Section 4.2 Precipitation Reactions Aqueous Reactions Precipitation Reactions Mixing ions that form insoluble compounds The insoluble solid formed is a precipitate Aqueous Reactions Solubility • The amount of substance that can dissolve in a given quantity of solvent at a given temperature Aqueous Reactions Solubility Guidelines Aqueous Reactions Steps for Predicting Precipitation 1. Note ions present in reaction 2. Consider possible combinations of cations and anions 3. Use Table 4.1 to determine if any of those combinations are insoluble Aqueous Reactions Example • Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed? Aqueous Reactions Step 1: Ions present • • • • Mg 2+ NO3 – Na + OH - Aqueous Reactions Step 2: Possible combinations • Mg 2+ with OH – • Na + with NO3 – Aqueous Reactions Step 3: Table 4.1 • Hydroxides generally insoluble, and Mg is not an exception • NaNO3 is soluble Mg(NO3)2 (aq) + 2NaOH (aq) Mg(OH)2 (s) + 2NaNO3 (aq) Aqueous Reactions Metathesis (Exchange) Reactions • Metathesis- Greek, “to transpose” • Pattern of precipitation formation AX + BY AY + BX AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Aqueous Reactions Metathesis (Exchange) Reactions AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Aqueous Reactions Balancing Metathesis Reactions 1. Determine ions present 2. Write chemical formulas of products by combining cation from one reactant with anion of the other 3. Use charges of ions to determine subscripts 4. Balance the equation Aqueous Reactions Sample Problem • Predict the identity of the precipitate that forms when solutions of BaCl2 and K2SO4 are mixed. Aqueous Reactions Step 1: Determine ions • BaCl2 and K2SO4 are mixed: • Ba 2+ • Cl •K+ • SO4 2Aqueous Reactions Step 2: Combine • Reactants: BaCl2 and K2SO4 BaCl2 + K2SO4 BaSO4 + K2Cl2 Aqueous Reactions Step 3: Subscripts Ba 2+ Cl - K+ SO4 2- BaCl2 + K2SO4 BaSO4 + KCl Aqueous Reactions Step 4: Balance BaCl2 + K2SO4 BaSO4 + 2KCl Did a precipitate form? Aqueous Reactions Solution BaCl2 (aq) + K2SO4 (aq) BaSO4 (s) + 2KCl (aq) Aqueous Reactions Molecular Equation • Reactants and products in their molecular form • NOT ionic character AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Aqueous Reactions Complete Ionic Equation • All soluble strong electrolytes shown as ions Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) AgCl (s) + K+ (aq) + NO3- (aq) • Spectator ions- appear as reactants and products in identical form Aqueous Reactions Net Ionic Equation • Omit spectator ions Ag+ (aq) + Cl- (aq) AgCl (s) Aqueous Reactions Writing Net Ionic Equations 1. Write a balanced molecular equation. 2. Dissociate all strong electrolytes. 3. Identify and cancel spectator ions Aqueous Reactions Sample Problem • Write the net ionic equation for mixing calcium chloride and sodium carbonate. Aqueous Reactions Step 1: Molecular equation • Calcium chloride and sodium carbonate CaCl2 + Na2CO3 CaCO3 + 2Na Cl Aqueous Reactions Step 2: Dissociate strong electrolytes Ca 2+ + 2Cl- + 2Na+ + CO3 2- CaCO3 + 2Na+ + 2Cl- All are strong electrolytes, but CaCO3 is insoluble in water Aqueous Reactions Step 3: Omit spectators Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s) Aqueous Reactions Homework • 4.19-4.24 on page 158 Aqueous Reactions