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HSC CHEMISTRY CORE TOPIC 2. THE ACIDIC ENVIRONMENT. INDICATORS. LE CHATELIER’S PRINCIPLE. TITRATION. pH. VOLUMETRIC ANALYSIS. NEUTRALISATION. CHEMICAL EQUILIBRIUM. THE ACIDIC ENVIRONMENT. HISTORY Lavoisier Davy Arrhenius. CARBOXYLIC ACIDS. ACIDIC OXIDES. BRONSTED - PowerPoint PPT Presentation
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HSC CHEMISTRY CORE TOPIC 2
THE ACIDIC THE ACIDIC ENVIRONMENTENVIRONMENT
THE ACIDIC ENVIRONMENT
VOLUMETRIC ANALYSIS
TITRATION
BRONSTEDLOWRYTHEORY
ESTERIFICATION
HISTORYLavoisier
DavyArrhenius ACIDIC
OXIDES
LE CHATELIER’S
PRINCIPLE
CARBOXYLICACIDS
ACID RAIN
pH
CHEMICAL EQUILIBRIUM
INDICATORS
NEUTRALISATION
Subsection 1Indicators were identified with the observation that the colour of some flowers depends on soil composition
Indicators
• substances that have distinctive colours in different types of chemical environments
• natural acid-base indicators are vegetable dyes that provide the colour of flowers and vegetables
• LITMUS is a pink mixture of compounds extracted from lichens grown mainly in the Netherlands
• today many indicators used are manufactured dyes
INDICATORS
INDICATOR pH RANGECOLOUR RANGE(low pH – high pH)
Methyl orange 3.1 – 4.4 red (orange) yellow
Bromothymol blue 6.0 – 7.6 yellow (green) blue
Phenolphthalein 8.2 – 10.0 colourless - crimson
Litmus 5.5 – 8.0 red - blue
INDICATORS
INDICATORS
Universal Indicator is a mixture of • thymol blue• methyl red• bromothymol blue• phenolphthalein• dissolved in methanol, propan-1-ol and water
INDICATORS
A solution gives the following colours in each indicator. Deduce the approximate pH
Indicator Colour
methyl orange yellow
bromothymol blue yellow
phenolphthalein colourless
>4.4
<6.0
<8.0
pH of solution is 4.4 – 6.0
INDICATORS
A solution gives the following colours in each indicator. Deduce the approximate pH
Indicator Colour
bromothymol blue blue
thymolphthalein colourless
phenol red red
> 7.6
< 9.5
> 8.4
pH of solution is 8.4 – 9.5
INDICATORS
USES• Universal indicator used to test soil
acidity/alkalinity (pH)• plants have preference for alkaline/neutral/acid
soils – choice of crop• diseases that affect plants thrive in soils with a
particular pH range• pH affects availability of nutrients
• Phenol red used to test acidity/alkalinity of a swimming pool – level of disinfection
Clay Minerals in Soil
Metal ions bind to negative surface
H+ are able to displace the surface cations from the clay. If aluminium ions are displaced by H+ the soil becomes toxic to crops growing in the soil.
