H2O (l) 2H2 (g) + O2 (g)

- Production of Sodium Hydroxide:

o Mercury Cell:

Electrolytic half-equations:

ANODE (titanium): 2Cl− (aq) Cl2 (g) + 2e−

CATHODE (mercury) : Na+ (aq) + e− Na (Hg)

Hydrolysis of sodium:

sodium + water sodium hydroxide + hydrogen

2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g)

o Diaphragm Cell:

Electrolytic half-equations:

ANODE (carbon): 2Cl− (aq) Cl2 (g) + 2e−

CATHODE (steel) : 2H2O (aq) + 2e− 2OH− (aq) + H2 (g)

o Membrane Cell:

Electrolytic half-equations:

ANODE (titanium): 2Cl− (aq) Cl2 (g) + 2e−

CATHODE (nickel) : 2H2O (aq) + 2e− 2OH− (aq) + H2 (g)

- Solvay Process:

o Brine Purification:

Ca2+ (aq) + CO3

2- (aq) CaCO3 (s)

Mg2+ (aq) + OH-

(aq) Mg(OH)2 (s)

o Carbon Dioxide Formation:

CaCO3 (s) CO2 (g) + CaO (s)

o Hydrogen Carbonate Formation:

Carbonic acid formation:

CO2 (g) + H2O (l) H2CO3 (aq)

Ammonia saturation:

H2CO3 (aq) + NH3 (aq) HCO3ˉ (aq) + NH4

+ (aq)

Overall:

CO2 (g) + NH3 (aq) + H2O (l) HCO3ˉ (aq) + NH4+

(aq)

NaCl (aq) + CO2 (g) + NH3 (aq) + H2O (l) NaHCO3 (s) + NH4Cl (aq)

o Sodium Carbonate Formation:

Thermal decomposition:

NaHCO3 (s) NaCO3 (s) + CO2 (g)

o Ammonia Recovery:

Lime-slaking

CaO (s) + H2O (l) Ca(OH)2 (aq)

Ammonia recovery:

NH4Cl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + NH3 (aq) + H2O (l)

o Overall Solvay Process:

2NaCl (aq) + CaCO3 (s) Na2CO3 (s) + CaCl2 (aq)

- Practicals

o Quantitative Analysis of Equilibrium:

Cobalt complex-ion equilibrium:

Co(H2O)62+

(aq) + 4Cl־(aq) CoCl42־(aq) + 6H2O (l) ∆H < 0

(PINK) (BLUE)

1. Industrial chemistry processes have enabled scientists to develop replacements for

natural products

Discuss the issues associated with shrinking world resources with regard to one

identified natural product that is not a fossil fuel, identifying the replacement

materials used and/or current research in place to find a replacement for the named

material (Refer to Chem Jac 2 Pages 360-363 & Reference Notes For More Case Studies)

Identify data, gather and process information to identify and discuss the issues

associated with the increased need for a natural resource that is not a fossil fuel and

evaluate the progress currently being made to solve the problems identified

Case Study – Fertilisers

- Natural Fertilisers

o Plants obtain their nitrogen from the soil by absorbing dissolved ammonium and

nitrate ions through their roots (this nitrogen passes through the food chain)

Thousands of years ago, farmers realised that crops could not be grown on the

same soil year after year unless the soil was fertilised to restore the

nutrients absorbed by plants

o Natural fertilisers were developed, but by the 20th century, the shrinking availability

of such resources could not meet the increasing demand for fertilisers to

increase food production, for a growing world population

Guano – Organic fertiliser, rich in nitrates and phosphates, used prevalently in

during 18th century

Made up of sunbaked bird droppings built up over thousands of

years on hot, dry Islands

Islands off the Peruvian Coast were major sources of bird guano, and

could also be derived from bat droppings that built up on floors of

caves and caverns, such as in southern USA.

Guano mined extensively and by the 1870s, the deposits became

largely depleted, meaning new sources had to be developed

Saltpetre – Powdered natural mineral, sodium nitrate

The nitrate (saltpetre) deposits of northern Chile became prevalent

sources of natural fertilisers after depletion of Guano

Became replaced by synthetic fertilisers in the 20th century, due to

growing demand for fertilisers to support growing populations

REFER TO PAGE 160 & 326 FOR EXEMPLAR ANSWER (+QUESTION)

- Synthetic Fertilisers

o Ammonium Nitrate

Between 1905-1914, Fritz Haber developed the Haber process for the

synthesis of ammonia (produced ammonia from nitrogen and hydrogen)

3H2 (g) + N2 (g) 2NH3 (g)

This ammonia could be oxidised to produce nitric acid (Ostwald process

developed to produce ammonia from nitric acid). When ammonia reacts with

nitric acid, a salt called ammonium nitrate is produced.

NH3 (aq) + HNO3 (aq) NH4NO3 (aq)

o Ammonium Sulfate

The ammonia could also react with sulfuric acid (Contact process developed to

produce sulfuric acid), to produce ammonium sulfate

2NH3 (aq) + H2SO4 (aq) (NH4)2SO4 (aq)

Both of these salts are excellent synthetic fertilisers

o Today, the above processes (Haber, Ostwald, Contact) are essential in producing

synthetic fertilisers adequately, on a large scale to meet the demands of the

world’s growing population

o Without synthetic fertilisers, the world could not maintain the intensive farming

practices required to produce sufficient food to sustain the large and growing world

population.

Issues associated with shrinking natural

fertiliser deposits

Evaluation of progress made to address

these issues

- Without fertiliser, soils become unable to support crop growth and thus a country’s ability to feed its population is threatened, and the possibility of famine exists. - Dwindling natural reserves of essential

materials can cause conflict between resource rich and -poor nations

- This is being addressed by the industrial production of fertiliser, such as ammonium sulfate and ammonium nitrate. NH3 (aq) + HNO3 (aq) NH4NO3 (aq)

2NH3 (aq) + H2SO4 (aq) (NH4)2SO4 (aq)

- This is an efficient way of helping to minimise famine and resource-based conflict. However it

is essential that poorer nations can afford to purchase fertiliser, and more work needs to be done on its fair distribution using foreign aid and grants

- Increased reliance on natural fertilisers puts pressure on ecosystems (e.g. mining operations to obtain fertilisers can destroy habitats and release atmospheric pollutants such

as CO2).

- Environmental Impact Analyses are used to predict the effect of mining on ecosystems, and develop strategies for rehabilitating affected areas when mining has ceased. They can be

very successful in reducing the impact of mining pollution, but are not as effective in minimising habitat loss

- Increased fertiliser production requires increased industrial activity, which consumes more fuel, releasing more pollutants

- Progress to reduce emissions of atmospheric pollutants (e.g. CO2) has been mixed. - Some countries have committed to the Kyoto

Protocol – An international treaty which commits its involved parties to reduce greenhouse gas emissions, by setting internationally binding emission reduction targets - Governments have put incentives in place

for companies to minimise CO2 emissions. There is ongoing research in all developed countries into alternative energy sources, however progress has not been rapid enough and more work needs to be done to minimise the extent and effects of global warming.

2. Many industrial processes involve manipulation of equilibrium reactions

Explain the effect of changing the following factors on identified equilibrium

reactions

o Pressure

o Volume

o Concentration

o Temperature

- Features of a Chemical Equilibrium

o Chemical equilibria are dynamic and involve reversible reactions

o Chemical equilibrium is achieved when the rate of the forward reactions equals

the rate of reverse reaction

Continual microscopic change occurs between reactants and products

o The concentrations of reactants and products are constant once equilibrium is

achieved. (NOT necessarily equal)

o Chemical equilibrium can only be achieved in a closed system (no matter or energy

can enter or leave the system)

o The position of an equilibrium describes the relative proportions of reactants and

products at equilibrium. When products dominate, the equilibrium lies to the right and

when reactants dominate, the equilibrium lies to the left

o Macroscopic properties (e.g. state, colour, temperature and pressure) will stay

constant when a system is at equilibrium

Changes in pressure, volume, temperature, concentration can alter the

position of an equilibrium

o Le Chatelier’s principle can be used to predict the response of an equilibrium

system to a change in pressure, volume, concentration or temperature

- Pressure & Volume

o Pressure and volume are related; decreasing pressure means increasing volume and

vice versa

Pressure changes only affect gaseous equilibrium systems

Volume changes not only affects gaseous systems but also ionic equilibria

o Gaseous Systems

Increase P - Equilibrium will favour the side that reduces pressure, that is,

creates less moles; thus opposing the change. One way pressure is increased

is by reducing the volume.

Decrease P- Equilibrium will favour the side that increases pressure, that is,

produces more moles; thus opposing the change. One way of pressure is

decreased by increasing the volume.

E.g. N2O4 (g) 2NO2 (g)

(1 mole) (2 moles)

o If the TOTAL pressure is increased, the equilibrium will shift to the left, to

decrease pressure, as there are less moles produce on the left.

o If the TOTAL pressure is decreased, the equilibrium will shift to the right,

to increase the pressure, as more moles are produced on the right.

o Ionic Equilibria

Many chemical reactions occur when chemical species are in solution

If volume of solvent is increased or decreased, the concentration of

dissolved particles will change and this will cause a shift in the equilibrium

- Concentration

o Increase – Equilibrium will shift towards the opposite side of the equation; this

opposes the change, as it reduces the concentration of the species by producing more

products on the opposite side.

o Decrease – Equilibrium will shift towards the same side of the equation the species is

on; this opposes the change, as it increases the concentration of the species by the

opposite reaction.

- Temperature

o Exothermic Reaction: A + B C + D + Heat (Consider heat as a product)

Increase – Equilibrium will shift to the left, to counteract the change by using

up excess heat on the product side, thus cooling the system.

Decrease - the equilibrium will shift to the right, as the forward reaction is

exothermic, and will heat the system to counteract the initial cooling

o Endothermic Reaction: A + B + Heat C + D

Increase – Equilibrium will shift to the right in order to use up excess heat

and cool the system

Decrease – Equilibrium will shift to the left as the reverse reaction is

exothermic, so as to heat the system to oppose the cooling

E.g. I2 (aq) + I-(aq) I3

-(aq)

o If volume of water is increased, there is a decrease in the concentration of

all species, so equilibrium shifts left to increase concentration of dissolved

ions.

o If volume of water is decreased (e.g. via evaporation), the concentration of

all species increases, so equilibrium shifts right to minimise the total amount

of dissolved ions in the system.

E.g. AgCl(s) Ag+(aq) + Cl-(aq)

o If H2S gas is bubbled through the above solution, it reacts with the silver ions to

produce Ag2S

o This reduces [Ag+] so equilibrium shifts to the right to dissolve more AgCl and

form more silver ions

E.g. CaCO3 (s) CaO (s) + CO2 (g) ΔH = 178 kJ/mol

1. If the temperature is INCREASED, the forward reaction will increase,

with equilibrium lying more on the right, as the endothermic forward

cooling opposes the imposed heating.

2. If the temperature is DECREASED, the reverse reaction will increase,

with equilibrium lying more on the left, as the exothermic reverse

heating opposes the imposed cooling.

