Section 4 2 Precipitation Reactions Aqueous Reactions Precipitation

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(NO 3)2 and Na. OH are mixed? Aqueous Reactions

Step 1: Ions present • • Mg 2+ NO 3 – Na + OH - Aqueous Reactions

Step 2: Possible combinations • Mg 2+ with OH – • Na + with NO 3 – Aqueous Reactions

Step 3: Table 4. 1 • Hydroxides generally insoluble, and Mg is not an exception • Na. NO 3 is soluble Mg(NO 3)2 (aq) + 2 Na. OH (aq) Mg(OH)2 (s) + 2 Na. NO 3 (aq) Aqueous Reactions

Metathesis (Exchange) Reactions • Metathesis- Greek, “to transpose” • Pattern of precipitation formation AX + BY AY + BX Ag. NO 3 (aq) + KCl (aq) Ag. Cl (s) + KNO 3 (aq) Aqueous Reactions

Metathesis (Exchange) Reactions Ag. NO 3 (aq) + KCl (aq) Ag. Cl (s) + KNO 3 (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 Ba. Cl 2 and K 2 SO 4 are mixed. Aqueous Reactions

Step 1: Determine ions • Ba. Cl 2 and K 2 SO 4 are mixed: • Ba 2+ • Cl • K+ • SO 4 2 Aqueous Reactions

Step 2: Combine • Reactants: Ba. Cl 2 and K 2 SO 4 Ba. Cl 2 + K 2 SO 4 Ba. SO 4 + K 2 Cl 2 Aqueous Reactions

Step 3: Subscripts Ba 2+ Cl - K+ SO 4 2 - Ba. Cl 2 + K 2 SO 4 Ba. SO 4 + KCl Aqueous Reactions

Step 4: Balance Ba. Cl 2 + K 2 SO 4 Ba. SO 4 + 2 KCl Did a precipitate form? Aqueous Reactions

Solution Ba. Cl 2 (aq) + K 2 SO 4 (aq) Ba. SO 4 (s) + 2 KCl (aq) Aqueous Reactions

Molecular Equation • Reactants and products in their molecular form • NOT ionic character Ag. NO 3 (aq) + KCl (aq) Ag. Cl (s) + KNO 3 (aq) Aqueous Reactions

Complete Ionic Equation • All soluble strong electrolytes shown as ions Ag+ (aq) + NO 3 - (aq) + K+ (aq) + Cl- (aq) Ag. Cl (s) + K+ (aq) + NO 3 - (aq) • Spectator ions- appear as reactants and products in identical form Aqueous Reactions

Net Ionic Equation • Omit spectator ions Ag+ (aq) + Cl- (aq) Ag. Cl (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 Ca. Cl 2 + Na 2 CO 3 Ca. CO 3 + 2 Na Cl Aqueous Reactions

Step 2: Dissociate strong electrolytes Ca 2+ + 2 Cl- + 2 Na+ + CO 3 2 - Ca. CO 3 + 2 Na+ + 2 Cl- All are strong electrolytes, but Ca. CO 3 is insoluble in water Aqueous Reactions

Step 3: Omit spectators Ca 2+ (aq) + CO 3 2 - (aq) Ca. CO 3 (s) Aqueous Reactions

Homework • 4. 19 -4. 24 on page 158 Aqueous Reactions