Subsection 2
While we usually think of the air around us as neutral, the atmosphere naturally contains acidic oxides of carbon, nitrogen and sulfur. The concentrations of these acidic oxides have been increasing since the Industrial Revolution
Oxides of Period 3
NaOH(s)
MgO(s) + H2O(aq)
S(s) + O2(g) SO2(g) + H2O(aq)
P4(s) + O2(g) P2O5(s)
P2O5(s) + H2O(aq)
Oxides of Period 3 NaOH(s) Na+
(aq) + OH-(aq)
MgO + H2O Mg(OH)2
S + O2 SO2
SO2 + H2O H2SO3 (sulfurous acid)
2H2SO3 + O2 2H2SO4 (sulfuric acid)
P4 + 5O2 2P2O5
P2O5 + 3H2O 2H3PO4 (phosphoric acid)
Cl2O7 + H2O 2HClO4 (perchloric acid)
Oxides of Period 3
Oxide/
HydroxideNa2O
(NaOH)
MgO Al2O3 SiO2 P2O5 SO2 Cl2O7
Bonding
Acid/base property
amphoteric acidic
Acid/Base Properties of Oxides
BASIC OXIDES• metal oxides and hydroxides (ionic
compounds)• soluble oxides react with water to form
alkaline/basic solutions Na2O(s) + H2O(l) 2NaOH(aq)
• react with acids/acidic oxides to form salts CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(l)
CaO(s) + SO2(g) CaSO3(s)
Acid/Base Properties of Oxides
ACIDIC OXIDES• generally oxides of non-metals (covalent
compounds• called acid anhydrides• react with water to produce an acidic solution
CO2(g) + H2O(l) H2CO3(aq)
• react with bases to form saltsCO2(g) + CaO(s) CaCO3(s)
2NaOH(l) + SiO2(s) Na2SiO3(s) + H2O(l)
Acid/Base Properties of Oxides
AMPHOTERIC OXIDES react with acids and bases Al2O3, BeO, ZnO, PbO, SnO
Al2O3(s) + 6HCl(aq) 2AlCl3(aq) + 3H2O(l)
Al2O3(s) + 2NaOH(aq) 2NaAlO2(aq) + H2O(l)
NEUTRAL OXIDES do not react with acids or bases CO, N2O, NO
Group A Oxides
105
Db107
Bh
basic amphoteric acidic3A 4A 5A 6A 7A
1A 8A
Group B
Salts
in general acids and bases form salts the type of salt is determined by the acid
HCl chloride salts HNO3 nitrate salts
H2SO4 sulfate salts
H3PO4 phosphate salts
A Saturated Solution of NaCl
Dissolution
NaCl(s) Na+(aq) + Cl-
(aq)
Solid NaCl
Solution with Na+ and Cl- ions
Na+
Cl-
NaCl
System at 25oC
A Saturated Solution of NaCl
Solid NaCl
Solution with Na+ and Cl- ions
Radioactive 24NaCl
Na+
Cl-
Radioactive 24NaCl is introduced into the beaker
A Saturated Solution of NaCl
Dissolution
NaCl(s) Na+(aq) + Cl_
(aq)
Recrystallisation
NaCl(s) Na+(aq) + Cl_
(aq)
CHEMICAL EQUILIBRIUM
Solid NaCl
Solution with Na+ and Cl- ions
Radioactive 24NaCl
Na+
Cl-
24NaCl
24Na+
24NaCl
Cl-
Chemical Equilibrium
CHEMICAL EQUILIBRIUM
• only occurs in a closed system• no interchange of matter between system and
surroundings• occurs in physical and chemical systems• temperature of the system remains constant• the rate of the forward reaction equals the
rate of the reverse reaction• refers to reversible reactions where the
forward reaction occurs simultaneously with reverse reaction
CHEMICAL EQUILIBRIUM
Le CHATELIER’S PRINCIPLE
• French chemist Henri Le Chatelier (1850-1936) studied changes in systems that were in a state of equilibrium
• If a stress is applied to a system in a state of chemical equilibrium, the system changes to relieve the stress
• may be changes in • concentration• volume and pressure• temperature
CONCENTRATION• increasing the concentration of a reactant or
product will cause the system to favour the direction which will decrease the concentration of that substance
• decreasing the concentration of a reactant or product means the rate of the reaction using up that substance will decrease in rate• the rate of the other reaction to produce that
substance will now have the faster rate
CONCENTRATION
equilibria with solids and pure liquids
e.g. C(s) + H2O(g) CO(g) + H2(g)
PRESSURE• changes in pressure have little effect on solids
and liquids as they are only very slightly compressible
• changes in pressure have significant effects on the concentration of GASES
• gas pressure is proportional to the number of molecules
• changing the partial pressure of a gas changes its concentration
Partial Pressure of a Gas
PRESSURE
Adding an inert gasdoes not change the partial pressures of any of the other gasesno effect on equilibrium
VOLUME CHANGES
reducing the volume of the gas by half doubles the pressure of the gas
VOLUME CHANGES
in the reaction above there will be a shift to the right as the forward reaction increases in rate to minimise the pressure changes
e.g. 2SO2(g) + O2(g) 2SO3(g)
• decreasing volume - reaction rate increases in the direction that produces the smaller number of molecules
• producing less molecules reduces the pressure• increasing the volume decreases the pressure
causes a shift to the left in the above reaction as this produces the larger number of molecules
CHEMICAL EQUILIBRIUM
analyse the changes made to the equilibrium system shown in the diagram at the left
2NO2 N2O4
TEMPERATURE• increasing the temperature increases the reaction rate• this is due to an increase in the fraction of collisions
in which the total kinetic energy of reacting particles is at least equal to the activation energy
