Acids and Alkalis http://www.gcsescience.com/index.html

When a substance dissolves in water,the solution may be acidic, neutral or alkaline.

What is an Acid?

An acid is any substance that produces H+ ions or H3O+

ions inwater(compare this with the definition of an alkalior base).

H+ ions are calledhydrogen ions, H3O+ ions are calledhydroxonium ions.

You will mostly see acids in reactions forming H+ ions.In reality, H+ is asingle proton, and does not exist on its

own.It always attaches to something, inwater it joins to H2O to

form H3O+ ions.

This tutorial will use either H+(aq) or H3O+

(aq) to mean acid in water

(this definition of an acid was given by Arrhenius, see also Lowry and Brønsted ).

Arrhenius, Lowry andBrønsted.

Arrhenius defined an acid as a substance that produces hydrogen ions in water.

The symbolfor a hydrogen ion in water is H+(aq)

(seeexamples).

Lowryand Brønsted defined an acid as a protondonorand a base as a protonaccepto

If you look at the reaction below

hydrochloric acid + water hydroxonium ion + chloride ionHCl(aq) + H2O(l) H3O+

(aq) + Cl-(aq)

According to Arrhenius, hydrochloric acid is an acidbecause it produces hydrogen ions in water

(hydrogen ions in water becomehydroxonium ions).

According to Lowry and Brønstedhydrochloric acid is an acid because it is a proton donor.

A proton is ahydrogen ion.A proton donor is a substance which gives a hydrogen

ionaway.If you look at the reaction above

hydrochloric acid gives a hydrogen ionto water.

A base is a protonacceptor.This means that a base willgain a hydrogen ion.

Water is a base when it is put withhydrochloric acidbecause water will gain a hydrogen ion to become H3O+.

acid + base acid + baseHCl(aq) + H2O(l) H3O+

(aq) + Cl-(aq)

On the right side of the arrow,H3O+ is anacidbecause it can give away a hydrogen ion to become H2O.

Cl- is a base because it can gain a hydrogen ionto become HCl.

Is water always a base according to Lowry and Brønsted ?

In the reaction below

ammonia + water ammonium ion + hydroxide ionNH3(aq) + H2O(l) NH4

+(aq) + OH-

(aq)

Ammonia is the base because it gains a hydrogen ion to become an ammoniumion.

Water is the acid because itgives away a hydrogen ion (to ammonia)

to become a hydroxide ion.

base + acid acid + baseNH3(aq) + H2O(l) NH4

+(aq) + OH-

(aq)

According to Lowry and Brønsted, water can be an acid or a base

depending on the substance reacting with it.

Examples of Acids.

The three common acids you will find in the laboratory are

Hydrochloric acid - HCl(aq), Nitric acid - HNO3(aq) and Sulfuric acid - H2SO4(aq)

They are all strong acids - seepH. They all ionise (make ions) in water

forming hydrogen ions (H+ions). This is what makes them acids.

Hydrochloric acidHCl(aq) H+

(aq) + Cl-(aq)

Nitric acidHNO3(aq) H+

(aq) + NO3-(aq)

Sulfuric acidH2SO4(aq) H+

(aq) + HSO4-(aq)

These three acids are all examples of hydrogen compounds withnon-metals.

Hydrochloric acid ishydrogen chloride (inwater).Nitric acid is hydrogen nitrate (inwater).

Sulfuric acid is hydrogen sulfate (inwater).Sulfuric acid is made using thecontact process.

The above acidsare called mineral acidsbecause they were originally obtained from minerals in

rocks.

Oxides of non-metalsare acidic - see CO2, NO and SO2

(compare these with metal oxides ).

See also carboxylic acids.

Hydrogen oxide (H2O) is Water - it isneutral, see Water.

What is the Difference between Strong and Weak Acids?

Acidsand alkaliscan be described asstrong or weak.This does not mean the same as concentrated ordilute.

The strength of an acid (or alkali) depends on how ionised it is in water.