Interpret the equilibrium constant expression (no units required) from the chemical

equation of equilibrium reaction

- Definition: K is the ratio of the product of the concentrations of the products to the product of

the concentration of the reactants, with each concentration value raised to the power of the

mole ratio of that substance present in a stoichiometric mixture at equilibrium

- For any reversible reaction, at its point of equilibrium, we can calculate a constant

o This constant is called the equilibrium constant (K)

- For any general equilibrium involving a moles of reactant A, b moles of reactant B, forming c

moles of product C and d moles of product D, the equilibrium equation is:

o aA + bB cC + dD

- When this reaction reaches equilibrium, the equilibrium constant is defined as:

- Rules for Chemical Equilibrium:

o The equilibrium constant is given as a number only (no units)

o The equilibrium constant (K) is only measured at constant temperature

o The position of a heterogeneous equilibrium does not depend on the amount

of pure solids or liquids present. If pure solid and liquid are involved in the

chemical reaction, their concentrations are ignored in the equilibrium expression. This

is because the concentrations for pure solids and liquids (not solutions) cannot

change.

o Water in a reaction occurring in dilute aqueous solution is omitted from the

equilibrium expression:

- The values of the equilibrium constant is used to give a quantitative indication of the

extent of the equilibrium: o If K is neither small nor large ( ~10 to 0.1) indicates that the equilibrium does not

lie strongly to the left or the right

o If K is large (>104) then equilibrium lies to the right; strongly favours the products

o If K is small (<10-4) then equilibrium lies to the left; strongly favours the reactants

- The Reaction Quotient (Q)

o Consider a hypothetical example of a gaseous reaction equilibrium:

o The system is initially at equilibrium and more W is added at constant temp.

o What effect would such a change (in concentration for example) have on the

equilibrium? We must first calculate the equilibrium constant for this reaction:

o Next, we must evaluate how this expression has changed on the addition of more W.

This calculation involves determining the reaction quotient, Q, which has the same

form as equilibrium constant expression but new values for new concentrations

o We can see that the reaction quotient is less than the equilibrium constant

This indicates that the system is no longer at equilibrium

To achieve equilibrium again (where Q = K), the system must respond by

shifting to the right to increase the reaction quotient

o The below table shows the final concentrations once the new equilibrium has

been reached and we can check that these new concentrations are at equilibrium by

evaluating the reaction quotient (Q=K)

Process and present information from secondary sources to calculate K from

equilibrium conditions (DO PRACTICE QUESTIONS)

EG: Calculate the equilibrium constant for the following reversible reaction, if at

equilibrium at 450ºC, in a 1.0 L container, there is 2.3 mol of H2, 2.8 mol of I2 and

17.1 mol of HI:

H2 (g) + I2 (g) 2HI (g)

For this reaction, the equation for the equilibrium constant is:

Firstly, find the concentrations of reactants and products:

[H2] = n/v = 2.3/1 = 2.3

[I2] = n/v = 2.8/1 = 2.8

[HI] = n/v = 17.1/1 = 17.1

However, note that we need [HI]2. This is because the species HI has a coefficient of 2 in

front of it, and this changes to an exponent of 2 in the equation:

[HI]2 = (17.1)2 = 292.41

Hence, at 450 ºC, the equilibrium constant is:

K = 292.41 / (2.3 × 2.8)

= 45.4

EG: HSC 2003; Q30 d. (iii): A 1 L reaction vessel initially contained 0.25 mol NO

and 0.12 mol O2. After equilibrium was established, there was only 0.05 mol NO.

Calculate the equilibrium constant for this reaction:

- Answer:

o Reaction: 2NO (g) + O2 (g) 2NO2 (g)

o Moles initially: 0.25 0.12 0.0

o Moles at equilibrium: 0.25 – 2x 0.12 – x 2x

o But at equilibrium NO = 0.05

So: 0.25 – 2x = 0.05, x = 0.1

o HENCE at equilibrium:

[NO] = 0.05

[O2] = 0.12 – 0.1 = 0.02

[NO2] = 2 × 0.1 = 0.2

o Thus, the equilibrium constant is:

o K = [NO2]2 / [NO2]2[O2] = 800.

NOTE: In order to form the expressions ‘0.25 – 2x’ etc., we note that at equilibrium, there

will be an amount of NO2. But the reaction ratio is 2:1:2 so we say that there will be ‘2x’

amount of NO2. But in order to get ‘2x’ NO2, ‘2x’ of NO would have had to react, and ‘x’ or

O2 would have had to react as well, according to the reaction ratios. Hence the amounts left

are ‘0.25 – 2x’ of NO and ‘0.12 – x’ of O2.

Identify that temperature is the only factor that changes the value of the equilibrium

constant (K) for a given equation

- The only factor that can change the value of the equilibrium constant, K, is temperature

o Changes in concentration, pressure or volume only change the position of the

equilibrium, however do not change the value of K for the reaction

o The addition of catalyst also does not change the value of K, simply reducing the

time needed to reach equilibrium

- Regardless of any change in concentration, pressure or volume, the value of K for a given

equilibrium reaction is constant if equilibrium temperature remains constant.

- For Exothermic reactions, if temperature is:

o Increased, then more reactants form (which are the denominator of the equation) so

hence the equilibrium constant will DECREASE.

o Decreased, then more products form (which are the numerator of the equation) so

hence the equilibrium constant will INCREASE.

- For Endothermic reactions, if temperature is:

o Increased, more products form (which are the numerator of the equation) so hence

the equilibrium constant will INCREASE.

o Decreased, then more reactants form (which are the denominator of the equation) so

hence the equilibrium constant will DECREASE

- Example – Haber Process

o For the Haber process, the value of the equilibrium constant changes as the

temperature changes

o Haber process is exothermic, so increasing temperature, decreases the value of

K

As the value of K decreases, the yield of ammonia at equilibrium decreases

Refer to Chem Jac 2, pages 375-376 for

more examples on temperature

dependence of K

Identify data, plan and perform a first-hand investigation to model an equilibrium

reaction

- Aim: To model an equilibrium reaction

- Equipment: 2 identical 100ml measuring cylinders labelled ‘A’ and ‘B’, 10ml graduated

pipette and 2ml graduated pipette, graph paper

- Method:

o Achieving Equilibrium

i. Measuring cylinder A was filled with 100ml of water

ii. The 10ml pipette was used to transfer 10ml of water to measuring cylinder B.

Note: This modelled the forward reaction of the equilibrium:

WaterA WaterB

iii. The 2ml pipette was then used to transfer 2ml of water from measuring

cylinder B to A.

Note: This modelled the backward reaction of the equilibrium:

WaterA WaterB

iv. Steps 2 and 3 formed one transfer cycle. The 2 processes were repeated for

30 transfer cycles, and after each transfer cycle, the volume of water in each

cylinder was measured

v. These two processes that were carried out were the reverse of one another.

When allowed to proceed, without water being added to the cylinders from

another source or taken away, reached a point of equilibrium

WaterA WaterB

o Disturbing the Equilibrium

i. The equilibrium was disturbed by adding an extra 10mL of water to measuring

cylinder A (upon the system reaching equilibrium)

ii. 20 transfer cycles were completed, with the volumes in each cylinder being

measured after each cycle

Eventually, a new equilibrium was established

- Discussion

Refer to Practical Sheet & Chem Jac 2 pages 392-393 for experimental Results

Choose equipment and perform a first-hand investigation to gather information and

qualitatively analyse an equilibrium reaction

- Aim: To qualitatively investigate an example of a chemical equilibrium

o The equilibrium analysed was:

Fe3+(aq) + SCN-

(aq) Fe(SCN)2+(aq) + heat

- Introduction:

o When iron(III) chloride solution is mixed with ammonium thiocyanate solution,

a deep red solution is formed

This is because a reaction take place between the Fe3+ ions and the SCN- ions

Fe3+ solution is pale yellow, SCN- solution is colourless, Fe(SCN)2+ solution

is deep red

Both the forward and reverse reactions shown in the equation above can occur

and an equilibrium can be established between the reactants and product

Due to the distinct colours of the chemical species, if we disturb the

system, we can readily detect the way the system responds by observing any

colour change that occur

- Equipment: 2ml of 0.1mol/L FeCl3 and NH4SCN solutions, dropper bottles of FeCl3, NH4SCN

and NaHPO4 solutions, warm water bath, ice bath, about 20ml distilled water, test tubes, test

tube rack

- Method:

1) Equal volumes (2ml) of 0.1mol/L iron(III) chloride and ammonium thiocyanate

solutions were mixed together, and a deep red colour formed.

2) This solution was diluted by a factor of 1/10 by making up the total volume to 20ml

with distilled water.

3) The solution was divided into equally between 6 test tubes, with one test tube as a

reference

- Results:

o The following changes were imposed on the solutions in the test tubes:

Drops of Fe3+ solution (0.5M iron chloride solution) were added to the

1st test tube. The solution became darker compared with the reference tube

(as more complex formed).

Drops of SCN- solution (0.5M ammonium thiocyanate solution) were

added to the 2nd test tube. Again the solution became darker.

Both these changes showed that increasing the concentration of

reactant in an equilibrium reaction pushes the equilibrium towards the

products.

Drops of 0.5M sodium hydrogenphosphate (NaHPO4) solution were

added to the 3rd test tube. The solution became colourless.

The phosphate ions react with Fe3+ to produce Fe(PO4)22-, showing that

removal of a reactant drives the equilibrium towards reactants.

The solution in the 4th test tube was heated; the solution became paler.

The solution in the 5th test tube was cooled; the solution became darker.

These two changes showed that the equilibrium moves in the reverse

direction as temperature increases, so the forward reaction is

exothermic.

- Risk Assessment:

o The iron (III) chloride and ammonium thiocyanate solutions are moderately poisonous.

If any of the solution is spilled, the affected area should be washed thoroughly with

water.

3. Sulfuric acid is one of the most important industrial chemicals

Outline three uses of sulfuric acid in industry

- Fertiliser Production – Most of the sulfuric acid produced is used to produce fertiliser

o Ammonium Sulfate

When linked with a Haber ammonia plant, ammonium sulfate fertiliser can be

produced readily

Ammonia waste gas is passed through sulfuric acid, which acts as ‘scrubber’,

forming ammonium sulfate

2NH3 (g) + H2SO4 (l) (NH4)2SO4 (s)

o Superphosphate Fertiliser

Readily produced by reacting 70% sulfuric acid with crushed calcium

phosphate

This forms a mixture of calcium dihydrogen phosphate and calcium sulfate,

which is dried and sold as superphosphate

Ca3(PO4)2 (s) + 2H2SO4 (aq) Ca(H2PO4)2 (s) + 2CaSO4.2H2O (s)

- Dehydrating Agent – This is used in the production of ethylene from ethanol

o Concentrated sulfuric acid is a strong dehydrating agent

Dehydration of ethanol:

ethanol ethylene + water

C2H5OH (l) C2H4 (g) + H2O (l)

- ‘Pickling’ Steel - Used to clean steel (or iron)

o Prior to galvanization, any oxide (e.g. iron oxide, rust, corrosion) that has formed on

the surface must be removed

o Sulfuric acid is very corrosive, and therefore has the ability to remove rust

FeO (s) + H2SO4 (aq) FeSO4 (aq) + H2O (l)

- Polymer Production

o Sulfuric acid is used in the production of many polymers (mainly condensation

polymers), such as rayon and cellophane.

o It is used to extrude the liquid mass into fibrous polymer threads.