• increasing the temperature favours the endothermic reaction
• decreasing the temperature favours the exothermic reaction
TEMPERATURE
Mass-Gas Volume
1. Calculate the mass of 18.25L of ammonia gas at 25.0oC and 100.0kPa.
2. At 100.0 kPa and 25.0oC, how many litres of carbon dioxide gas will be produced when 75.0g of calcium carbonate is decomposed into calcium oxide?
Mass-Gas Volume
3. Solid lithium hydroxide has been used in space craft to remove carbon dioxide from air. Lithium carbonate and water are formed.
What mass of lithium hydroxide would be needed to remove 250.0 L of carbon dioxide at 100.0 kPa and 25.0oC?
Acid RainSulfur dioxide
natural and man-made sources reactions to produce it
sulfur compounds in coal S(s) + O2(g) SO2(g)
smelting of sulfide ores ZnS(s) + O2(g) Zn(s) + SO2(g)
oxidation of H2S – decay and industrial
2H2S + 3O2(g) 2H2O(l) + 2SO2(g)
reactions to produce acidic solutions effects
living things and environmentcorrosion metals, limestone buildings (CaCO3)
Acid RainNitrogen oxides NOx (NO, NO2)
natural and man-made sources reactions to produce it
high temperature engines
N2 + O2 2NO (neutral oxide)
2NO + O2 2NO2 (acidic oxide)
reactions to produce acidic solutions 2NO2 + H2O HNO2 + HNO3
effectsliving things and environment
Production of Ozone
photodecomposition
NO2 NO + O
ozone formation
O + O2 O3
regeneration of nitrogen dioxide
O3 + NO O2 + NO2
Ozone is a secondary pollutant in the troposphere
Photochemical Smog
What is acid rain?
More appropriate term is“acidic deposition”-snow, fog, sleet, haze, dry deposition
What is Acid Rain?Pure water: pH 7Natural rain: pH 5-6Acid rain pH < 5
Acid Rain
1730s – originated at height of Industrial Revolution
1872 – Robert Smith, an English chemist, coined the phrase “acid rain”
1950s – lake acidification first described
1960s – became more noticeable and subsequently became worse in rural areas
tall chimneys on factories allow wind to transport pollutants far away from sources of production
Acid Rain
Out west, in the Rocky Mountains scientists are finding that power plant emissions are saturating high-elevation watersheds in Colorado with acid-causing nitrogen. Evergreen forests are losing their needles and tree health is declining throughout the forest range
Acid Rain
Acid rain damage Blue Ridge Mountains North Carolina
Acid Rain
Acid rain damage on monumentCaCO3 (s) + H2SO4 (aq) CaSO4 (aq) + CO2 (g) + H2O (l)
Acid Rain
Tasmania - Queenstown emerged as a boomtown of the 1890s when gold and minerals were discovered at Mount Lyell. The strange but arresting 'moonscape' that surrounds the town was caused by acid-rain during the mining era.