What is a Strong Acid?

A strong acid (for example hydrochloric acid) is completely(100%) ionised.A strong acid has a pHof 1.

hydrogenchloride (in water) hydrogen ion + chloride ionHCl(aq) H+

(aq) + Cl-(aq)

Allof the hydrogen chloride molecules becomehydrogen ions and chloride ions

when they are dissolved in water (see examples of other strong acids).

What is a Weak Acid?

A weak acid (for exampleethanoic acid) is only partly (less than 100%)ionised.

A weak acid has a pHof 3 to 5.

ethanoic acid(in water) hydrogen ion + ethanoic ionCH3CO2H(aq) H+

(aq) + CH3CO2-(aq)

Someof the ethanoic acidmolecules become ions in waterbut most of them stay as molecules.

The reaction is reversible (shown by the arrow).

See alsoConcentrationand Reaction Rates of Strong and Weak Acids.

What is the Difference between Acid Strength andConcentration?

A strong acid does not become a weak acid just because it is diluted.

Concentrated hydrochloric acid and dilutehydrochloric acid are both

strong acids because they are bothcompletely ionised in water.

Concentrated ethanoic acid and diluteethanoic acid are both weak acids

because they are only partly ionised in water.

You can tell the difference between a strong acid and a weak acid

by using universal indicator to see whatpH it is.

A strong acid and a weak acid of the sameconcentrationwill react at different rates with the samemetal

Strong and Weak Acids have DifferentReactionRates.

Magnesium will react with an acid and make hydrogen gas.The more concentrated the acid, the faster the magnesium

will react

and you will see bubbles of hydrogenbeing produced more quickly.

A strong acid and a weak acid of the sameconcentrationwill react atdifferent rates with the same metal.

You can see the difference between a strong acid and a weak acid

of thesame concentration by looking at the reaction withmagnesium.

The strong acid reacts faster and you seemore bubbles of hydrogen.

This is because the strong acid hasmore hydrogen ions in the solution

even though it is at thesame concentration as the weak acid.

A piece of magnesium the same size should be usedin both acids for the comparison to be fair.

The weak acid will produce the same amount ofhydrogen as the strong acid

from the same amount of magnesium but the weak acid will take longer to do it.

The ionisation of a strong acid is complete. A weakacid is only partly ionised .

The ionisation of a weakacid (for example ethanoic acid) is reversible.

ethanoic acid (in water) hydrogen ion + ethanoic ionCH3CO2H(aq) H+

(aq) + CH3CO2-(aq)

The number of hydrogen ions produced by a weak acid issmall.

When they have reacted with (for example) magnesiummore of the ethanoic acid molecules will ionise to producemore hydrogen ions and ethanoic ions (see Le Chatelier's

Principle).Eventually all of theethanoic acid moleculesionise and

react with magnesiumand so the same amount of product (hydrogen gas) will be

produced.

Manufacture of Sulfuric Acid - The Contact Process

Sulfuric acid is made using the contact process.

Sulfuric acid has many uses in industry.The amount of sulfuric acid which a country uses inone

yearcan be a measure of that country'seconomic development

(how modern or wealthy it is).

Raw Materials to Make Sulfuric acid.

The raw materials (the things you need) are sulfur, air andwater.

Sulfur is available fromfossil fuels and sulfide ores.

When sulfur in fossil fuels is burned it produces sulfur dioxide gas.

This is the balanced chemical equation for the reaction ofsulfur with oxygen.

sulfur + oxygen sulfur dioxide.S(s) + O2(g) SO2(g)

Sulfide ores are heated inairto produce the metal oxide andsulfur dioxide gas, for

example

leadsulfide + oxygen lead oxide + sulfur dioxide. 2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g)

The metal is then extracted from themetal oxide.

Sulfur is oxidised in these reactions to become sulfur dioxide.

Both of the above reactions are exothermic.The sulfur dioxide gas produced from both of the above

methodsis used to make sulfur trioxide.