- Electrolyte in Car Lead-Acid Batteries

For More Uses, Refer to Chem Jac 2 pages 380-381

Describe the processes used to extract sulfur from mineral deposits, identifying the

properties of sulfur which allow its extraction and analysing potential environmental

issues that may be associated with its extraction

- Sulfuric acid is manufactured from pure, elemental sulfur

- Most sulfur is present in the Earth’s crust as sulfide and sulfate minerals as well as minor

components of coal, crude oil and natural gas

- The Frasch Process – This process is used to extract vast deposits of elemental sulfur from

mineral deposits

o There exists a series of 3 concentric pipes which are

placed into drill holes, forced down into sulfur deposits

o Superheated and pressurized liquid water (160-

170oC / 1.5MPa) is injected into the sulfur deposit

through the outer pipe

This high temperature water rapidly melts the

sulfur

The melted sulfur and water form an emulsion

(A colloidal dispersion of one liquid in another)

o Compressed (pressurized) air is injected through the

innermost tube, which then forces the sulfur-water

emulsion to the surface through the middle tube

o At the surface, the emulsion is collected in large vats,

and as the dispersion cools, it separates water and

solid sulfur (sulfur obtained is 99.5% pure)

- Properties of Sulfur Allowing for its Extraction:

o Low melting point (113oC) due to weak dispersion

forces – this allows the superheated water to easily melt the sulfur.

o Low density (2.07 g/cm3) forms a water-sulfur emulsion that can be easily lifted to

the surface by the compressed air.

o Insolubility of sulfur in water allows the sulfur to be easily separated from the water

after cooling to produce a very pure product.

o Inert, non-volatile, non-toxic and odourless – no special precautions need to be

taken to protect the health of miners

- Environmental Issues:

o The sulfur produced must be cooled quickly to avoid oxidation to sulfur dioxide or

reduction to hydrogen sulfide, both of which are serious air pollutants.

o The recovered water contains dissolved minerals, so it must be recycled to avoid

contamination of local ecosystems, and cooled to avoid thermal pollution.

o Extraction of large amounts of sulfur creates underground caverns that are prone to

subsidence (collapse) and are difficult to backfill

Outline the steps and conditions necessary for the industrial production of H2SO4

from its raw materials

Gather, process and present information from secondary sources to describe the

steps and chemistry involved in the industrial production of H2SO4 and use available

evidence to analyse the process to predict ways in which the output of sulfuric acid

can be maximised (Refer to above dot point & next 2 dot points)

The Contact Process is the main method used to produce sulfuric acid from elemental sulfur.

The raw materials are sulfur, oxygen and water

- The Contact Process

o Stage 1 – Production of SO2 (S SO2)

Elemental sulfur is melted and sprayed under pressure into an excess of

dry air in the combustion chamber of the sulfur furnace

Dry air has been scrubbed of water vapour by passing through 99%

sulfuric acid (dehydrating agent)

Air must be dry to avoid acid mist and corrosion in downstream

pipes

An excess of air (and oxygen) ensures sulfur reacts completely

Sulfur burns in oxygen to produce sulfur dioxide:

S(l) + O2 (g) SO2 (g) ∆H = -297kJ/mol

Combustion releases considerable heat and heat exchangers are used to

remove and recycle heat (before catalytic oxidation stage, temp reduced from

1000oC to 400oC)

Recycled heat used to remelt more sulfur

Used to produce steam in a waste heat boiler. Steam is then used to

power turbines that generate electrical energy for factory

Alternatively, SO2 can be obtained through the smelting of metal oxide ores

(for the feedstock for stage 2):

2ZnS (s) + 3O2 (g) 2ZnO (s) + 2SO2 (g)

o Stage 2 – Catalytic Oxidation of SO2 (SO2 SO3)

This process occurs in the Catalytic Tower (Converter)

Clean, dry sulfur dioxide is the feedstock for this stage.

Electrostatic precipitators are often used to remove any ash from

the gas stream

SO2 gas is passed into a catalytic tower, which consists of 3 or 4 catalytic

beds, each impregnated with vanadium pentoxide (V2O5)

The gas stream passes over the 3 or 4 catalytic beds, and SO2 is

oxidised to produce SO3

o 2SO2 (g) + O2 (g) 2SO3 (g) ∆H = -99kJ/mol

First Catalytic Bed – Here, the heat liberated in the oxidation, raises

the temperature from 400 to 550oC, increasing rate of conversion

(collision theory), achieving 70% conversion rate

Second Catalytic Bed – Before gas

enters here, heat exchanger is used to

lower temp. to 400-425oC. Reaction rate

reduced but yield (conversion

percentage) increased to 97%

(pushes equilibrium to right in this

exothermic reaction)

Third Catalyst Bed – Emerging gas

stream from SCB is cooled to 400-425oC

and is passed over this bed, for more

conversion

Fourth Catalyst Bed - Removal of SO3:

Before gas mixture enters FCB, SO3 is removed by absorption into

98% sulfuric acid in First Absorption Tower, producing oleum, which

on dilution forms sulfuric acid

o Removal of SO3 helps shift equilibrium to the right

Remaining gases pass through fourth catalyst bed at 400-425oC,

leading to effective yield of SO3 to be 99.7%, which then passes into

Second Absorption Tower

o Stage 3 – Absorption of SO3 (SO3 H2S2O7)

This process occurs mainly in Second Absorption Tower

Cooled SO3 (from FCB) enters the bottom of SAT and sulfuric acid is sprayed in

at the top

Acid trickles down special vertical plates than increase the SA of

contact of acid and gas

Product of this reaction is an oily liquid called oleum

o SO3(g) + H2SO4(l) H2S2O7(l)

This is preferable to dissolving directly SO3 into water, which is

very exothermic and forms sulfuric acid mists which are difficult to

separate from the residual gas

o Stage 4 – Conversion of Oleum to Sulfuric Acid (H2S2O7 H2SO4)

This process occurs in the Diluter

Oleum is diluted with water to produce 98% sulfuric acid (18M)

H2S2O7(l) + H2O(l) 2H2SO4(l)

------------------------------------------------------------------------------------------------------------------

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Describe the reaction conditions necessary for the production of SO2 and SO3

Apply the relationship between rates of reaction and equilibrium conditions to the

production of SO2 and SO3

- Production of Sulfur Dioxide (SO2)

o This involves the combustion (oxidation) of S to SO2

o The reaction is exothermic and goes to completion:

S(l) + O2 (g) SO2 (g) ∆H = -297kJ/mol

o Conditions for Production

Reaction must be carried out in a combustion furnace

Must be sprayed into excess dry, oxygen-rich air

However, to increase the rate of reaction, the sulfur is liquefied (which

increases its surface area), and an excess of oxygen is used.

o The production of SO2 is a reaction that proceeds to completion, and hence there are

no equilibrium considerations to be made

- Production of Sulfur Trioxide (SO3)

o Involves catalytic oxidation of SO2 to SO3

o The production of SO3 is a reversible reaction, and the conditions chosen for its

production are made after considering equilibrium factors

o Equilibrium Considerations:

Gas Pressure: High pressures favour the forward reaction (increased yield),

as there are less moles of gas as products compared to reactants

Temperature: Lower temperatures favour the forward reaction (increased

yield), as the reaction is exothermic

At 400oC, K=100, so the equilibrium lies very much to the right and

yield of sulfur trioxide is very high (98%)

At 550oC, K=1, still favouring formation of sulfur trioxide

Oxygen Concentration: Excess of oxygen will drive the equilibrium to the

right and increase the yield

o Reaction Rate Considerations:

Temperature: High temperatures are favourable, as the rate of reaction will

increase due to more molecular collisions

Catalyst: The vanadium pentoxide catalyst is used to increase the

reaction rate by lowering the activation energy for the reaction

o The catalyst is impregnated uniformly onto the surface of

porous silica pellets and the high SA ensures rapid reaction

o Compromise Conditions:

A balance between the equilibrium yield and rate of reaction has to be

made in the industrial synthesis of sulfur trioxide, in order to allow for an

overall efficient, cost-effective process:

Compromise temperature range of 400-550ºC is used, which

provides a balance between yield and rate of reaction.

High pressure equipment is expensive and dangerous; a compromise

pressure of 100-200 kPa, sufficiently low to avoid expense of high-

pressure apparatus but sufficiently high for adequate equilibrium yield

The conditions chosen give an economically practical balance

between yield of products at equilibrium and the rate of

chemical reaction

2SO2 (g) + O2 (g) 2SO3 (g) ∆H = -99kJ/mol

Describe, using examples, the reactions of sulfuric acid acting as:

o An oxidising agent

o A dehydrating agent

- The ionisation equilibrium in which sulfuric exists is as follows:

o H2SO4(l) + 2H2O(l) SO42-

(aq) + 2H3O+(aq)

- Sulfuric acid as an oxidising agent:

o In dilute solutions of sulfuric acid, hydronium ions act as oxidants, and reactive

metals (higher than hydrogen in the activity series) are oxidised, releasing H2 gas

E.g.

NOTE: some metals (e.g. Al, Ni, Cr) are not readily attacked by sulfuric acid

due to the passivating oxide layer on their surfaces (an inert surface layer

than prevents corrosion)

o In concentrated sulfuric acid, the sulfate ion is the oxidant, and it can only oxidise

metals above below lead in the activity series

E.g. Hot, concentrated sulfuric acid oxidises copper metal to copper (II)

sulfate, sulfur dioxide and water

Half Equations:

o

The sulfate ion is usually reduced to sulfur dioxide, but under certain

temperature conditions and certain reductants, hydrogen sulfide or sulfur may

be produced

More Examples:

- Sulfuric acid as a dehydrating agent:

o Concentrated sulfuric acid has a strong affinity for water – it rapidly absorbs water

from the air.

It is used to dry the air to be used for combustion of sulfur in the Contact

process.

o Hydrated crystals can be dehydrated by concentrated sulfuric acid.

Blue copper (II) sulfate crystals turn white when concentrated sulfuric acid is

added

o Carbohydrates such as sugars can be dehydrated by concentrated sulfuric acid

Sucrose (C12H22O11) is readily dehydrated in this way. The mixture caramelizes

and turns brown then black

The black, porous solid is mainly carbon and the heat from the reaction causes

water to be produced as steam, which causes the black mass to expand

o Concentrated sulfuric acid is also used to dehydrate ethanol to produce ethylene

Perform first-hand investigations to observe the reactions of sulfuric acid acting as:

o An oxidising agent

o A dehydrating agent

NOTE: In the above copper granule test, moist BLUE litmus paper was

placed over the forming gas and it turned RED, indicating the gas was indeed

SO2 (as SO2 is an acidic oxide). The blue signified the production of copper

sulfate.

Extra:

- As a Dehydrating Agent:

o In the fume cupboard, 10ml of concentrated sulfuric acid was

added to a beaker containing 50 grams of sugar (sucrose)

and stirred

In the beaker (dehydration), the sugar turned black,

and then began to smoke, and form a large tower of

black carbon. This proved that dehydration occurred:

C12H22O11 (s) 12C (s) + 11H2O (l)

Describe and explain the exothermic nature of sulfuric acid ionisation

- Recall: Breaking bonds absorbs energy, creating bonds releases energy.

- The ionisation and dilution (a form of ionisation, as when molecular H2SO4 is placed in

water, it ionises) of concentrated sulfuric acid in water generates considerable amounts of

heat

o H2SO4 ionises in 2 steps, and each step is an exothermic process

H2SO4 (l) + H2O (l) HSO4−

(aq) + H3O+ (aq) ΔH = -90 kJ/mol

HSO4−

(aq) + H2O (l) SO42−

(aq) + H3O+ (aq)

o The 1st reaction, H2SO4 is completely ionised, the reaction goes to completion, and is

strongly exothermic

o The 2nd reaction, ionisation only occurs slightly or partially

- Explanation for Exothermic nature:

o The ionisation of sulfuric acid is exothermic because the energy released by the

solvation of the ions (bond forming) is greater than the total energy required to

ionise the molecule and to overcome the hydrogen bonds that hold the water

molecules together (bond breaking)

o In concentrated sulfuric acid, there are very few ions (almost 100% molecular), as

most of the water is tied up as hydrates (e.g. H2SO4.H2O). Upon ionisation, the

breaking of the covalent bods, leads to the release of a vast amount of energy

Identify and describe safety precautions that must be taken when using and diluting

concentrated sulfuric acid

- When USING concentrated H2SO4

o To protect your skin, eyes and clothes, always wear protective gloves, long-sleeved

clothing (lab coat), enclosed shoes, safety goggles.