Acid Rain1984 – reported almost half of Germany’s Black
Forest damaged by acid rain
Other areas
- acidification of lakes in Scandinavia
- Taj Mahal and many statues in Europe increased deterioration due to acid rain
- substantial problem in Europe, China and Russia as burn higher S-containing coal to generate electricity
- aluminium
Acid RainSulfur dioxide
natural and man-made sources reactions to produce it
sulfur compounds in coal smelting of sulfide ores oxidation of H2S – decay and industrial
reactions to produce acidic solutions effects
living things and environmentcorrosion metals, limestone buildings (CaCO3)
Acid Rain
SO2 Pollution
Killer smogs of London – 1952, 1956, 1957, 1962
Acid Rain
Acid RainNitrogen oxides NOx (NO, NO2)
natural and man-made sources reactions to produce it
high temperature engines
N2 + O2 2NO (neutral oxide)
2NO + O2 2NO2 (acidic oxide)
reactions to produce acidic solutions 2NO2 + H2O HNO2 + HNO3
effectsliving things and environment
http://www.csiro.au/promos/ozadvances/series14acidrainmovb.htm
Changes in Sulfate across the USA
http://nadp.sws.uiuc.edu/data/amaps/so4/amaps.html
NADP Annual Maps
Production of Ozone
photodecomposition
NO2 NO + O
ozone formation
O + O2 O3
O3 + NO O2 + NO2
Ozone is a secondary pollutant in the troposphere
Photochemical Smog
TABLE 4.4 TEXT P. 124
Common Acids
Acetic acid
Phosphoric acid
Sulfuric acid
3. Acids occur in many foods, drinks and even within our stomachs
Naturally occurring acetic/ethanoic (vinegar) citric/2-hydroxypropane-
1,2,3-tricarboxylic acid (citrus fruit)
hydrochloric (stomach)
3. Acids occur in many foods, drinks and even within our stomachs
Acids
Aspirinacetylsalicylic acid
Amino acids
Acids
Manufactured/Synthetic sulfuric acid
car batteries, fertiliser (NH3)2SO4, detergents, catalyst production ethanol and esters
nitric acid fertilisers , explosives
BasesNaturally Occurring ammonia NH3
also manufactured to produce fertilisers (Haber process)
metal oxides – Fe2O3, CuO
carbonates CO32- (Na2CO3, CaCO3)
Manufactured/Synthetic sodium hydroxide – soap, Draino (NaOH) calcium oxide, calcium hydroxide
Bases
Acids & Bases
Text p. 131-133
Self-Ionisaton/Autolysis of H2O in a sample of pure water a very small amount
of the molecules react with each other this is called the self-ionisation of water.
H2O(l) + H2O(l) H3O+(aq) + OH–
(aq)
at 25oC [OH-] = [H3O+] = 1.0x10-7 mol/L
KW = [OH-] x [H3O+] = 1.0 x 10-14
in any aqueous solution the [OH-] and [H3O+] are interdependent but KW is constant
aqueous solutions are neutral, acidic, basic
Using Kw
If the hydroxide ion concentration of a sodium
hydroxide solution is 1.5 x 10-3mol/L at 25oC,
what is the hydrogen ion concentration?
Using Kw
At 25oC an aqueous solution has a hydrogen ion concentration of 2.4 x 10-3mol/L.
What is the hydroxide ion concentration in this solution?
The pH Scale
proposed in 1909 by Danish scientist Soren Sorensen
pH means: power of the Hydrogen ion pH = -log[H+] the negative logarithm of the
hydrogen ion concentration neutral, acidic, basic solutions to obtain the [H+] given the pH
[H+] = 10-pH
pH – A measure of acidity Nitric acid (HNO3) is used in the production of
fertilizer, dyes, drugs, and explosives. Calculate the pH of a HNO3 solution having a hydrogen ion concentration of 0.76 M.
The pH of a brand of orange juice is 3.33. Calculate the H+ ion concentration.
The OH– ion concentration of a blood sample is 2.5 x 10–7 M. What is the pH of the blood?
pH
Show that a change in pH from 4.75 to 3.75
corresponds to a tenfold increase in hydrogen ion
concentration.
Problems with B-L Theory
The theory works very nicely in all protic solvents
but fails to explain acid-base behavior in aprotic solvents and some non-solvent situations.
A more general concept of acids and bases was proposed by G.N. Lewis at about the same time Bronsted-Lowry theory was proposed.
4.2 Bronsted-Lowry theory
3.2.2 plan and perform a first-hand investigation to measure the pH of identical
concentrations of strong and weak acids
HA
H+
A-
100% ionisation of HA
Would the solution conduct (be an electrolyte)?
