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 + CO 2 CaCO 3 + H2OH2O

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 (NO 3 – ) ions are spectator ions. This means the ionic equation can be simplified to: Na I (aq)AgNO 3 (aq) Ag I (s) NaNO 3 (aq) + + Na + (aq) Ag I (s) + I – (aq) + Ag + (aq) + NO 3 – (aq) + Na + (aq) + NO 3 – (aq) I – (aq) + Ag + (aq) Ag I (s)

Which ions are spectators?

Silver halides The different silver halide precipitates can be distinguished by their differing colours. white AgCl precipitate cream AgBr precipitate yellow Ag I precipitate chloridebromide 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) + AgNO 3 (aq)AgCl (s) + NaNO 3 (aq) Cl – (aq) + Ag + (aq)AgCl (s)

Identifying negative ions: sulfate Sulfate ions (SO 4 2– ) 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: SO 4 2– (aq) + Ba 2+ (aq)BaSO 4 (s) sodium sulfate Na 2 SO 4 (aq) + + BaCl 2 (aq) barium chloride BaSO 4 (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 CuSO 4 and sodium hydroxide NaOH iron II chloride FeCl 2 and sodium hydroxide NaOH iron III chloride FeCl 3 and sodium hydroxide NaOH potassium chloride KCl and silver nitrate AgNO 3 potassium bromide KBr and silver nitrate AgNO 3 potassium iodide KI and silver nitrate AgNO 3 potassium sulphate KSO 4 and barium chloride BaCl 2 lead nitrate Pb(N0 3 ) 2 and potassium iodide KI

Equations CuSO 4 + NaOH FeCl 2 + NaOH FeCl 3 + NaOH KCl + AgNO 3 Cu(OH) 2 + Na 2 SO 4 2 Fe(OH) 2 + NaCl 2 2 Fe(OH) 3 + NaCl 33 KNO 3 + AgCl (aq) (s) (aq) (s)(aq) (s)(aq) (s) Finish symbol equation – balance – add state symbols

Equations KBr + AgNO 3 KI + AgNO 3 K 2 SO 4 + BaCl 2 Pb(NO 3 ) 2 + KI KNO 3 + AgBr KNO 3 + AgI KCl + BaSO 4 2 KNO 3 + PbI 2 (aq) (s) (aq) (s) (aq) (s)(aq) (s) Finish symbol equation – balance – add state symbols 2 2

State symbols

Chemical analysis