Concentrated H2SO4 causes serious damage to the skin and eyes, behaving in

the same way towards skin as it does towards sucrose. Skin will char rapidly

o If sulfuric acid contacts the skin, it must be washed off rapidly with copious

amounts of water. If larger amounts spilled on the skin, it is best to wipe excess away

rapidly with paper towel and then wash with large volumes of water

This minimises the large amount of heat that would be produced on dissolution

with water

o If sulfuric acid is spilt whilst using, onto bench or floor, area should be isolated

quickly to prevent it spreading. If fumes present, evacuation should be undertaken

Sand can be spread over spillage to absorb acid

Neutralise any spills immediately using a suitable neutralizing agent such as

sodium hydrogen carbonate

o When pouring sulfuric acid into a beaker, pour the acid slowly down a glass rod to

avoid splashing.

- When DILUTING concentrated H2SO4

o Always wear protective clothing (safety goggles, gloves, lab coat, enclosed shoes)

o Add small amounts of ACID to larger amounts of WATER (this allows the heat of

ionisation to disperse throughout the water, to prevent boiling).

o Continuously stir the mixture to disperse the heat evenly, to aid a smooth dilution

Use available evidence to relate the properties of sulfuric acid to safety precautions

necessary for its transport and storage

- Storage:

o Sulfuric acid is hydroscopic, meaning it absorbs water from the surroundings

Well-sealed containers should be used to prevent sulfuric acid from

absorbing water from atmosphere

o Sulfuric acid is very corrosive

Smaller bottles (such as 1 or 2.5L) are more suitable and safer for regular

use

Bottles should be stored in a secure, cool, ventilated room in

plastic trays in case of breakage

Smaller reagent bottles can be placed in small plastic trays to ensure

than any drips do not contact the workbench while acid is being used

o Sulfuric acid should be stored well away from metals (particularly metal powders),

bases and water

Sulfuric acid reacts strongly exothermically with such substances

o Sulfuric acid should not be stored in contact with wooden shelves

Any spillage of concentrated acid on the wood will lead to a highly exothermic

dehydration reaction

- Transportation:

o During transportation, large volumes of concentrated acid should be stored in

steel containers

Steel is much stronger than glass or plastic, which are more likely to

rupture and hence cause spillage if there is an accident

Concentrated sulfuric acid (98%) is virtually all molecular (no hydronium

ions) and so the acid does not attack the steel containers

Also due to the formation of a passivating layer (which prevents

corrosion)

o When storing or transporting concentrated , care must be taken to avoid

contamination with water, as significant ionisation and heat build-up will occur

Steel will be attacked under these circumstance and hydrogen gas will be

released

o Dilute sulfuric acid, however, contains hydronium ions, and vigorously attacks

metals like iron and steel.

Dilute acid must be stored in glass or plastic containers – making it more

difficult to transport.

4. The industrial production of sodium hydroxide requires the use of electrolysis

Explain the difference between galvanic cells and electrolytic cells in terms of energy

requirements

- Galvanic Cells:

o Galvanic cells use oxidation and reduction half-reactions to convert stored chemical

potential energy into electrical energy

o Galvanic cells are constructed from two half-cells which are connected electrically to

produce a complete circuit

The difference in electronegativity (and reactivity) of the two metal

electrodes used, enable spontaneous redox reactions to occur where there is a

transfer of electrons, and hence electricity is generated

Cathode is the site of reduction, anode is site of oxidation

Recall Function of Salt Bridge

- Electrolytic Cells:

o Electrolytic cells convert external DC electrical energy into chemical energy in

order to drive non-spontaneous redox reactions

An external source of DC power is connected to the two electrodes which are

situated within the same compartment (as opposed to 2 in a galvanic cell)

The polarities of the anode and cathode in this cell, however are reversed.

The negative electrode is the cathode, the positive electrode is the

anode

Cathode and anode are still sites of reduction and oxidation respectively

but the polarities of each electrode are reversed

Comparison

Cell Feature Galvanic Cell Electrolytic Cell

Function - Spontaneous chemical reaction

converts chemical energy into electrical

energy

- Electrical energy is converted into

chemical energy to bring about a non-

spontaneous chemical reaction

Eo Value - The voltage of the cell must be positive for a reaction to occur - Eo is positive

- Reactions are forced by an applied voltage - Eo is negative

Site of Reduction

- Cathode (Positively charged) - Cathode (Negatively charged)

Site of Oxidation

- Anode (Negatively charged) - Anode (Positively charged)

Electron Flow in External Circuit

- Electrons flow from anode to cathode (negative terminal to positive terminal)

- Electrons flow from negative battery terminal to cathode and from anode to positive battery terminal

Construction - Two half-cells are used with separate electrolytes so that electron flow can

be collected in an external circuit

- Consist of one compartment - Electrodes are usually immersed in a

common electrolyte

Electrical Energy

Produced Required

- E.g. Pictured Galvanic Cell Above:

o The more active metal, zinc, is the stronger reductant and forms the anode of the cell.

o Electrons flow through the external circuit from the zinc anode to the copper

cathode

o The cell will produce 1.10V from the spontaneous redox reaction:

o Copper will be deposited on the cathode while the zinc anode will oxidise

- E.g. Pictured Electrolytic Cell Above:

o An external DC power source, that provides a voltage greater than 1.10V, will

cause a non-spontaneous redox reaction that is the reverse of the cell reaction in the

galvanic cell:

o Zinc will be deposited on the cathode while the copper anode will oxidise

Outline the steps in the industrial production of sodium hydroxide from sodium

chloride solution and describe the reaction in terms of net ionic and full formulae

equations

- Industrially, sodium hydroxide is produced by electrolyzing brine (concentrated NaCl

solution)

o Hydrogen and chlorine gas are produced as by-products

- At the anode, chlorine ions are oxidised to gaseous chlorine:

o

- At the cathode, water is reduced to form hydroxide ions and hydrogen gas

o

- Net ionic equation:

o

- Full Equation: (adding in the Na+ spectator ions)

o

- The three methods used to industrially produce sodium hydroxide from sodium

chloride solution are explained in detail below

Distinguish between the three electrolysis methods used to extract sodium

hydroxide:

o Mercury process

o Diaphragm process

o Membrane process

By describing each process and analysing the technical and environmental difficulties

involved in each process

Salt Purification:

- Before brine solutions can be used to manufacture sodium hydroxide, they must be

extensively purified to remove impurities, which have the potential of interfering with the

below processes

o Impurities such as Ca2+ and Mg2+ ions precipitate as the NaOH concentration increases

and these precipitates block the pores and prevent ion diffusion in the diaphragm

and membrane processes

- Ca2+ is removed by adding Na2CO3, forming CaCO3

- Mg2+ is removed by adding NaOH, forming insoluble Mg(OH)2

- Fe3+ is removed by adding NaOH, forming Fe(OH)3

- SO42- is removed by adding barium chloride or calcium chloride, forming BaSO4 or CaSO4

Mercury Cell (Castner-Kellner Cell):

- This cell was designed to ensure that the products of electrolysis are kept separate (H2

and Cl2 are explosive mixtures and so must not be allowed to mix)

- A direct electric current is passed through a saturated, purified brine solution

(electrolyte, 26%(w/w))

o DC allows the electrodes to maintain a constant charge. AC would mean each

electrode would alternate as anode and cathode

- The anode consists of inert titanium plates

o Chloride ions (from brine solution) are oxidised at the anodes to form chlorine gas,

which is removed from the cell

o The chlorine is washed and dried before it is compressed and stored in cylinders

- The cathode consists of liquid mercury, flowing in a long steel trough.

o Sodium ions (from brine solution) are preferentially reduced at the cathode (instead

of water):

o The sodium metal rapidly dissolves in the flowing mercury to form an amalgam

(alloy of Na and Hg)

o The amalgam flows into the ‘decomposer’ or ‘denuder’ which contains pure water,

with graphite balls acting as a catalyst

The sodium amalgam reacts with water to form NaOH and hydrogen gas,

releasing the mercury (which is recycled in the cell):

o The hydrogen gas that forms is collected and compressed into cylinders or piped to

factories

o The sodium hydroxide produced is a highly pure (50%) solution. It may be

evaporated and dried and sold as solid flakes

- Overall Cell Reaction:

o 2NaCl(aq) + 2H2O(l) H2(g) + Cl2(g) + 2NaOH(aq)

- Environmental Issues o Although the mercury is recycled, there is a mechanical loss of mercury to the

environment

Waste water containing mercury can be converted to a highly toxic

compound called methyl-mercury.

o This compound is taken up by organisms and passed along the food chain with

biological amplification (biomagnification) at each stage, and can greatly affect the

health of humans

Mercury is a toxic heavy metal which poisons the central nervous system

(neurotoxin), leading to mental degeneration and loss of motor control

o Mercury can be removed by reacting it with sulfide to form insoluble mercury sulfide:

- Technical Aspects & Considerations

o The Mercury cell operates at the highest working voltage (3.9-4.2V), so the cost of

electricity is the greatest for this cell

Mercury is also a very expensive metal, which adds to the operating costs

of these cells

o The Mercury cell has the highest energy consumption (3360 kwh/mt cl2)

o The Mercury cell produced the highest purity sodium hydroxide (50%)

o Considerations:

At the anode, chlorine gas is evolved; it is a corrosive, powerful oxidising

agent. Hence an INERT anode of titanium plates is used.

For the cathode, mercury is used because:

It is a liquid, and can be pumped around a system.

It readily forms alloys (amalgams) with many metals (e.g. sodium).

When using a mercury cathode, sodium ions, not water molecules, are

preferentially reduced, as opposed to using other cathodes.

The titanium plates are held very close to the mercury surface to allow

for very large current flows; this promotes electrolysis:

3 to 4 volts are used, but with tens of thousands of amperes of

current.

The raw materials (brine and water) must be purified before use.

The depleted brine must be continuously replaced with fresh brine.

Diaphragm Cell:

- The diaphragm cell consists of 2 compartments, separated by a porous diaphragm

o Traditionally this diaphragm was composed of asbestos, but in more recent times, has

been replaced by safer and superior diaphragms

o Diaphragms such as Polyramix (PMX) or Tephram are resistant to chemical attack

by Cl and NaOH and more durable than asbestos

o These diaphragms are about 3mm thick and are permeable to both cations and

anions

- The electrolyte in both compartments is saturated (26%), purified brine

- The anode is titanium (or titanium-steel alloys), sometimes coated with a mixture of oxides

(Ruthenium oxide and iridium oxide), which improve the stability of the anodes during

electrolysis

o Chlorine ions (from the brine) is oxidised at the anode to produce chlorine gas, which

is pumped out of the cell as it is formed

o To prevent Cl- ions diffusing into the catholyte, fresh brine constantly replaces spent

brine as the anolyte

- The cathode is made of steel mesh

o Here, water (present in brine solution) is reduced to form hydrogen gas and

hydroxide ions:

o Hydrogen is pumped out of the cell as it forms

o Na+ ions migrate (due to an imbalance of charge as more and more Cl- is oxidation)

from the anode chamber through the diaphragm to the cathode chamber, to balance

the charge of OH- formed there. Cl- ions also diffuse into cathode chamber (NaCl is

then present)

- The level of anolyte is above the catholyte level to reduce the mixing of hydroxide and

chlorine ions by diffusion

o A slight positive pressure is maintained in the anode chamber (due to higher level)

to prevent the migration of OH- from the catholyte

o This mixing is undesirable as the it forms sodium hypochlorite, which reduces the yield

of NaOH and chlorine

2NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

- Overall Cell Reaction:

o 2NaCl(aq) + 2H2O(l) H2(g) + Cl2(g) + 2NaOH(aq)

- After Electrolysis: Catholyte contains 10% NaOH and 15% NaCl

o To remove unwanted NaCl, solution is fractionally crystallised by cooling (NaCl is

less soluble than NaOH and so precipitates out), leaving a solution of 50% NaOH and

1-2% NaCl

- Environmental Issues:

o The original diaphragm cell used a porous asbestos diaphragm

Asbestos was identified as a carcinogenic substance, leading to lung

diseases such as mesothelioma.