Strong Acid
HA H+ + A-
unionised acid molecule
hydrogen ion
anion from acid
Strong Acid
For example
HCl(aq) H+(aq) + Cl-
(aq)
OR HCl(g) + H2O(l) H3O+(aq) + Cl-
(aq)
HNO3(aq) H+(aq) + NO3
-(aq)
OR
HNO3(l) + H2O(l) H3O+(aq) + NO3
-(aq)
HA
H+
A-
Partial ionisation of HA
Would the solution be conductive?
Weak Acid
At any one time, only a fraction of the molecules are ionised
HA H+ + A-
Weak Acid Note the use of the double arrowdouble arrow The unionised acid molecules are in
EQUILIBRIUM with the ionised hydrogen ion and anion from the acid
CH3COOH(aq) H+(aq) + CH3COO-(aq)
OR
CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-
(aq)
HA H+ + A-
Acids and Bases
STRONG ACIDS HCl, HBr, HI, H2SO4, HNO3, HClO3, HClO4
WEAK ACIDS organic acids, and H2SO3, HNO2, H3PO4, H2CO3
STRONG BASES ionise completely in water to produce OH- ions
LiOH, Na2O, KOH, Ba(OH)2 ALKALIS – strong soluble bases
WEAK BASES NH3, CO32-, HCO3
-
Acids and Bases
Weak bases like NH3 react with water to produce hydroxide ions
This also forms an EQUILIBRIUM
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
ammonium
ion
Acids
If the degree of ionisation of a weak acid is known then the pH of the solution can be determined.
e.g. If a solution of 0.037M hydrofluoric acid, HF, is 12.9% ionised what is the pH of the solution? HF H+ + F-
[H+] = 12.9/100 x 0.037 M = 0.00477 M
pH = -log(0.00477) = 2.32
Finish worksheet on p.100 in SSB
Acids and Bases
strength weak – limited
ionisation forming an equilibrium system
strong – complete (100%) ionisation
concentration dilute concentrated
water (solvent) solute
concentrated, Minitial
diluted, Mfinal
adding water lowers the solute concentration
moles of solute remain constant
Vinitial
Vfinal
molesinitial = molesfinal
Mfinal x Vfinal = Minitial x Vinitial
Dilution
Acid Concentration
dilute solution of a strong acid low number of moles of acid molecules per L of
solution all acid molecules completely ionised
concentrated solution of weak acid higher number of moles of acid molecules per L
of solution acid molecules only partially ionised
Monoprotic Acid
contains only one ionisable hydrogen HCl, HNO3, CH3COOH
Diprotic Acid
contains 2 ionisable hydrogens 2-step ionisation
First ionisation
H2SO4 H+ + HSO4- (complete)
Second ionisation
HSO4- H+ + SO4
2- (partial)
Triprotic Acid
contains 3 ionisable hydrogens phosphoric acid H3PO4
First ionisation
H3PO4 H+ + H2PO4- (partial)
Second ionisation
H2PO4- H+ + HPO4
2-
Third ionisation
HPO42- H+ + PO4
3-
HA
H+
A-
Strong Acid
unionised acid molecule
hydrogen ion
anion from acid
HA
H+
A-
Weak Acid
Gas-neutralisation Problems
1. At 25oC and 100 kPa, 2.5 litres of hydrogen chloride gas is bubbled through a sodium hydroxide solution. If the solution is 0.50M what volume would be needed to completely neutralise the gas? balanced equation moles of HCl moles of sodium hydroxide needed volume of solution
Gas-neutralisation Problems
2. 3.0 litres of carbon dioxide is bubbled through 200.0 mL of 0.15 M calcium hydroxide solution at 25oC and 100 kPa. What mass of calcium carbonate precipitate will form?
3. If 350.0 mL of a solution of potassium hydroxide completely neutralises 5.0 L of sulfur dioxide gas at 25oC and 100 kPa, what is the concentration of the solution?
Gas-neutralisation Problems
4. What volume of 0.25M barium hydroxide solution would completely neutralise 10.0 L of hydrogen chloride gas at 25oC and 100 kPa?