The Reactionof Oxygen with Sulfur Dioxideto make Sulfur Trioxide.

Sulfur dioxide is made by reacting sulfur with oxygen.

Sulfur dioxide can be made to react with more oxygen to make sulfur trioxide.

This is another example of oxidation.

The reaction is reversible (shown by the arrow).The forward reaction (from left to right) is exothermic.

This is the balanced chemical equation for the reaction ofsulfur dioxide with oxygen.

sulfurdioxide + oxygen sulfur trioxide.2SO2(g) + O2(g) 2SO3(g)

Theindustrial conditions for the chemical reaction are1) The Temperature is 450 °C.

2) The Pressure is between 1 and 2 atmospheres.3) A Vanadium ( V ) oxide (V2O5) catalyst is used.

About99% of the sulfur dioxideis converted into sulfur trioxide under these conditions.

The sulfur trioxide is used to make sulfuric acid.

The effect on the rate of the reaction andthe yield which you get from a reversible reaction

depends on the conditions (temperature, pressure and catalyst).

These have been summarized for theHaber Process.

1. Increasing the temperature favours the endothermic reaction.

2. Increasing the pressure favours the smaller volume.

3. Using a catalyst gives the equilibrium conditionsmore quickly.

The Effect of Temperature on the Formation of Sulfur Trioxide.

The formation ofsulfur trioxide is reversible.

The forward reaction (from left to right) is exothermic(compare this with the formation of ammoniain theHaber

process).

sulfurdioxide + oxygen sulphur trioxide2SO2(g) + O2(g) 2SO3(g)

Increasing the temperature will increase the rate of the reaction

butdecrease the amount of sulfur trioxide in the equilibrium mixture.

The temperature of 450 °Cis a compromise betweentheamount of sulfur trioxide in the equilibrium mixture

and the rate at which sulfur trioxide is formed.

The temperature of450 °C gives the best yield ofsulfur trioxide

The Effect of Pressure on the Formation of Sulfur Trioxide.

The formation ofsulfur trioxide is reversible.

sulfurdioxide + oxygen sulfur trioxide2SO2(g) + O2(g) 2SO3(g)

Increasing the pressure will increase the rate of the reaction

andincrease the amount of sulfur trioxide in the equilibrium mixture.

A highpressure will give the best yield ofsulfur trioxide.

The reaction is run at between 1 and 2 atmosphereswhich gives 99% conversion ofsulfur dioxide to sulfur

trioxide.

The Effect of using a Catalyst on the Formation of Sulfur Trioxide.

The catalyst for this reaction isVanadium ( V ) oxide (V2O5).Transition metals often make goodcatalysts.

The formation of sulfur trioxide is reversible.

sulfurdioxide + oxygen sulfur trioxide2SO2(g) + O2(g) 2SO3(g)

We know from reaction rates that a catalyst can increase the rateof a reaction.

With areversiblereaction, a catalyst will increase the rateof both theforward and the backward reaction.

This is useful because thecatalyst will cause the reaction mixture

to reach its equilibrium composition more quickly.Note, the catalyst will not change the equilibrium

compositionbut it will increase the yield in a particular time.

In the contact processthe reactants can be pushed through the

reaction vessel more quickly in the presence of a catalyst.Without the catalystthe process would take longer and

bemore expensive(compare this with the formation of ammoniain the Haber

process).

How is Sulfur trioxide made into Sulfuric Acid?

Sulfur trioxide will dissolve inwater to make sulfuric acid.

This is the balanced chemical equation for the reaction ofsulfur trioxide with water.

sulfur trioxide + water sulfuric acidSO3(g) + H2O(l) H2SO4(aq)

Thereaction is highly exothermic and produces an acid mist

which is difficult to condense intosulfuric acid.

In industry it is easier to dissolve sulfur trioxidein concentrated (98%)sulfuric acid to form oleum(oleum is sometimes called fuming sulfuric acid).