The manufacture and use of asbestos diaphragms was eventually banned by

legislation and more efficient and inert polymer diaphragms developed

o There may be hypochlorite (ClO-) (formed from the combination of Cl- and OH- ions),

a strong oxidant and disinfectant, in the waste brine solution, which must be

removed before brine is discharged into environment

2NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

o Recently there have been concerns about the long-term persistence of chlorinated

waste products in the environment, which have led to greater requirements on

industry to reduce the release of chlorinated wastes

o The high energy needs of the chlor-alkali industry is also an environmental concern as

the production of electricity requires burning of fossil fuels, releasing CO2 into

the atmosphere and contributing to global warming

- Technical Aspects & Considerations:

o The Diaphragm operates at the lowest working voltage (2.9-3.5V), so the cost of

electricity is the least for this cell

o The Diaphragm cell has medium energy consumption (2720 kwh/mt cl2)

o The Diaphragm cell produces sodium hydroxide that is of reasonable purity

(produced as a 50% solution, or as flakes, with 1-2% salt impurity) and less purer

than Mercury process

o Considerations:

A large system of many small cells is employed, instead of one large cell,

in order to keep the distance between the anode and cathode small, in order

to maximise current:

3.5-5 volts used, with tens of thousands of amperes of current.

Brine must be initially purified to remove impurities such as compounds of

Ca, Mg and Fe and which could block the diaphragm

Level of Anolyte higher than level of Catholyte (Explained above)

Purification of final NaOH solution – Fractional Crystallisation (Explained

Above)

Membrane Cell:

- The membrane cell is similar to a diaphragm cell, possessing an improved diaphragm called a

‘membrane’

o The new, inert membrane is made from perfluorosulfonic acid multilayer polymer

which has been modified to incorporate anionic groups (such as carboxylate) so that it

acts as ion exchange membrane

- Only Na+ ions (not OH- or Cl-) are selectively allowed to diffuse from the anolyte into the

catholyte.

o This ensures that the NaOH produced is more pure and concentrated than in a

diaphragm cell

The membrane cell typically produces sodium hydroxide with a salt impurity

of only 0.2%

- Electrolyte:

o The anode chamber contains saturated, purified brine

o The cathode chamber contains pure water or dilute NaOH solution (from which

concentrated NaOH is produced)

- The anode is still composed of activated titanium

o Here, Cl- ions are attracted to the positive anode, oxidised and removed as chlorine

gas:

o This results in an imbalance of Na+ and Cl- ions

Excess positive Na+ ions move across through the membrane into the cathode

chamber to equalise the charge

- The cathode is still composed stainless steel mesh

o In this chamber, water (from dilute NaOH solution or pure water) is still reduced to

hydrogen gas and hydroxide ions:

o The Na+ and OH- ions form the NaOH, increasing the concentration of NaOH in this

chamber

- Action of the Membrane:

o Ion selectivity is achieved by creating a thin, porous polymer membrane coated with

anionic groups that repel anions but not cations

This prevents OH- ions from entering the anolyte, and thus the formation

of hypochlorite ions

Prevent chloride ion diffusion into catholyte and increased formation of

NaCl in the catholyte

Leaves a pure NaOH solution that is concentrated to 50% in a falling-film

evaporator (research) using heat from steam

- Overall Cell Reaction:

o 2NaCl(aq) + 2H2O(l) H2(g) + Cl2(g) + 2NaOH(aq)

- Environmental Issues

o Because of the nature of the membrane, and the inert electrode used, there are very

few environmental issues associated with this cell

o Issues include the leakage of chlorine gas and NaOH into the environment

Recently there have been concerns about the long-term persistence of

chlorinated waste products in the environment, which have led to greater

requirements on industry to reduce the release of chlorinated wastes

- Technical Aspects & Considerations

o The Membrane cell operates at a relatively low working voltage (3.0-3.6V),

o The cell has the lowest energy consumption (2650 kwh/mt cl2)

The operating costs for this cell are hence the lowest

o The cell produces sodium hydroxide of high purity and concentration (50% with

0.02% salt impurity))

o Considerations:

Just as with the diaphragm cell, a large system of many small cells is

employed, instead of one large cell, in order to maximise the current:

3.5-5 volts used, with tens of thousands of amperes of current.

The polymer has many useful properties, such as its selective

permeability, as well as its ability to resist chemical attack in strongly basic

solution

Explain this technical consideration (using above)

Refer to Falling-film evaporator

More Needed on Technical Difficulties

- Non-Specific Technicalities

o Product Purity & Quality Control

Industrial chemists working in the chlor-alkali industry need to control the

quality of feedstocks, brine solutions and final products. The techniques

used to monitor quality include:

Acid-base titration to monitor the alkalinity of NaOH

Atomic absorption spectrophotometry (AAS) to determine the level of

Mg and Ca ion impurities in brine feedstocks

Gravimetric analysis to determine the moisture content of Cl

produced

Ion-selective electrodes to monitor the concentration of ions, such

as Cl- on solution

o Choosing an Industrial Site

The location of a chlor-alkali plant is a technical consideration that must be in

accordance with the following factors:

Location of Markets: Plant should be sited near other industries that

use one or more of the products of brine electrolysis

Energy: Plants use vast amounts of electrical energy and so this must

be readily available (needs to be cheap due high consumption)

Raw Materials & Feedstocks: Brine or salt must be readily available.

Such plants are hence located in coastal areas

Transport Networks: Site must be close to transportation such as

road, rail or shipping. Raw materials need to be efficiently transported

to such sites via established transport links

Workforce: A large workforce (Industrial chemists, technicians, site

workers) is required and many chemical jobs require tertiary

qualifications. Siting near a city or large town will provide this labour

force

Identify data, plan and perform a first-hand investigation to identify the products of

the electrolysis of sodium chloride

Refer to Chem Reference Notes (Idil & Siu). Do Last

Analyse information from secondary sources to predict and explain the different

products of the electrolysis of aqueous and molten sodium chloride

Refer to Chem Reference Notes (Idil & Siu). Do Last

5. Saponification is an important organic industrial process

Describe saponification as the conversion in basic solution of fats and oils to glycerol

and salts of fatty acids

- Glycerol

o Glycerol (1,2,3- propanetriol) is a triol, an alkanol with 3

hydroxyl groups

o Glycerol is derived from propane

- Fatty Acids

o Fatty acids are long chain carboxylic acids (-COOH) that occur in fats and oils

Saturated fatty acids contain only C-C single bonds forming fatty esters

which are solid fats

E.g. Stearic Acid CH3(CH2)16COOH, lauric acid CH3(CH2)10COOH,

myristic acid, palmitic acid

Unsaturated fatty acids contain at least one C=C bond and form esters that

are liquid oils

E.g. Oleic Acid CH3(CH2)7CH=CH(CH2)7COOH, Linoleic acid,

Linolenic acid

- Fats & Oils:

o Fats and oils (triglycerides) are fatty esters formed between fatty

acids and glycerol

The natural formation of fats and oils (fatty esters) is

from an esterification reaction

At room temperature, fats are solid, while oils are liquid

A triglyceride is a compound in which all 3 hydroxyl

groups of a glycerol molecule have been esterified with

carboxyl groups from 3 fatty acid chains

o E.g. Glyceryl Tristearate:

NOTE: When naming triglycerides; the glycerol is

changed to glyceryl, and the acid is changed from “-ic” to “-ate”, and the

prefix “tri-” is added.

Hence, by applying these rules, we can deduce that this fat was

formed by the reaction of glycerol with 3 chains of stearic acid

- Saponification – Process used to make soap

o Saponification is the hydrolysis, in alkaline solution, of fats and oils (fatty esters/

triglycerides) to produce glycerol and the salts of fatty acids (soap)

Essentially it is the reversal of esterification

Involves the reaction between a fatty ester and an alkali:

The fatty ester is mixed with an alkaline solution and heated

The alkali attacks the fat or oil molecules and produces salts of the

fatty acids present in the fatty ester (These fatty acid salts are called

soap)

Usually NaOH and KOH alkaline solutions are used

o E.g. Sodium Stearate

Glyceryl Tristearate is heated with concentrated sodium hydroxide solution

to form glycerol and sodium stearate

The NaOH splits the triglyceride along the ester bond (-COOC-)

o The 3 OH- groups attach to the glyceryl forming glycerol

The fatty acids (carboxylate ions) react with the Na+ ions (from

NaOH solution) to form sodium stearate (a common soap)

Describe the conditions under which saponification can be performed in the school

laboratory and compare these with industrial preparation of soap

Perform a first-hand investigation to carry out saponification and test the product

(Refer to Practical completed in class & Siu/Idil Reference Notes)

- School Laboratory Saponification:

o A pure fat or oil (most commonly coconut fat/oil) is mixed with an excess of NaOH

solution

Weigh approximately 10g of fat into a 500ml beaker

Mix 25ml of water with 25ml of methylated spirits in a flask. Dissolve

approximately 12g of NaOH in the water-alcohol mixture

Methylated spirits is added to emulsify the fat or oil into micelles so

that there is larger area in contact with NaOH solution (hence

increased reaction rate)

This NaOH solution is then stirred into the fat in the 500ml beaker.

The mixture is then heated on a hotplate for 30mins, the mixture being

stirred periodically to ensure good contact between base and the fat

More water-methylated sprits is also added periodically, to maintain

the volume of the liquid

After reaction, beaker is removed from heat and allowed to cool

Formed soap curds are ‘salted out’ using excess NaCl solution

Increasing the concentration of ions (using excess NaCl) causes the

soap to precipitate out and form the layer of soap curds (as NaCl is

more soluble than soap)

After vacuum filtration and washing with water to remove any alkali, the

crude soap can be collected and tested

- Industrial Saponification: Kettle Boiled Batch Process

o This process is performed in large, heated, pressurized steel containers called

kettles (may contain up to 120 tonnes of material)

o Fats or oils are first blended and mixed with concentrated NaOH in the kettles

Some salt and soap from a previous batch is left in kettle to assist with mixing

as it promotes the formation of emulsions and assists in the solidification of

the soap as it forms

o The mixture is boiled using high-pressure and high-temperature steam

o Additional hot brine and steam are added at the end of the saponification reaction

to ‘salt out’ the soap and wash it free of glycerol

Glycerol is extracted by neutralising excess NaOH, distilling off the water, and

then distilling the glycerol under low pressure

o The mixture settles for several days and the soap curd gradually collects at the

surface

Soap curd is washed with water to remove excess salt and alkali

Soap curd is then sprayed and vacuum dried (in the end containing 12%

water)

o Soap is then converted into small pellets that are combined with fragrances and

colours before reblending and reprocessing into the desired form (liquid, powder or

bars)

- Comparison of SLS & KBBP

School Laboratory Saponification Kettle Boiled Batch Process

Similarities - Fats and oils are mixed with concentrated alkali and heated

- Concentrated, excess brine (NaCl solution) is used to separate the soap from the aqueous phase (‘salting out’) - The crude soap is washed free of glycerol

Differences - One pure fat or oil is used - A simple hotplate is used to heat the

mixture and a glass rod is used to stir - Some glycerol still remains in the soap - The soap is relatively crude and

processed and collected rapidly - Methylated spirits is added to help

emulsification (an emulsion is a dispersion of one liquid in another) - No fragrances or colours were

added to the soap (not processed highly)

- A blend of fats or oils is used - High-pressure and high-

temperature steam is used to heat and stir the mixture - Glycerol is removed and purified for use

- Longer process; settling of soap occurs over several days

- A previous batch of soap and salt (as opposed to methylated sprits) is left in the kettle to emulsify the new reactants

- Fragrances and colours are combined with the soap (processing occurs into desired forms of soap such as bars, liquid, powders, etc.)

