Question

The lattice enthalpy (formation of ionic solid from ions in the gas phase) for AgCl(s) is -916 kJ/mol and the hydration enthalpy (dissolution of gaseous ions into water) is - 850 kJ/mol. How much heat (in Joules) is involved in forming 1L of saturated AgCl solution (1.8 × 10-4 g / 100 mL water) by dissolving AgCl(s)? Assume solution volume does not change much upon dissolution. The equations are given below. Ag+ (g) + Cl− (g) Æ AgCl(s) Ag+ (g) + Cl- (g) Æ Ag+ (aq) + Cl- (aq)

Answer 1

1. Ag+ (g) + Cl− (g) ===> AgCl(s)    dH = -916kJ/mol

2. Ag+ (g) + Cl- (g) ===> Ag+ (aq) + Cl- (aq)      dH = -850kJ/mol

3. Ag+ (aq) + Cl- (aq) ===> Ag+ (g) + Cl- (g)    dH = 850kJ/mol

So, 1 + 3 equations

gives Ag+ (aq) + Cl- (aq) ===> AgCl(s) dH = -66kJ/mol

100mL water has mass AgCl = 1.8 x 10^-4grams

1L water has mass AgCl = 1.8 x 10^-3 grams

moles = mass/molar mass = 1.8 x 10^-3 grams/ 143.32g/mol = 1.2559 x10^-5moles

So, heat released in 1 mol = 66kJ = 66000J

heat released in 1.2559 x10^-5moles = 1.2559 x10^-5moles x 66000 = 0.83J