1. Making salts (3): precipitation

2. Soluble or insoluble? Insoluble salts are made by mixing two soluble compounds. The solid obtained when solutions of two soluble compounds are mixed is called precipitate. We have to know which compounds are soluble in water and which are insoluble. We are using solubility rules, shown in the table below.

3. If we want to make an insoluble salt, e.g. lead chloride, we have to:  identify the ions present in the insoluble salt – lead (Pb 2+ ) and chloride (Cl - )  use solubility rules to choose soluble compounds including these ions – e.g. lead nitrate for lead and sodium chloride for chloride  add one solution to the other  filter off the precipitate then wash and dry the solid Soluble compoundsInsoluble compounds all salts of Group I elements all nitrates all ammonium salts most chlorides, bromides and iodideschlorides, bromides and iodides of silver and lead most sulphatessulphates of calcium, barium and lead Group I hydroxides and carbonates are soluble (calcium hydroxide is slightly soluble) most hydroxides and carbonates Group I and II oxides react with watermost metal oxides

5. What happens in a precipitation reaction? By looking at the reaction between lead nitrate and sodium chloride as an example, we can explain why a solid precipitates. Pb(NO 3 ) 2(aq) + 2NaCl → PbCl 2(s) + 2NaNO 3(aq) In a solution of ionic compound, the ions are free to move. Solution of lead nitrate contains lead ions and nitrate ions that are separate from one another, moving freely and randomly throughout the water. The water molecules help to keep them in solution. A similar thing happens with solution of sodium chloride. When we mix the solutions, lead nitrate is precipitated. The lead ions in solution have a greater attraction for chloride ions than the water molecules that keep them in solution.

6. The lead ions and chloride ions come together in large numbers and form a three-dimensional ionic lattice. The sodium ions and nitrate ions remain in solution. They are spectator ions. Pb 2+ (aq) + 2NO 3 - + 2Na + + 2Cl - → PbCl 2(s) + 2Na + + 2NO 3 - Pb 2+ (aq) + 2Cl - → PbCl 2(s)