Refer to Chem Jac 2 pages 417-418 for 2 other industrial saponification processes

Gather, process and present information from secondary sources to identify a range

of fats and oils used for soap-making

Fat or Oil Properties

Tallow - Very common animal fat derived from beef processing - Produces hard, greasy soap (unless other oils such as coconut oil are blended with it before alkaline hydrolysis)

Lard - Derived from pigs

- Produces hard soap that lathers quickly but does not readily dissolve in water

Coconut Oil (mainly lauric acid)

- Derived from pressed, dried fruit of coconut palm (relatively pure oil) - Often blended with tallow to produce softer soap that dissolves faster in water

- Produces soap that lather in salt or hard water

Palm Oil - Derived from the flesh of the palm fruit - Long hydrocarbon chains give it properties more similar to tallow than

vegetable oils

Olive Oil (mainly

oleic acid)

- Derived from crushed fruits of olive trees

- High percentage of unsaturated fatty acid

Account for the cleaning action of soap by describing its structure

- RECALL that like dissolves like:

o Polar substances dissolve in other polar substance

o Non-polar substances dissolve in other non-polar substances

- Water is polar, while grease and dirt are non-polar; they do not mix

o The force that keeps two immiscible liquids separated from each other is called

interfacial tension

- A surfactant (surface acting agent) is an agent that lowers the interfacial tension

between two liquids, allowing them to mix

o SOAP is a surfactant, allowing polar water and non-polar grease/dirt to mix

o The structure of soap is responsible for its role as a cleaning agent:

- Structure of Soap:

o Soap is an ionic compound.

The positive ion is usually a sodium (Na+) or potassium (K+) ion

The negative ion consists of:

Tail – long hydrocarbon tail

Head – Carboxylate end group

o The surfactant (cleansing) property of soap is ONLY caused by the structure of the

negatively charged fatty-acid chain; the cation has no effect

The head is said to be hydrophilic (water-loving) as it has a strong affinity

for water (readily forms hydrogen bonds with water)

The tail is said to be hydrophobic (water-hating), as the non-polar tail has a

little affinity for water (rather dissolving in non-polar grease)

- Cleaning Action of Soap:

o When stained clothing is added to a soap-water mixture, the long, non-polar

hydrocarbon tails form dispersion forces with the non-polar grease, dissolving in

it

The polar anionic heads remain at the surface of the grease, interacting

(dipole-dipole interactions) with water molecules

o Agitation allows grease layer to lift off the material, and form droplets or micelles

dispersed throughout the water

The hydrocarbon tails are buried deep in the grease or dirt particles, allowing

them to be lifted and carried away from clothing or skin

This stabilised mixture of grease, soap and water form an emulsion – soap

being the emulsifier

The negative heads of the micelles repel each other and prevent them from

clumping and joining together again

As the mixture is agitated, more grease is lifted of the surface of the dirty

object, and more grease/soap particles are formed

o

o Excess soap forms a lather of soap, water and air, which helps suspend the

particles of grease

o The last step is to rinse the lather and the soapy emulsion of grease and water

away from the material (using fresh water) leaving the material clean

EXTRA

- In dilute solutions, the soap anion forms a surface layer, where the hydrophobic tail is

out of the water and the hydrophilic head is stabilized in the water by ion-dipole

attractions.

o This monolayer of soap anions at the water surface disrupts the hydrogen

bonding between water molecules and lowers the water’s surface tension

This increases the ability of water to ‘wet’ a surface.

- Thus, soap is acting as a surfactant (or ‘surface active agent’).

- Another way that soap anions can stabilize themselves is to form structures called

micelles in the bulk of the soap solution

o The long hydrocarbon chains cluster to form an oily central core where the chains

are stabilised by dispersion forces.

o The negative head groups are exposed on the surface of the micelle where they

interact through ion-dipole attractions with the positive ends of the water dipoles

Explain that soap, water and oil together form an emulsion with the soap acting as

an emulsifier

- An emulsion is a dispersion of small droplets of one liquid throughout another liquid

o Two normally immiscible substances are stabilised by an emulsifier

o Emulsifiers are the same as surfactants – they reduce the surface tension to allow

miscible liquids to mix

- In a mixture of oil and water:

o If we try to mix oil and water by vigorous agitation, we form a temporary dispersion

of oil throughout the water

Eventually the oil droplets join together to form larger droplets that rise to the

surface to form a separate layer of oil

o The presence of soap, however, will create an emulsion upon agitation, with soap

acting as an emulsifier

The hydrocarbon tails of the soap anions dissolve in the oil and promote

droplet formation

These droplets repel each other, due the negatively-charged carboxylate

heads surrounding the droplets

- Oil-in-Water Emulsions:

1) Consist of oil colloidal particles dispersed in water

2) Oil in water emulsions have a greater proportion of water than

oil

Dissolve better in polar solvents, such as water,

rather than non-polar solvents such as kerosene

3) These emulsions have a non-greasy feel on the skin

4) Examples: Milk, mayonnaise, cream, cosmetic cleaners

- Water-in-Oil Emulsions

1) Consist of colloidal droplets of water dispersed in an oil

2) Oil-in-water emulsions mix better with non-polar solvents

3) They have a greasier feel due to the high proportion of oil

4) Examples: Butter, sunscreen, lipstick, cold cream, hand cream

Perform a first-hand investigation to gather information and describe the properties

of a named emulsion and relate these properties to its uses

- Aim: To prepare mayonnaise as an emulsion and test its properties. Mayonnaise is an

emulsion of vegetable oil and egg yolk, with the emulsifier being the lecithin found naturally in

the egg yolk.

- Equipment: vegetable oil, egg, vinegar.

- Method:

1) The mayonnaise was made by slowly adding oil to the egg yolk, while whisking

vigorously to disperse the oil. The lecithin stabilised the mixture by binding the oil and

water together.

2) A little water was added if the mixture was too thick.

3) Vinegar was added for flavour.

- Properties in relation to uses:

o Slow filtration of mayonnaise resulted in some oil droplets remaining in the filter paper,

indicating that mayonnaise is a dispersion of oil particles through aqueous

solution.

o Mayonnaise is a very stable emulsion, due to the strong emulsifying properties of

lecithin. It does not readily separate into its component layers.

This property is useful as it is a food product and it needs to stay in an edible

condition, in storage, for relatively long periods of time.

o Mayonnaise has the property of having a smooth creamy texture (with a

homogenous appearance) that does not feel oily (due to being an oil-in-water

emulsion)

This contributes to its appeal as a food product, because most people prefer

the creamy taste of mayonnaise over the sensation of eating pure oil.

- Risk Assessment:

o The emulsion formed is tested by eating, so care must be taken in regards to

cleanliness and refrigeration to avoid infection.

Refer to Chem Jac 2 Pages 446-447 and In-Class Practical For MORE

Perform a first-hand investigation to demonstrate the effect of soap as an emulsifier

- Aim: To demonstrate the effect of soap as an emulsifier.

- Equipment: test tube, cooking oil, soap solution.

- Method:

o A test tube was half filled with hot water, and then cooking oil was added.

o The tube was shaken vigorously and allowed to settle. The oil formed a separate layer

on top of the water as the two liquids were immiscible.

o A few drops of soap solution in water were added to the test tube, and the tube was

again shaken vigorously and allowed to stand until most of the bubbles had broken up.

- Results:

o The mixture now appeared homogenous (no separate layers) and had a cloudy

appearance.

o This suggested that an emulsion of invisibly small oil droplets dispersed through the

water had formed.

o This demonstrated that soap can act as an emulsifier, a substance that can help form

an emulsion between two normally immiscible liquids.

Distinguish between soaps and synthetic detergents in terms of:

o The structure of the molecule

o Chemical composition

o Effect in hard water

Soap:

- Structure:

o Soap molecules consist of a cation (e.g. Na+) which has no effect on the cleaning

action of soap

o It also consists of a long hydrocarbon tail with an anionic (negatively charged)

carboxylate head

- Chemical Composition:

o Soap is the salt of a fatty acid (most soaps are sodium stearate, stearic acid being

the fatty acid)

o It is a hydrocarbon molecule consisting of:

The head, a carboxylate anion (-COO) which is hydrophilic (water loving)

The tail, a long hydrocarbon chain which is hydrophobic

o Chemical composition is usually CH3(CH2)nCOO- Na+

- Effect in Hard Water :

o Soap (most commonly sodium stearate) does not function well in hard-water

Hard water is water that contains high levels of Mg2+ and Ca2+ ions

Soap is unable to produce a lather of bubbles in hard water

o When soap is used in hard water, the soap (stearate) anions react with high levels of

Mg2+ and Ca2+ ions to produce insoluble precipitates

These precipitates occur as a greyish ‘soap scum’ which stains ceramic sinks,

baths as well as clothes, reducing overall effectiveness of the soap (by

reducing the number of available soap molecules)

o E.g. Precipitation of calcium stearate

The calcium stearate sticks to and stains clothes if they are washed repeatedly

with soap in hard water

Anionic Detergents:

- Structure:

o Like soaps, anionic detergents consists of a long hydrocarbon tail and anionic head

group:

o These detergents are called ‘anionic’ because the actual surfactant ion (everything

except Na+ or the cation) has a negative charge

o Different to soaps, in the way that the structure and chemical composition of the

head is different

- Chemical Composition:

o Differ from soaps in that the head is a different ion to

carboxylate

o Anionic detergents are mostly the salts of alkyl benzene

sulfonates

The consist of long hydrocarbon tail (alkyl

group) connected to a benzene ring, bonded

to a ‘sulfonate’ group (-SO3-)

The sulfonate group is polar and hence hydrophilic

o Other common detergents in this class include alkyl

sulfates, alkyl ether sulfates and alkyl phosphates

o Typical composition is CH3(CH2)n(C6H6)O(SO2)O- Na+

- Effect in Hard Water:

o The efficiency of anionic detergents is decreased slightly by hard water, but not nearly

as great as for soap

This is not due to the formation of insoluble precipitates, but rather because

the surfactant anions form ‘soluble complexes’ with Mg2+ and Ca2+. This

reduces the amount of available surfactant.

o However, anionic detergents are still very effective cleaning agents, even in high-

ion hard water

o Lather in hard water well except alkyl sulfates and alkyl phosphates (form precipitates

in low pH water)

Cationic Detergents:

