Precipitation Reactions Section 4.2 Precipitation Reactions

Precipitation Reactions Mixing ions that form insoluble compounds The insoluble solid formed is a precipitate

Solubility The amount of substance that can dissolve in a given quantity of solvent at a given temperature

Solubility Guidelines

Steps for Predicting Precipitation Note ions present in reaction Consider possible combinations of cations and anions Use Table 4.1 to determine if any of those combinations are insoluble

Example Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed?

Step 1: Ions present Mg 2+ NO3 – Na + OH -

Step 2: Possible combinations Mg 2+ with OH – Na + with NO3 –

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)

Metathesis (Exchange) Reactions Metathesis- Greek, “to transpose” Pattern of precipitation formation AX + BY  AY + BX AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Metathesis (Exchange) Reactions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Balancing Metathesis Reactions Determine ions present Write chemical formulas of products by combining cation from one reactant with anion of the other Use charges of ions to determine subscripts Balance the equation

Sample Problem Predict the identity of the precipitate that forms when solutions of BaCl2 and K2SO4 are mixed.

Step 1: Determine ions BaCl2 and K2SO4 are mixed: Ba 2+ Cl - K +

Step 2: Combine Reactants: BaCl2 and K2SO4 BaCl2 + K2SO4  BaSO4 + K2Cl2

Step 3: Subscripts Ba 2+ Cl - K + SO4 2- BaCl2 + K2SO4  BaSO4 + KCl

Step 4: Balance BaCl2 + K2SO4  BaSO4 + 2KCl Did a precipitate form?

BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq) Solution BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq)

AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Molecular Equation Reactants and products in their molecular form NOT ionic character AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Complete Ionic Equation All soluble strong electrolytes shown as ions Spectator ions- appear as reactants and products in identical form Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl (s) + K+ (aq) + NO3- (aq)

Ag+ (aq) + Cl- (aq)  AgCl (s) Net Ionic Equation Omit spectator ions Ag+ (aq) + Cl- (aq)  AgCl (s)

Writing Net Ionic Equations Write a balanced molecular equation. Dissociate all strong electrolytes. Identify and cancel spectator ions

Sample Problem Write the net ionic equation for mixing calcium chloride and sodium carbonate.

Step 1: Molecular equation Calcium chloride and sodium carbonate CaCl2 + Na2CO3  CaCO3 + 2Na Cl

Step 2: Dissociate strong electrolytes Ca 2+ + 2Cl- + 2Na+ + CO3 2-  CaCO3 + 2Na+ + 2Cl- All are strong electrolytes, but CaCO3 is insoluble in water

Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s) Step 3: Omit spectators Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s)

Homework 4.19-4.24 on page 158