This is the balanced chemical equation for the reaction ofsulfur trioxide with sulfuric acid.

sulfur trioxide + sulfuric acid oleumSO3(g) + H2SO4(aq) H2S2O7(l)

Water is carefully added to oleum to remake concentrated (98%)sulfuric acid.

This is the balanced chemical equation for the reaction ofoleum with water.

oleum + water sulfuric acidH2S2O7(l) + H2O(l) 2H2SO4(aq)

Sulfuric acid has many uses in industry.

What are the Properties of Acids?

All acids have a pH of less than7 (see pH)and all acids make hydrogen ionsin water.

All acids will make litmus paper turn from blue to redand universal indicatorwill turn red, orangeor yellow.

What are the Chemical Reactions of Acids?

All acids will react with

1) Any alkalior base, seeneutralisation.

2) Any metal above hydrogenin the reactivity series.The metal will fizz, giving off hydrogen gas,

and leaving the metal salt in solution.It is not safe to put a metalinto an acid which is

above magnesium in the reactivity series.

Any chloride or sulfatecan be safely madeby reacting the appropriatemetal

(from lead to magnesium in the reactivity series)with hydrochloric acid to make thechloride

or sulfuric acid to make thesulfate (see examples).

3) Any metal carbonate ormetal hydrogen carbonate.The metal carbonate or metalhydrogen carbonate will

bubblegiving off carbon dioxide gas, leaving the metal salt and

water.

Any chloride or sulfatecan be madeby reacting the appropriatemetal carbonate or hydrogen

carbonatewith hydrochloric acid to make thechloride

or sulfuric acid to make thesulfate (see examples).

What are the Uses ofAcids?(see also the uses of Sulfuric Acid).

Acids are used both in school and in the chemical industry(see some examples of the chemical reactions of acids).

Otheruses of acids are listed below.

1) Steel used in construction is treated with acid before being painted.

Dilutesulfuric orhydrochloric acid will remove anysurface rust

which would otherwise spread under thepainted surface.Rust remover which is used to repair cars is

dilutephosphoric acid -H3PO4.

2) Acids are used to make fertilizers.

3) Lime scale removers containweak acids .Lime scale is the name

given to the calcium carbonate that forms in kettles and pipes.

The formation of lime scale is also called furring. To remove lime scale

you can try usinglemon juice (citric acid) or vinegar (ethanoic acid) .

4) Baking powder used in cooking contains tartaric acid.

Uses 2, 3 and 4 are examples of neutralisation.

What are the Usesof Sulfuric Acid?

1) The acid in a carbattery.2) Making Detergents.

3) Metal treatment and anodising.4) A Catalyst

5) A Dehydrating agent .6) Making Fertiliser.7) Paints and dyes.

8) To measure economic development .

Dehydration.

Dehydration means removing water from a substance.Concentrated sulfuric acid is very good at dehydration.

Example 1.Blue copper(II)sulfate crystals contain water.

Five water molecules surround each coppersulfate particle.

Concentrated sulfuric acid takes away the water molecules

and the copper sulfatebecomes white.

concentratedblue copper(II)sulfate white copper(II)sulfate

CuSO4(5H2O) sulfuric acid CuSO4

White copper(II)sulfate is called anhydrous and is used as a test for water.

Example 2.Glucose is a sugar which is made in the leaves of

plantsduring photosynthesis.It does not contain water moleculesbut does contain

hydrogen and oxygenwhich are the elements of water. Glucose has the formula

C6H12O6.Concentrated sulfuric acid will take away the elements of

waterfrom glucose leaving only carbon.

concentratedglucose carbon

C6H12O6 sulphuric acid C

The water which is removed in these examplesdissolves in the concentrated sulfuric acid and makes it

more dilute.

See also the dehydration of ethanol .

What is an alkali?

An alkali is any substance that produces OH- ions in water(compare this with the definition of an acid).

OH- ions are called hydroxide ions (see examples).

Alkali is pronounced like alcohol, with 'lie' at the end instead of 'hol'.