- Structure:

o Cationic detergent molecules consist of a long hydrocarbon tail with a positively

charged head group:

o Called ‘cationic’ because the surfactant ion has a net positive charge ( as opposed to

the net negative charge of soap and anionic detergents)

o Like most surfactants, they consist of a hydrophobic, hydrocarbon tail connected

to a charged, hydrophilic head

- Chemical Composition:

o The negative ion is often a halide ion such as chloride or bromide (Note: this anion

has no effect in the cleaning action of CD)

o The positively charged head in cationic

detergents is commonly the alkyl ammonium

group (also called the quaternary ammonium

group) or the alkyl pyridinium group

Quaternary ammonium cations are

ammonium (NH4+) derivatives; that

is, the four hydrogens (hence

quaternary) of an ammonium ion have

been replaced by alkyl groups; usually, 3

hydrogens are replaced by methyl

groups, and one hydrogen is replaced by

a long hydrocarbon chain.

o Typical composition is CH3(CH2)nN+(CH3)3Cl-

- Effect in Hard Water:

o Cationic detergents are highly effective in hard water

Their cleaning action is not affected by the high concentration of Ca2+ and

Mg2+ ions in hard water

Due to the positively charged head group of the CD molecule, ions within

hard water do not bond to these molecules (to form insoluble precipitates

or complex such as those formed with soap and AD)

o Able to lather in hard water and soft water

Non-Ionic Detergents:

- Structure:

o Unlike soap and ionic detergents, non-ionic detergents have no distinct, charged

head or tail, but rather possess hydrophilic groups:

o Consist of a long hydrocarbon tail, connected to another hydrocarbon chain,

containing oxygen atoms, ending in a hydroxyl group

- Chemical Composition:

o Non-ionic surfactants are also called ethoxylates, because they are formed by joining

together many ‘ethoxy’ groups (-CH2-CH2-O-)

o Consisted of molecules, not ions

The presence of oxygen makes certain sections of the molecule polar

The oxygen atoms form hydrogen bonds with water, while the non-polar tail

bonds to the dirt/grease

The detergent molecules then surround the grease molecule, similar to

the way ionic detergents surround grease.

o Typical Composition is CH3(CH2)nO(CH2CH2O)n(CH2)2OH. The second n varies from

5 to 50

- Effect in Hard Water:

o Non-ionic detergents are highly effective in hard water

Their cleaning action is not affected by the high concentration of Ca2+ and

Mg2+ ions in hard water

Non-ionic detergent molecules do not react at all with the Mg2+ and the

Ca2+ ion, so completely unaffected by hard water

o Able to lather in hard water and soft water

Distinguish between anionic, cationic and non-ionic synthetic detergents in terms of:

o Chemical composition

o Uses

Refer to above dot point for comparison between Chemical Composition of the different

synthetic detergents

Uses:

- Anionic Detergents:

o Most common cleaning agents, used in general cleaning purposes including:

Liquid and powder laundry detergents

Dishwashing detergents

Hand-soaps

Shampoo

Toothpaste

Heavy-duty engine lubricants

o Effective in removing grease and stains from natural fibres

o Examples:

Sodium dodecylbenzene sulfonate - Most commonly used in laundry

detergents as it lathers well and can be dried to a stable powder during

manufacturing

Barium tetradecylbenzene sulfonate – Used in heavy-duty engine

lubrications

Sodium alkyl ether sulfates – Used in shampoos as well as dishwashing and

laundry liquids

- Cationic Detergents:

o The cationic head group binds strongly to negatively charged surfaces. Accounts for use

in:

Hair conditioners

Positive heads attach to negatively charged hair, and the surface of

the hair becomes coated with long hydrocarbon tails sticking out,

leaving a smooth, waxy feel and shiny appearance

Fabric Softeners (e.g. Distearyl dimethyl ammonium chloride)

Positive heads bond to negative fabric fibres, leaving a soft, smooth

feel

o Disinfectants (e.g. Monoalkyl quaternary ammonium detergents; lauryl dimethyl

benzyl ammonium chloride)

Antiseptic properties of such detergents make them very useful in many

household disinfectants and sanitisers including antiseptic soaps,

mouthwashes, and lozenges for sore throats

o Also used in water treatment facilities as flocculating agents

o Cationic detergents are not used as soaps or in dishwashing liquids as they leave

the surface with a greasy feel.

- Non-Ionic Detergents

o Mainly used in paints, adhesives and cosmetics (where improved contact between

polar and no-polar substances is required)

o They produce less foam than other detergents, so they are also used in low foam

applications, such as dishwashing powders and front-loading machines

o Also used in insecticides, herbicides, and pharmaceuticals

Solve problems and use available evidence to discuss, using examples, the

environmental impacts of the use of soaps and detergents

- Biodegradability:

o Soaps are biodegradable as they are single-chain hydrocarbons (derived from fatty

acids)

They are easily broken down into carbon dioxide and water by the action of

natural decomposers in the environment (such as bacteria)

o Synthetic detergents that were developed (to overcome the shortcomings of soaps)

posed an environmental risk, associated with their non-biodegradability

Branched-chain anionic detergents were developed (extensive branching

of hydrocarbon tails)

They were not biodegradable and persisted in the environment

as they were not decomposed by microbes in the few days it took soap

residues to decompose

Rivers and dams became covered

in foam from a build-up of detergent.

Foams reduce diffusion between water

and the air, making waterways

unsuitable for aquatic life

The following table compares the time

for branched-chain detergents and

soaps to decompose in the natural

environment

Linear detergents were then developed by

synthesising detergents with non-branching tails

These are much more biodegradable than branched detergents, and

so solved the foam problem

- Phosphates:

o Synthetic detergents do not contain any phosphates (perhaps trace amounts)

As detailed above, anionic detergents are affected slightly by the

presence of Mg2+ and Ca2+ ions in hard water (surfactant anions forming

complex)

o Substances called ‘builders’ are added to detergent powders to increase their

efficiency

Builders include phosphate and polyphosphate compounds (e.g. sodium

tripolyphosphate Na5P3O10)

Builders are used to complex the ions in hard water, preventing them

from interfering with the washing process (effectively softening the water and

increasing cleaning power of detergents):

o However, increased use of ‘builders’ adds to increasing level of phosphates in

waterways, making such use a major environmental problem:

Excess phosphates in waterways lead to eutrophication and algal blooms,

which lead to the depletion of oxygen in the water system.

This causes severe degradation of water quality, the promotion of

anaerobic conditions and the disturbance of the natural ecosystem

o Sodium zeolite is now being used to replace phosphates in detergents

Zeolites help remove Ca2+ and Mg2+ ions from hard water by exchanging Na+

ions for Ca or Mg ions.

These zeolites do not cause the eutrophication problems associated with

phosphates.

- Biocidal Properties:

o Cationic detergents have mild biocidal properties

They are attracted to the membrane surfaces of bacterial cells where

they disrupt the cellular processes

o The presence of these cationic detergents in wastewater and in sewage treatment

facilities can alter the balance of bacterial decomposers

At high concentrations of CD’s, the bacteria that decompose sewage are

killed

o Cationic detergents represent a small percentage of detergents used, so environmental

impact is minimal

6. The Solvay process has been in use since the 1860s

Describe the uses of sodium carbonate

Sodium Carbonate is used in many industries:

- Glass Manufacture

o The main use of SC is in the production of glass for windows and bottles

o Glass is produced when SC is heated at very high temperatures, in combination with

sand (SiO2) and calcium carbonate (CaCO3), and cooled very rapidly

o SC acts as a flux in glass production as it lowers the melting point of SiO2 and CaCO3

- Soap & Detergent Production

o SC can be used as a base in soap and detergent manufacture, in place of stronger

alkalis, such as NaOH

o SC is also used to make sodium phosphate and sodium silicates that act as

builders

- Softening Agent

o SC can be used to soften hard water as the carbonate ions precipitate Ca and Mg

ions

o Some laundry powders contain SC to lower hardness of water

- Baking Soda Production

o NaHCO3 is produced as part of the Solvay process (process for manufacture of Sodium

Carbonate). It is commonly called baking soda

o Baking soda is used in cooking to produce carbon dioxide as a raising agent.

o It is also used in CO2 fire extinguishers.

- Borax Manufacture

o Borax (Na2B4O7) is the salt formed when boric acid (H3BO3) neutralizes SC

solution

o Borax is used in the manufacture of glazes and glass

o It is also used as a cleaning agent in laundry products

Identify the raw materials used in the Solvay process and name the products

- The Solvay Process is the major chemical industrial process used to manufacture sodium

carbonate (Na2CO3)

- Raw Materials:

o Brine (saturated sodium chloride solution, NaCl)

o Limestone (calcium carbonate, CaCO3)

o Ammonia (NH3)

- Products:

o Soda Ash (Sodium carbonate, Na2CO3)

o Calcium chloride (CaCl2)

Identify, given a flow chart, the sequence of steps used in the Solvay process and

describe the chemistry involved in:

o Brine purification

o Hydrogen carbonate formation

o Formation of sodium carbonate

o Ammonia recovery

- Step 1 – Brine Purification

o Brine for this process is obtained from:

Salt water stored deep underground

Underground rock salt deposits

Sea Water

o Before brine solution can be used, it must be purified to remove impurities such as

Ca and Mg salts which could interfere with the crystallisation of sodium hydrogen

carbonate

Sodium carbonate (previously produced) and NaOH are added to precipitate

out insoluble CaCO3 and Mg(OH)2

Ca2+ (aq) + CO3

2- (aq) CaCO3 (s)

Mg2+ (aq) + 2OH-

(aq) Mg(OH)2 (s)

A flocculent is added and the insoluble CaCO3 and Mg(OH)2 precipitates are

flocculated and filtered off

o This whole process of purification, leaves behind a purified sodium chloride solution

(purified brine) which is concentrated to about 30% (w/w)

- Step 2 – Sodium Hydrogen Carbonate Formation

o Carbon dioxide is obtained by the thermal decomposition of limestone (CaCO3) in

the vertical-shaft lime kiln leaving calcium oxide (CaO) as a by-product to be used

in ammonia recovery.

CaCO3(s) CaO(s) + CO2(g) ……………………………… (1)

o In the ammonia absorber tower, the purified brine solution is saturated with

ammonia (obtained from the Haber process)

During this exothermic process, considerable heat is released so the tower

must be cooled with cold water pipes to ensure that the brine is fully

ammoniated

o The ammoniated brine solution is fed into the carbonator.

Here, carbon dioxide gas (from the lime kiln as well as recycled CO2) is

pumped under pressure (about 300kPa) into the base of the carbonator

The ammoniated brine is fed into the top of the carbonator and trickles

downwards over the surfaces of serrated plates inside the carbonator which

provide a high SA for reaction

Through a series of reversible reactions, sodium hydrogen carbonate is

formed:

Firstly, rising CO2 reacts with water in ammoniated brine solution to

form carbonic acid:

o CO2 (g) + H2O (l) H2CO3 (aq)

This weak acid then reacts with the weak base ammonia, forming the

hydrogen carbonate ion and ammonium

o H2CO3 (aq) + NH3 (aq) HCO3ˉ (aq) + NH4

+ (aq)

The low temperature (0oC) and high salt concentration in the bottom

half of the carbonator promotes the crystallisation of sodium

hydrogen carbonate. The Na+ ions form a precipitate with the HCO3-

ions, as NaHCO3 is less soluble than NH4Cl, leaving a solution of

ammonium chloride behind

o Na+ (aq) + HCO3ˉ

(aq) NaHCO3 (s)

Overall ionic equation in carbonator tower:

o Na+ (aq) + CO2 (g) + NH3 (aq) + H2O (l) NaHCO3 (s) +

NH4+

(aq)

Adding in the chloride spectator ion, gives complete equation:

o NaCl (aq) + CO2 (g) + NH3 (aq) + H2O (l) NaHCO3 (s) +

NH4Cl (aq)

o Only about 75% of the original salt is converted to sodium hydrogen carbonate.

