Precipitate Reactions

L.O.

• I can state which are spectator ions in a precipitation reaction.• I can make a clean dry precipitate and write the ionic equation for the formation of the precipitate

Precipitation reactions

An insoluble solid that forms during an aqueous reaction is called a precipitate. A reaction which forms a precipitate is called a precipitation reaction.

The limewater test for carbon dioxide is a precipitation reaction.

Limewater is actually a dilute solution of calcium hydroxide.

The calcium hydroxide reacts with carbon dioxide to form calcium carbonate, which is insoluble in water:

calcium hydroxide

Ca(OH)2

+ carbon dioxide

calcium carbonate

+ water

+ CO2 CaCO3 + H2O

Uses of precipitation reactions

Precipitation reactions have a number of other uses:

production of coloured pigments for paints and dyes

removal of toxic chemicals from water

separation of reaction products.

Most precipitation reactions are very fast reactions that occur between ions.

This makes them very useful for identifying specific ions based on the type of precipitate formed.

A lead iodide precipitate.

Isolating the precipitate

The precipitate from a precipitation reaction can be separated from the reaction mixture by filtration.

Buchner funnel

filter paper

vacuum pump

Buchner flask

A Buchner funnel and flask can be used to accelerate the process.

This apparatus uses a vacuum pump to draw the mixture through the filter.

The filtrate is finally washed and dried.

Spectator ions

In ionic precipitation reactions there are often ions that are not involved in the reaction. These are known as spectator ions.

The spectator ions are easily identified using the ionic equation.

This equation shows that the silver and the iodine ions have reacted, joining together to make the precipitate.

The sodium (Na+) and nitrate (NO3–) ions are spectator ions.

This means the ionic equation can be simplified to:

NaI (aq) AgNO3 (aq) AgI (s) NaNO3 (aq)+ +

Na+ (aq)

AgI (s)+ I– (aq)+Ag+

(aq)+NO3–

(aq)+ Na+

(aq) + NO3–

(aq)

I– (aq) + Ag+ (aq) AgI (s)

Which ions are spectators?

Silver halidesThe different silver halide precipitates can be distinguished by their differing colours.

white AgCl precipitate

cream AgBr precipitate

yellow AgI precipitate

chloride bromide iodide

Identifying negative ions: halides

Halide ions are formed from the Group VII elements, the halogens. Halides are detected using silver nitrate solution.

The substance to be tested is first acidified with a small amount of nitric acid before adding the silver nitrate solution.

The precipitates formed are silver halides:

If halides are present, a precipitate will form.

sodium chloride

+ silver nitrate

silver chloride

+ sodium nitrate

NaCl (aq) + AgNO3 (aq) AgCl (s) + NaNO3 (aq)

Cl– (aq) + Ag+

(aq) AgCl (s)

Identifying negative ions: sulfate

Sulfate ions (SO42–) are identified by adding a few drops of

barium chloride solution. The solution must be acidified first with a few drops of hydrochloric acid.

A white precipitate of barium sulfate forms.

The ionic equation for this reaction is:

SO42–

(aq) + Ba2+ (aq) BaSO4 (s)

sodium sulfate

Na2SO4 (aq) +

+

BaCl2 (aq)

barium chloride

BaSO4 (s)

barium sulfate

+

+

2NaCl (aq)

sodium chloride

Method

Add the chemicals together and write the symbol equations for each one, remember to include state symbols

copper sulphate CuSO4 and sodium hydroxide NaOHiron II chloride FeCl2 and sodium hydroxide NaOH

iron III chloride FeCl3 and sodium hydroxide NaOH

potassium chloride KCl and silver nitrate AgNO3

potassium bromide KBr and silver nitrate AgNO3

potassium iodide KI and silver nitrate AgNO3

potassium sulphate KSO4 and barium chloride BaCl2lead nitrate Pb(N03)2 and potassium iodide KI

Equations

CuSO4 + NaOH

FeCl2 + NaOH

FeCl3 + NaOH

KCl + AgNO3

Cu(OH)2 + Na2SO42

Fe(OH)2 + NaCl22

Fe(OH)3 + NaCl33

KNO3 + AgCl

(aq) (aq) (s) (aq)

(aq) (aq) (s) (aq)

(aq) (aq) (s) (aq)

(aq) (aq) (aq) (s)

Finish symbol equation – balance – add state symbols

Equations

KBr + AgNO3

KI + AgNO3

K2SO4 + BaCl2

Pb(NO3)2 + KI

KNO3 + AgBr

KNO3 + AgI

KCl + BaSO42

KNO3 + PbI2

(aq)(aq) (s)(aq)

(aq) (aq) (s)(aq)

(aq) (aq) (s)(aq)

(aq) (aq) (aq) (s)

Finish symbol equation – balance – add state symbols

22

State symbols