What is a Base?

A substance which will neutralise anacid,but does not dissolve in water, is called a base.

For example,copper(II) oxide,iron(II)oxide and zinc carbonate arebases,they do not dissolve in water(see salts made from bases

or carbonates).

Any base that dissolvesin water is called an alkali.For example,

sodium hydroxide ,potassium hydroxide and sodium carbonate arealkalis.

What is the Difference between Strong and Weak Alkalis?

Alkalisand acids can be described asstrong or weak.This does not mean the same as concentrated ordilute.

The strength of an alkali (or acid) depends on how ionised it is in water.

An alkali forms hydroxide ions (OH- ions) in water.

What is a StrongAlkali?

A strong alkali (for example sodium hydroxide) is completely(100%) ionised.

A strong alkali has a pHof 14.

sodiumhydroxide (in water) sodium ion + hydroxide ionNaOH(aq) Na+

(aq) + OH-(aq)

Sodiumhydroxide exists only as ionsboth as a solid and dissolved in water

(see examples of other strong alkalis ).

What is a Weak Alkali?

A weakalkali (for example ammonia) is only partly (less than 100%)ionised.

A weak alkali has a pHof 11 or 12.

ammonia + water ammonium ion + hydroxide ionNH3(g) + H2O(l) NH4

+(aq) + OH-

(aq)

Someof the ammonia molecules become ions in waterbutmost of them stay as molecules.

Ammonia in water makes ammonium hydroxide solution (NH4OH(aq)).

This is also called ammonia solution.

Examples of Alkalis.

The three common alkalis you will find in the laboratory are

1) Sodium hydroxide solution - NaOH(aq).

2) Calcium hydroxide solution - Ca(OH)2(aq) is called lime water

(lime water is used in the test for carbon dioxide ).

3) Ammonia solution - NH3(aq).

1 and 2 are strong alkalis , 3 is a weak alkali - see pH.They all ionise in water to form hydroxide ions (OH-ions).

1) NaOH(aq) Na+(aq) + OH-

(aq)

2) Ca(OH)2(aq) Ca2+(aq) + 2OH-

(aq)

3) NH3(aq) + H2O(l) NH4+

(aq) + OH-(aq)

NH4+ is called an ammonium ion.

Any metal oxide orhydroxide is a base.If the base dissolves in water it is called an alkali

(compare this with non-metal oxides).

What are the Propertiesof Alkalis?

All alkalis have a pH greater than7 and make hydroxide ions in water.

All alkaliswill make litmus paper turn from red to blue and universal indicatorwill turn blue or purple.

All alkalis react with acids to form a salt and water, see neutralisation.

What are the Usesof Alkalis?

1) Sodium hydroxide is used in the manufacture of paper, soap andceramics.

Ceramics include plates,cups, bricks and tiles.Sodium hydroxide is used as an alkali cleaner and to treat

aluminium

before anodising. It is also usedto neutraliseacids to make salts.

2) Calcium oxide or calcium carbonate is used to remove acidic gases

from a power station chimney to reduceacid rain.Calcium carbonate has many other uses.

3) Calcium hydroxide (called 'slaked lime', or just 'lime'),is added to soils or lakes to make them less acidic

(seeacid rain).Calcium hydroxidesolution (lime water) is used in the test

for carbon dioxide ).

4) Ammonia is used in the Haber process to make fertiliser.

5) Indigestion may be caused by too much hydrochloric acid in the stomach.

Indigestion tablets contain a basesuch as magnesium oxide or calcium carbonate to

neutralise the acid.

Why is Water Neutral?

In a sample of watera very small number of water molecules will form ions.

water hydrogen ion + hydroxide ion.H2O(l) H+

(aq) + OH-(aq)

This ionisation is reversible (shown by the arrow).

The hydrogen ion is acidic. The hydroxide ion isalkaline.Water forms equal amounts of bothions, and so water is

neutral.

Compare this reaction withneutralisation.