5. 500.0mL of hydrogen chloride gas at 25oC and 100kPa is bubbled through 800.0mL of distilled water. Assuming all the hydrogen chloride reacts, what is the pH of the solution?
pH of Solutions
1. 175.0mL of a 0.085M solution of sodium hydroxide is mixed with 150.0mL of a 0.15M solution of hydrochloric acid. Determine the pH of the final solution.
2. 250.0mL of a 0.15M solution of potassium hydroxide is mixed with 275.0mL of a 0.085M solution of nitric acid. Determine the pH of the final solution.
pH of Solutions
3. 50.0mL of a 0.050M solution of barium hydroxide is mixed with 75.0mL of a 0.100M solution of hydrochloric acid. Determine the pH of the final solution.
pH Meter tests the voltage of the
electrolyte converts the voltage to
pH very cheap, accurate must be calibrated with
buffer solutions non-destructive testing –
does not change solution being tested
pH of Acid Solutions
Acid: Molarity: pH (±0.1): [H+]:
HCl 0.1 1.0 0.10
C6H8O7 0.1 1.5 0.032
CH3COOH 0.1 2.9 0.00013
3.3.2 plan and perform a first-hand investigation to measure the pH of identical concentrations of strong
and weak acids
Molecular Structure and Acid Strength
the strength of an acid depends on its tendency to ionize.
for general acids of the type H–X:1. The stronger the bond, the weaker the acid.
2. The more polar the bond, the stronger the acid.for the hydrohalic acids, bond strength plays the key role giving: HF < HCl < HBr < HI
Molecular Structure and Acid Strength
The electrostatic potential maps show all the hydrohalic acids are polar. The variation in polarity is less significant than the bond strength which decreases from 567 kJ/mol for HF to 299 kJ/mol for HI.
Acids
1. Write equations to show the 2-step ionisation in water of the weak sulfurous acid, H2SO3
Acids as Food Additives
acidulant – gives a sharp/tart taste to food antimicrobials – lowers pH to inhibit growth
of bacteria, yeasts or molds antioxidants – slows oxidation which causes
spoilage e.g. fats and oils inhibit/block enzymes that continue natural
ripening after harvest – causes browning
3.3.6 Identify data, gather and process information from secondary sources to identify examples of naturally
occurring acids and bases and their chemical composition
Name: Formula: pH in natural form: Naturally found in:
Acetic CH3COOH 3-5 Vinegar, grapes, wine
Ascorbic C6H8O6 2-3 Fruit (esp. citrus), vegetables
Carbonic H2CO3 2-3 Acid rain
Citric C6H8O7 2-3 Citrus fruits
Formic CHOOH 3-5 Poison of stinging ants/insects
Hydrochloric HCl 0.1-2 Gastric juice in stomach
Ammonia NH3 9-11 Volcanic gases, decomposed plant/animal matter
Caffeine C8H10N4O2 8-10 Coffee beans, cola nuts
Nicotine C8H14N2 8-10 Tobacco leaves
Limestone CaCO3 8-10 Limestone
Acids and Bases
OPERATIONAL
DEFINITION based on observed
properties what do they do?
