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