What is pH?

pH is a measure of how acidic or howalkaline a solution is.

The pH of a solution depends on the degree of ionisation of an acidor alkali

as well as how concentratedor dilute it is.The pH scale goes from 1 to 14

with 1 being very strongly acidicand 14 being very strongly alkaline.

A pH of 7is neutral.

You can measure the pH of asolution using universal indicator.

Just as litmus paper will bered for an acid and blue for analkali,

so universal indicator is amixture of indicatorsthat will give a different colour for a different pH.

Any acid will have a pH of less than 7 .Any alkali will have a pH of more than 7 .

Strong acids (HCl or H2SO4 or HNO3) have a pH of 1 (red).

Weak acids have a pH of 3 to 4 (orange).Examples of weak acids are ethanoic acid (vinegar),

citric acid(lemon juice) and rain water.Rain water has a natural pH of 5·5 (see carbonic acid).

Water andsalts are neutral. They have a pH of 7 (green).

A weak alkali(ammonia) has a pH of11 to 12(blue).

A strong alkali(NaOH) has a pH of 14 (purple).

Neutralisation and Salts.

What is Neutralisation?

A pH of 7is neutral. An acid has a pH of less than 7 .

An acid is neutralisedwhen it reacts with an alkali, a base a carbonate or a metal.

When neutralisation is complete there is no more acidleft, the pH is 7 and

the solution will contain a neutral salt.Neutralisation reactions are exothermic.

What is a Salt?

When an acid is neutralisedthe hydrogen ionof the acid is replaced

by a positive ion from thealkali, base, carbonate or metal.The ionic compound which is formed is called a salt (see

ionic equations ).Some salts are used to make fireworks.

A salt can be soluble(it dissolves in water)or insoluble (it does not dissolve in water).

Click on the links below for examples of

1) Salts made from an acid with a metal .

2) Salts made from an acid and a base .

3) Salts made from an acid and a carbonate .

4) Soluble salts made from an acid and an alkali .

5) Insoluble salts made from an acid or alkali or another salt .

6) Reactions of elements to make a salt .

Neutralisation.

An acid plus an alkali makes a salt plus water.

Hydrochloric acid makes chlorides.Sulfuric acid makes sulfates. Nitric acid makes nitrates.

This is the balanced chemical equation for the reaction ofhydrochloric acid and sodium hydroxide.

hydrochloric acid + sodium hydroxide sodium chloride + water.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

This is the balanced chemical equation for the reaction ofsulfuric acid and potassiumhydroxide.

sulfuric acid + potassium hydroxide potassium sulfate + water.

H2SO4(aq) + 2KOH(aq) K2SO4(aq) + 2H2O(l)

When ammoniais used as the alkali, ammonium hydroxide is formed in solution.

This is the balanced chemical equation for the reaction ofsulfuric acid and ammoniumhydroxide.

sulfuric acid + ammoniumhydroxide ammonium sulfate + water.

H2SO4(aq) + 2NH4OH(aq) (NH4)2SO4(aq) + 2H2O(l)

The saltformed here is ammonium sulfate. Ammonium salts are used asfertilisers.

All of the salts made above aresoluble.Pure samples of the salt can be made by using a titration.

What is an Ionic Equation?

If a reactioninvolves substances that form ionsin water, we can write

the equation showing only the ions. This is called an ionic equation.

What is the Ionic Equationfor Neutralisation?

In the reaction betweenhydrochloric acid and sodium hydroxide

(see the previous page)

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

We can write it in terms of ions,since both hydrochloric acid and sodium hydroxide form

ions in water.

H+(aq) + Cl-(aq) + Na+

(aq) + OH-(aq) Na+

(aq) + Cl-(aq) + H2O(l)

Chloride ions (Cl-(aq)), and sodium ions (Na+(aq)) appear on

both sides of theequation. They are not changed by thereaction

(see spectator ions ) and so they may be left out of theequation.

