SCH 3U1 1

Solubility of Ionic Compounds 2 All solutes will have some solubility in water. “Insoluble” substances simply have extremely low solubility. low partial high solubility solubility solubility ( 1 g/100mL) “insoluble” “soluble”

Dissociation in Water Ionic compounds dissociate (break up) into ions when they form a solution:dissociate MgCl 2 (s) Mg 2+ (aq) + Cl - (aq) Positive ions are called cations. Negative ions are called anions. Dissociation equations show the number of each cation and anion produced by a compound when it dissolves in an aqueous solution. They must be balanced! 3 2

Examples: Na 3 PO 4 (s) Mg(NO 3 ) 2 (s) (NH 4 ) 2 SO 4 (s) 4 3 Na + (aq) + PO 4 3- (aq) Mg 2+ (aq) + 2NO 3 - (aq) 2NH 4 + (aq) + SO 4 2- (aq) Note that water (H 2 O) does not appear in dissociation equations!

Factors Affecting Solubility of Ionic Compounds in Water 1) The Charge on an Ion as ionic charge increases, the attraction between ions increases therefore as charge increases, solubility ________________ e.g. salts containing fluoride (F - ) ions are __________ soluble than salts containing oxide (O 2- ) ions 5 decreases more Solubility at 25 0 C: MgO : g/ 100 mL water MgF 2 : g / 100 mL water

Factors Affecting Solubility of Ionic Compounds in Water 2) Ion Size smaller ions form shorter ionic bonds these bonds are _____________ than bonds between larger ions with the same charge therefore as ion size increases, solubility ___________ e.g. fluoride salts are ________ soluble than chloride salts 6 increases stronger less Solubility at 25 0 C: MgF 2 : g/ 100 mL water MgCl 2 : 54 g / 100 mL water MgBr 2 : 101 g / 100 ml water

Predicting Solubility Predict which compound will be more soluble: Correct Answer? a) NaF vs. CaF 2 b) Ca(NO 3 ) 2 vs. CaSO 3 c) NaBr vs. NaCl 7 NaF Ionic charge: Na + vs. Ca 2+ Ca(NO 3 ) 2 NaBr Ionic charge: NO 3 - vs. SO 3 2- Ion size: Br - > Cl -

The Solubility Rules The following rules are useful for predicting if a salt is soluble or insoluble: Soluble: Compounds containing ammonium (NH 4 + ) or alkali metal ions (Li +, Na +, K +, Rb +, Cs + ) are soluble. Compounds containing the nitrate ion (NO 3 - ) are always soluble. Compounds containing Cl -, Br - or I - ions are soluble… except with Ag +, Pb 2+, Hg 2+ Compounds containing the sulfate ion (SO 4 2- ) are soluble…except with Ca 2+, Sr 2+, Ba 2+, Ra 2+, Pb 2+. 8

The Solubility Rules Insoluble: Compounds containing carbonate, phosphate or hydroxide ions (CO 3 2-, PO 4 3-, OH - ) are insoluble…. except with NH 4 +, Li +, Na +, K +, Rb +, Cs + 9

The Solubility Rules on the Periodic Table InsolubleSoluble CO 3 2-, PO 4 3-, OH - except with NH 4 +, Li +, Na +, K +, Rb +, Cs + NO 3 - NH 4 +, Li +, Na +, K +, Rb +, Cs + Cl -, Br -, I - except with Ag +, Pb 2+, Hg +. SO 4 2- except with Ca 2+, Sr 2+, Ba 2+, Ra 2+, Pb

Making Predictions Use the solubility rules to predict if the following compounds are soluble or insoluble. a) Mg(NO 3 ) 2 c) (NH 4 )CO 3 b) NaCld) Ca 3 (PO 4 ) 2 11 soluble insoluble

Mixing Solutions What will happen if two solutions are mixed? e.g. sodium chloride (aq) + potassium nitrate (aq) NaCl (aq) Na + (aq) + Cl - (aq) KNO 3 (aq) K + (aq) + NO 3 - (aq) Answer: NOTHING!!!!! All possible salts soluble! 12

Mixing Solutions What will happen if two solutions are mixed? e.g. iron (III) nitrate (aq) + potassium hydroxide(aq) Fe(NO 3 ) 3 (aq) Fe 3+ (aq) + 3 NO 3 - (aq) KOH (aq) K + (aq) + OH - (aq) Answer: A PRECIPITATION OCCURS 13

Precipitation Reactions These are double displacement reactions that occur when 2 solutions are mixed and a solid (insoluble) product is formed. Using the solubility rules, we can predict if a precipitation reaction will occur and the product(s) formed. e.g.1 sodium carbonate (aq) + barium chloride (aq) e.g.2 potassium nitrate (aq) + copper (II) chloride (aq) 14