Acids • taste sour• change the colour of indicators e.g. blue
litmus to red• neutralise bases and basic oxides• some are corrosive• react with active metals such as zinc,
magnesium giving off hydrogen gas• aqueous solutions of acids conduct electricity –
they are ELECTROLYTES
Bases
• taste bitter• change the colour of indicators e.g turn red
litmus blue• neutralise acids and acidic oxides• some are corrosive• solutions of soluble bases in water are
electrolytes
Acids and Bases
CONCEPTUAL
DEFINITIONS a theoretical framework to
explain observed properties
more likely to change as our knowledge increases
4.1 Outline the historical development of ideas about acids
1778 – Antoine Lavoisier• oxides of P and S combined with water to
produce acidic solutions
S + O2 SO2 + H2O H2SO3
• oxygen is responsible for acidity• named oxygen from Greek “oxys” =sharp/sour
and “genes” = born/form (acid former)
4.1 Outline the historical development of ideas about acids
1811 – Sir Humphrey Davy• acids contain the element hydrogen - so
hydrogen is responsible for acidity
4.1 Outline the historical development of ideas about acids
1887 Svante Arrhenius acidic and basic solutions conduct
electricity so electrolytes (ions) acids react with metals to produce
hydrogen so ions involved developed ionic theory of electrolytes for
which he received a Nobel Prize in 1903
4.1 Outline the historical development of ideas about acids
1887 Svante Arrhenius acids are substances that release H+
in aqueous solutione.g. HCl(aq) H+
(aq) + Cl-(aq)
H2SO4(aq) 2H+(aq) + SO4
2-(aq)
bases are substances that release OH- ions in aqueous solution e.g. NaOH(aq) Na+
(aq) + OH-(aq)
Ba(OH)2(aq) Ba2+(aq) + 2OH-
(aq)
4.1 Outline the historical development of ideas about acids
Neutralisation
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
H+(aq) + OH-
(aq) H2O(l)
Problems with Arrhenius Theory
the role of the solvent? – is an acid an acid in any solvent
all salts should produce neutral solutions – neither acidic nor basic
the need for hydroxide as the base
e.g. NH4OH as the base and not NH3
The Hydrogen Ion
a proton with a 1+ charge and extremely small mass/volume high charge density and intense electric field
too reactive to exist independently in a very polar solvent like water
the hydronium ion, H3O+
Subsection 4Because of the prevalence and importance of
acids, they have been used and studied for hundreds of years. Over time, the definitions
of acid and base have been refined
4.2 Outline the Bronsted-Lowry theory of acids and bases
in 1923 a more general theory of acid-base behaviour was independently proposed by Danish chemist J Bronsted and English chemist T Lowry
Bronsted-Lowry theory defines: an acid as a species from which a proton
can be removed (acids are proton donors) a base as a species that can remove a
proton from an acid (bases are proton acceptors)
4.2 Bronsted-Lowry theory
HCl(g) + H2O(l) H3O+(aq) + Cl-
(aq)
+ -
hydronium ion
4.2 Bronsted-Lowry Theory
CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-
(aq)
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
an acid-base reaction is one in which a proton is transferred from an acid to a base
a proton-transfer reaction
4.2 Bronsted-Lowry theory
The role of the solvent:
Hydrogen chloride in liquid ammonia
4.2 Bronsted-Lowry theory
a broader definition which shows the complementary nature of acids and bases
shows the role of the solvent which can be a proton acceptor or proton donor
includes more species that Arrhenius Theory - molecules and ions
acid must contain hydrogen to have a proton removed
4 Bronsted-Lowry theory
each B-L reaction involves two acid-base pairs called CONJUGATE PAIRS - two species that differ by a proton
conjugate means “coupled or joinedcoupled or joined”
4 Bronsted-Lowry theory
HCl(g) + H2O(l) H3O+(aq) + Cl-
(aq)
+ -
acid conjugate base
base conjugate acid
Acids and Bases
1. What is the pH of a solution made by diluting 2.50mL of 6.0M HCl to 500.0mL?
2. What is the pH of a 0.035M solution of Ba(OH)2 ?
3. The pH of a HCl solution is 1.25. If 200.0mL of this solution is diluted to 500.0mL, what is the pH of this new solution?
4 AMPHIPROTIC SPECIES
Molecules or ions that can accept OROR donate a proton
Act as acids or act as bases
e.g. H2O(l) + H2O(l) H3O+(aq) + OH–
(aq)
4 AMPHIPROTIC SPECIES
hydrogen carbonate ion and a strong acid and base
HCO3-(aq) + OH-
(aq) CO32-
(aq) + H2O(l)
acid base
HCO3-(aq) + H3O+
(aq) H2CO3(aq) + H2O(l)
base acid
H2CO3(aq) CO2(g) + H2O(l)
4 AMPHIPROTIC SPECIES
hydrogen carbonate ion and a weak acid/base
HCO3- + H2O CO3
2- + H3O+
acid base
HCO3- + H2O H2CO3 + OH-
base acid
4 AMPHIPROTIC SPECIES
The hydrogen sulfate ion is amphiprotic.
a) Write balanced equations to show this behaviour. (use H3O+ and OH-)
b) A solution of sodium hydrogen sulfate in water turns blue Litmus red. Use an equation to explain this behaviour.