This leaves the ionicequation

hydrogen ion + hydroxide ion waterH+

(aq) + OH-(aq) H2O(l)

(compare this reaction with the ionisation of water).

This is the reaction that always occurs when an acid + alkali salt + water.

It is called the ionic equationfor neutralisation.

The hydrogen ion of the acid+ thehydroxide ionof thealkalicombine to form a water molecule,

leaving the metal from the alkali and the non-metal from the acid

to form a salt solution .

How much acid is needed to neutralise an alkali?

What is a Titration?

A titration adds a carefully measured amount of one solutionto another.

To do a titrationyou need to use a burette, pipette and a conical flask.

Click herefor an example of a titrationof an acid with an alkali.

A titration is used to calculate the concentration of another solution or

to get a pure dry sample of a soluble salt by allowing a solution to crystallise.

What is the End Point in a Titration between an Acid and an Alkali?

In a titration an acid can be added drop by drop to an alkalinesolution.

The pH of the alkaline solution will decrease asmore acid is added.

When the pH goes down to 7 the solution is neutral. This is called the end point.

If more acid is added to the solution after the end pointthe pH will continue to go down and the solution becomes

acidic.

The end point can be found by using one of the 4 methods below.

1) Use an indicator. 2) Use a pH meter.

3) Measure theconductivityof the solution.

4) Measure theamount of heat released into the solution.

What is an Indicator?

An indicator is a substance that changes colourat the end point of a titration.

An indicator is added to the solution in the conical flask (see an example).

Different indicatorsare used depending on the strengthof the acid or alkali.The indicator used for a particular titration is given in the table below.

Indicators used for weak Acids or Alkalis.Indicator Titration Colour inAcid Colour inAlkali

Methyl OrangeStrong Acid

+Weak Alkali

Red Yellow

PhenolphthaleinStrong Alkali

+Weak Acid

Colourless Pink

Either Indicator can be used for a Strong Alkali +Strong Acid.

Universal indicator is not usually used for a titrationbecause it changes gradually giving different colours for a

different pH.Methyl orange or phenolphthalein are used

because they give a sudden change in colour at neutralisation

which makes it easier to see the end point of the titration.

A Titration using anAcid and an Alkali.

The exact amount of acid needed toneutralise analkali can be found

by titration. This technique can be used to makepure crystals of a soluble salt(one that dissolves in water).

In the example below, anacid and an alkali react to make sodium chloride.

hydrochloric acid + sodium hydroxide sodium chloride + water.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

The burette is filled with hydrochloric acid.A known quantity of alkali (say50 cm3

sodium hydroxide) is released from

a pipette into theconical flask. The tap on the burette is turned open to allow

the acid to be added drop by drop into the alkali. The alkali contains an

indicator (phenolphthalein) which is pink in an alkali and colourless in an acid.

When enough acid has been added to neutralise the alkali, the

indicatorchanges from pink to colourless. This is the end point of thetitration.

The titration can be repeated using the same amounts of acidand alkali

but without the indicator. Pure salt crystals which are free from indicator

can then be crystallised from theneutral solution.

Using a pH meter.

You can use a pH meter to find the end point. At neutralisation the pH is 7 .

A pH meter allows you to make a pure salt sample quickerthan an indicator because you do not have to repeat the

titration

Using Conductivity to find the End Point of a Titration.

At the start of this titration the conical flaskcontains a strong alkali that

is fully ionisedin water. If electrodes are placed inside the conical flask

the ions in the water will conduct electricityand a current will flow.

Conductivity is a measure of how well the solution in the conical flask

conducts electricity. The more ions there are the better the conductivity and

the higher thecurrent will be. The current can be measured using an ammeter.

As acid is added to the alkalihydrogen ions and hydroxide ions react together to

formwater molecules.The number of ions in the conical flask starts to decreaseand the current flowing through the solution will be seen to

decrease.

At neutralisation all of the hydrogen ions and hydroxide ions have

reacted together to form water molecules. The neutral solution contains

only salt ions dissolved in water molecules. The solution will still conduct

electricity because of the saltions but the current will be at a minimum.