Unreacted salt is recycled

- Step 3 – Formation of Sodium Carbonate

o The solution leaving the carbonator is filtered and the NaHCO3 is collected.

Rotary vacuum drum filters are used to separate the NaHCO3 from the

filtrate (containing NH4Cl solution), which is returned to the ammonia

recovery distillery

The crystals are washed and then dried by drawing in air through the

crystalline mass

Any excess ammonia (NH3) that is adsorbed, or NaCl, or CO2 is recycled

o The dried, NaHCO3 crystals are then heated in a rotating calciner (heat obtained

from contact with steam-heated tubes)

Sodium hydrogen carbonate decomposes to produce anhydrous sodium

carbonate, water vapour and carbon dioxide

2NaHCO3 (s) Na2CO3 (s) + CO2 (g) H2O(g)

o The solid NaHCO3 could also have been packaged and sold as baking soda (as

opposed to placing it in a calciner)

- Step 4 – Ammonia Recovery

o Ammonia is considered to be a catalyst of the Solvay Process because it is not

consumed in the reaction, but allows the reaction to proceed (Note: the input of

ammonia is a once-only process as the ammonia is continuously recycled)

o The by-product of calcium oxide (from thermal decomposition of CaCO3 in the lime

kiln) is ‘slaked’ (reacted with water) in the slaker to form slaked lime (calcium

hydroxide, Ca(OH)2)

CaO (s) + H2O (l) Ca(OH)2 (aq)

o The residue solution (filtrate) from the rotary vacuum drum filter is an ammonium

chloride solution (also containing CO2 and NH3 which are removed from the filtrate

through distillation and then recycled)

o In the ammonia recovery distillery, ammonia is recovered.

Ammonium chloride solution is heated with the calcium hydroxide (from

slaker) to recover the ammonia

2NH4Cl (aq) + Ca(OH)2 (aq) CaCl2 (aq) + 2NH3 (aq) + 2H2O (g)

The ammonia released is distilled off using large quantities of low-pressure

steam to ensure that little ammonia is lost

The only waste product is calcium chloride (CaCl2) which can be recovered

and sold but most of it is released into the ocean.

- Overall:

o Adding up all the above equations, the Solvay Process can be summarised as:

2NaCl (aq) + CaCO3 (s) Na2CO3 (s) + CaCl2 (aq)

Solvay Process Flow Charts & Diagrams

Perform a first-hand investigation to assess risk factors and then carry out a

chemical step involved in the Solvay process, identifying any difficulties associated

with the laboratory modelling of the step

- Step A – Decomposition of Sodium Hydrogen Carbonate

o Aim: To model the decomposition of sodium hydrogen carbonate

o Equipment: 1 teaspoon of NaHCO3, 150ml lime water, Bunsen burner, ammonium

chloride and calcium hydroxide solutions.

o Method:

1) 1 teaspoon NaHCO3 of was weighed and placed into a pre-weighed test tube

2) The test tube was stoppered and clamped to a retort stand

3) The test tube was heated with a Bunsen flame and the evolved gases were

passed through a rubber delivery tube into a beaker of limewater

4) The test tube was cooled and weighed again to determine the loss in weight.

o Results:

o

o Water vapour formed around the mouth of the test tube

o The lime water turned cloudy, indicating presence of CO2 gas

o The test tube decreased in mass, indicating the loss of water vapour and carbon

dioxide

o Risk Assessment (Factors):

Na2CO3 is a product of the reaction and is slightly toxic if ingested and also a

skin irritant.

Care should be taken when handling this substance, and hands should be

washed immediately if they come into contact with the chemical. Wear

safety gloves, goggles, lab coat

The Bunsen burner has potential risks of causing burns to skin and causing

a fire in the school laboratory.

All flammable materials should be removed from the work area when

heating.

If burns occur, rinse the affected area with cold water for 10 minutes

Wear protective clothing and use Bunsen on safety flame when not

heating

o Difficulties associated with the laboratory modelling of this step:

The difficulty encountered in this experiment was that it was not clear when

the reaction had gone to completion, because the reactant and product

were both the same colour (white)

- Step B – Ammonia Saturator

o To model this step, 10 mL of concentrated ammonia solution (30%) was mixed with

10 mL of brine (saturated sodium chloride solution).

o 10g of ammonium chloride crystals were dissolved into the solution, and then more

continuously added until no more would dissolve.

o The clear solution was then decanted into a conical flask.

o Risk Factors

Ammonia is highly toxic. It was handled in the fume cupboard

o Difficulties

A higher concentration of ammonia is used in the Solvay Process.

This is hard to attain in the school laboratory (and unsafe), so instead,

ammonium crystals were used to facilitate the carbonation

Sample Solvay Process Questions

Process information to solve problems and quantitatively analyse the relative

quantities of reactants and products in each step of the process (MUST REFER TO

CHEM JAC 2, PAGES 434-435 FOR PRACTICE QUESTIONS)

E.g. What masses of limestone (88.7% CaCO3) and brine (30.9% NaCl) are needed to

produce a tonne of SODIUM CARBONATE?

- Firstly, we state the chemical equation to determine the reaction ratios:

o 2NaCl (aq) + CaCO3 (s) Na2CO3 (s) + CaCl2 (aq)

- Finding the molar mass of sodium carbonate:

Molar mass (Na2CO3) = 2 (22.99) + (12.01) + 3 (16.0)

= 106.0 g/mol

- Finding the number of moles of Na2CO3 in tonne:

Moles (Na2CO3) = mass / molar mass

= (1000000) / (106.0)

= 9433.96

- Finding the mass of limestone used:

Moles (CaCO3) = moles (Na2CO3); reaction ratio of 1:1

= 9433.96

Molar mass (CaCO3) = (40.1) + (12.0) + 3 (16.0)

= 100.1 g/mol

Mass (CaCO3) = moles × molar mass

= 9433.96 × 100.1

= 944339.40 g

o BUT limestone is 88.7% CaCO3. Hence, the mass of limestone is:

Mass (limestone) = 944339.40 / 88.7 × 100 = 1.06 tonnes

- Similarly, finding the mass of brine used:

Moles (NaCl) = 2 × moles (Na2CO3); reaction ratio of 2:1

= 2 × 9433.96

= 18867.92

Molar mass (NaCl) = (23.0) + (35.5)

= 58.5 g/mol

Mass (NaCl) = moles × molar mass

= 18867.92 × 58.5

= 1103773.32 g

o BUT brine is 30.9% NaCl. Hence, the mass of brine is:

Mass (brine) = 11.3773.32 / 30.9 × 100 = 3.57 tonnes

Discuss environmental issues associated with the Solvay process and explain how

these issues are addressed

- Disposal of Calcium Chloride:

o The Solvay process produces calcium chloride as a waste product.

At the end of the ammonia recovery process, a solution of CaCl2 (~15%

(w/w)) containing some salt, slaked lime and suspended lime-stone particles

remain

o Although a certain amount of it is recovered and purified for sale (used as a drying

agent and in de-icing roads), far more is produced than is required for these purposes

o Coastal Plants: In Australia’s only Solvay plant (in Osborne, Adelaide, located close

to the sea)

Residual CaCl2 (slightly alkaline due to presence of calcium hydroxide) is

discharged safely into the ocean with suitable dilution.

This does not significantly increase the concentrations of Ca2+ or Cl-

ions in the oceans, as they already exist in high concentrations.

Hydrogen carbonate ions, naturally present in sea water, can also neutralize

excess alkalinity in the waste solution:

o Inland Plants:

In inland plants (where no ocean is available) CaCl2 waste solutions cannot be

discharged very easily

Could not be discharged into rivers and lakes as this would

significantly increase the salinity, concentration of ions (Ca and

Cl), causing disruption to ecosystems

CaCl2 can raise the salinity levels in the soil, contaminating ground

water

How Issues Are Being Addressed:

Solvay companies in such locations contain their waste in settling

ponds before discharging solutions

In some cases, the solutions are evaporated to dryness, and the

minerals buried underground; it is suitable, but very expensive.

- Thermal Pollution:

o Some of the reactions in the Solvay process are exothermic and produce large

amounts of heat.

o Water from nearby rivers or lakes is used as a coolant to absorb waste heat.

If this heated water was returned directly to the river or lake, it could cause

thermal pollution, resulting in lower levels of dissolved oxygen (as the

solubility of oxygen in water decreases as temperature of water body

increases)

This would destroy aquatic life and disturb the ecological balance.

o How Issue Could be Addressed:

To reduce this problem, the water is cooled in cooling ponds before it is

discharged into the environment.

Heat diffusers can be used to cool the water, but this is an expensive

process.

- Ammonia Leakages:

o There is often some loss of ammonia to the atmosphere from Solvay plants (e.g.

from the ammonia recovery unit).

Ammonia is an air pollutant, particularly if discharged near cities.

It can cause coughing and irritation of sensitive organs (e.g. eyes and

throat) and severe burns if high concentrations of ammonia come into

contact with skin.

o How Issue is Being Addressed:

Careful design and monitoring of Solvay plants keeps ammonia leakages at an

acceptable level.

For example, scrubbers in chimneys remove ammonia gases:

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EXTRA

- Mining & Farming

o Limestone

The limestone required for the Solvay process is mined

Most of these mines are open-cut and the normal issues associated

with mining and environmental damage should be addressed

o Salt

Establishment of salt farms to evaporate sea water needs to be regulated so

that minimal damage occurs to local environment

- Particulate Emissions:

o Particulate emissions from kilns and dryers are controlled by scrubbers and

electrostatic precipitators

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Use available evidence to determine the criteria used to locate a chemical industry

using the Solvay process as an example

The criteria used for selecting a location for an industrial plant are:

- Availability of Raw Materials:

o Raw materials must be readily available either locally or by establishment of

transport links

o Raw materials should be available at a low cost

o Solvay Process example:

Salt is obtained by solar evaporation of sea water in salt farms near the plant

Limestone is mined locally and transported on a dedicated rail line

Ammonia is transported as liquid ammonia in rail tankers from Haber plants

Water pipelines linking a fresh water reservoir to Solvay plant are

established

- Availability of Energy:

o Electricity and heat are required for the plant

o Rail transport of coal and coke should be established

o Solvay Process Example:

Coke is used to decompose limestone and for heating

Coal is used to generate electricity and as a fuel to create steam

The Solvay process is a relatively low energy process so it is more

important to locate the plant close to raw materials than to energy supplies

- Proximity to Markets:

o The products of the chemical industry should be readily marketable locally,

domestically and internationally

o Solvay Process Example:

Sodium carbonate has a universal market, as it is a high demand product

Soda ash is a major component of glass manufacturing, so local and

domestic industries are important to ensure profitability

Markets for baking soda (NaHCO3) are also important

- Environmental Issues & Waste Disposal

o Waste disposal strategies must be established to prevent environmental pollution

o Solvay Process Example:

This is a major consideration when locating a Solvay plant, as large amounts

of calcium chloride are produced.

CaCl2 waste can be diluted and discharged into the ocean with minimal

effect on the aquatic environment.

Therefore, coastal plants are more practical than inland plants

(associated with a greater potential to harm the environment)

Heat exchangers can be used to recycle excess heat so that thermal

pollution of waterways is minimised

- Transport:

o Raw materials need to be transported to the factory and products readily transported

to markets

o Needs to be in close proximity to established transport links

o Solvay Process Example:

Dedicated rail links bring in raw materials such as limestone

Rail links can also supply coke and coal

Products can be exported through coastal ports and transported intrastate

and interstate

- Personnel:

o Factories should be located near towns or cities to obtain the necessary skilled

workforce including technicians and industrial chemists