4.2.8 & 4.3.3 TITRATIONS
Validity & ReliabilityTITRATION - VALIDITY appropriate reaction – acid and base primary std or standardised secondary std appropriate indicator for type of titration accurate measuring instruments – volumetric
glassware – volumetric pipette, burette correct washing procedures and use e.g. method of
operating the pipette and burette
RELIABILITY 3 or more trials – reproducible average titre
4.2.4 Identify a range of salts which form acidic, basic and neutral solutions and explain their
acidic, neutral or basic nature
0.1M Salt SolutionpH Universal
Indicator pH Probe
NaCl 6-7 6.5
NH4Cl 4-5 4.5
NaCH3COO 8-9 9.5
NaNO3 6-7 7.2
Na2CO3 10-11 9.9
TEXT p. 154 TABLE 5.4
Summary of salts formed from different types of acids & bases
4.2.4 Identify a range of salts which form acidic, basic and neutral solutions and explain their
acidic, neutral or basic nature
Indicators
Phenolphthalein is a commonly used indicator for titrations, and is a weak acid.
the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of
equilibrium to the left, and turns the indicator colourless.
Adding hydroxide ions removes the hydrogen ions from the equilibrium which shifts to the right to replace them - turning the indicator pink.
Strong Acid with Strong Base
HCl + NaOH NaCl + H2O
Strong Acid with Strong Base
pH starts low
Equivalence
point pH = 7
pH finishes high
HCl + NaOH NaCl + H2O
8.3-10
3.1-4.4
Weak Acid with Strong Base
Volume of base added (mL)
CH3COOH + NaOH NaCH3COO + H2O
phenolphth
methyl orange
Weak Acid with Strong Base
Volume of base added (mL)
pH starts higher
Equivalence point
pH finishes high
CH3COOH + NaOH NaCH3COO + H2O
phenolphth
methyl orange
Weak Base with Strong Acid
Volume of acid added (mL)
NH3 + HCl NH4Cl + H2O
phenolphth
methyl orange
Weak Base with Strong Acid
Volume of acid added (mL)
pH starts moderately high
Equivalence point
pH finishes low
NH3 + HCl NH4Cl + H2O
phenolphth
methyl orange
Weak Acid with Weak Base
pH
4.2.4 Identify a range of salts which form acidic, basic and neutral solutions and explain their acidic, neutral or basic
nature
For each of the salts below,
i) give the formula
ii) state the acid and base that produced the salt
iii) state whether you would expect 0.1M aqueous solutions to be neutral, acidic or basic
iv) explain why, giving appropriate equations where necessary
1. barium nitrate 2. sodium methanoate
3. sodium carbonate 4. ammonium nitrate
5. sodium sulfite 6. potassium bromide
4.2.7 Neutralisation
a proton transfer reaction exothermic reaction for example
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(aq)
H = -56.1 kJmol-1
4.3.5 Neutralisation – safety measure and minimise damage in chemical spills
Factors to consider type of acid or base – weak or strong,
concentrated or dilute volume – few mL on laboratory bench or
much larger volume in more public place
4.3.5 Neutralisation – safety measure and minimise damage in chemical spills
weak acids and bases are safer to use neutralise acids
Na2CO3 – solid, cheap, easy to use, excess does not present problems of disposal
neutralise acids and alkalis NaHCO3 – amphiprotic
HCO3- + OH- CO3
2- + H2O
HCO3- + H+ H2CO3 CO2 + H2O
Booklet p. 142-144
Sources of H+ in the Body
Ketone bodies
Acetone
Betahydroxybutyric acid
Acetoacetate
(CH3COCH2COOH)
4.2.9 BUFFERS
A buffer is a solution that resists a change in its pH when acid (H3O+) or base (OH-) is added to it. based on chemical equilibrium
A solution of a weak acid and its conjugate base OR a weak base and its conjugate acid nearly all biochemical reactions are influenced
by the pH of their fluid environment maintaining the pH of blood