As more acid is added the current will start to increase because there

will now be unreacted hydrogen ions in the solution as well as the

salt ions. The solution is now no longer neutral but has become acidic.

If you draw a graph of current against the amount of acid added

you can see where the minimum is. This is the end pointof the titration

at neutralisation.

Saltsmade from an Acid and a Base.

Metaloxides are bases. A base will neutralise an acid. If the

metal oxide dissolves in water it forms hydroxide ions and is called an alkali.

Any chloride or sulfate can be safely made by reacting thecorrect metal oxide with hydrochloric acid to make

thechlorideor sulfuric acid to make thesulfate.

This is the balanced chemical equation for the reaction ofsulfuric acid and ironoxide.

sulfuric acid + iron(II) oxide iron(II) sulfate + waterH2SO4(aq) + FeO(s) FeSO4(aq) + H2O(l)

This is the balanced chemical equation for the reaction ofhydrochloric acid and copperoxide.

hydrochloric acid + copper(II)oxide copper(II)chloride + water

2HCl(aq) + CuO(s) CuCl2(aq) + H2O(l)

The base can be added a little bit at a time to the acidand it will dissolve to form thesolublesalt.

When no more base dissolves, the acid has been neutralised.

The undissolved base can be removedfrom the solution by filtration.

Pure salt crystals can then be crystallised from the neutral solution.

You can also use a carbonate to makesalts in the same way.

Saltsmade from an Acid and a Carbonate.

An acid can be neutralised by a metalcarbonate.A carbonate can be an alkali or a base .

The metal carbonate will bubblegiving off carbon dioxide gas

and the reaction makes the salt and water.This can be used as a test to identifycarbonate ions .

Any chloride or sulfate can be safely made by reacting thecorrect metalcarbonate with hydrochloric acid to make

thechlorideor sulfuric acid to make thesulfate.

For example

hydrochloric acid + zinccarbonate zinc chloride + carbon dioxide + water

2HCl(aq) + ZnCO3(s) ZnCl2(aq) + CO2(g) + H2O(l)

hydrochloric acid+ magnesium carbonate magnesiumchloride +carbon dioxide + water

2HCl(aq) + MgCO3(s) MgCl2(aq) + CO2(g) + H2O(l)

sulfuric acid + copper carbonate copper sulfate + carbon dioxide + water

H2SO4(aq) + CuCO3(s) CuSO4(aq) + CO2(g) + H2O(l)

sulfuric acid + calcium carbonate calcium sulfate + carbon dioxide + water

H2SO4(aq) + CaCO3(s) CaSO4(aq) + CO2(g) + H2O(l)

Calcium carbonate islimestone. This reaction occurs with acid rain .

The carbonate can be added a little at a time to the acid.It will bubble and dissolve to form the soluble salt .

When no more carbonate dissolves, the acid has been neutralised.

The undissolved carbonate can be removed from the

solution by filtration.Pure salt crystals can then be crystallised from the neutral

solution.

You can also use a metal oxide to make salts in the same way.

Precipitation Reactions.

The process of making asolid come from a solution is called precipitation.

The solid itself is called aprecipitate.

An insoluble salt (one that doesn't dissolve)can be made by reacting the appropriate soluble salt

with an acid oralkali oranother salt.

For examples of precipitation, see tests for ions includingprecipitation using the alkali sodium hydroxide

the barium chloride test for sulfate ionsand the silver nitrate test for bromide,chloride and iodide

ions.

Precipitation reactions can be used toremove ions from water.

Sodium carbonate can be used toprecipitate calcium carbonate.

This is a way of making hard water into soft water.

calcium chloride+ sodium carbonate calcium carbonate +sodium chloride.

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

The ionic equation is Ca2+(aq) + CO3

2-(aq) CaCO3(s)

A precipitate can be separated from the solution by filtration.

The precipitate can then be left somewhere warm to dry.