Precipitation Reactions Chapter 4

Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction. 2

Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate (solid) is formed. 3

Precipitation Reactions How to identify Two reactants are an aqueous solution (aq) One product has to be solid (s) Metathesis reaction (double replacement) Example: AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Molecular Equation The molecular equation lists the reactants and products in their molecular form. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) 5

Ionic Equation In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl (s) + K+ (aq) + NO3- (aq) 6

Writing Net Ionic Equations Write a balanced molecular equation. Dissociate all strong electrolytes. Cross out anything that remains unchanged from the left side to the right side of the equation. (these substances are called spectator ions) Write the net ionic equation with the species that remain. 7

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq) 8

Net Ionic Equation Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq) 9

Practice Problem Write the molecular, ionic and net ionic equation for the reaction: Aqueous sodium hydroxide reacts with aqueous copper (II) sulfate to precipitate copper (II) hydroxide 2 NaOH (aq) + CuSO4 (aq)  Na2SO4 (aq) + Cu(OH)2 (s) 2 Na+ + 2 OH- + Cu 2+ + SO42-  2 Na+ + SO42- + Cu(OH)2 (s) Cu2+ + 2OH-  Cu(OH)2 (s)

Practice Problem 2 Write the molecular, ionic and net ionic equation for the reaction: Aqueous potassium carbonate reacts with aqueous silver nitrate to precipitate silver carbonate. K2CO3 (aq) + 2AgNO3 (aq)  2 KNO3 (aq) + Ag2CO3 (s) 2 K+ + CO32- + 2Ag+ + 2NO3-  2 K+ + 2 NO3- + Ag2CO3(s) 2Ag+ + CO32-  Ag2CO3(s)