Question

1) Calculate the standard enthalpy change for the following reaction at 25 °C.

Mg(OH)2(s)+2HCl(g)------>MgCl2(s)+2H2O(g) answer in kj/mol

2) The oxidation of copper(I) oxide, Cu2O(s), to copper(II) oxide, CuO(s), is an exothermic process,

2Cu2O(s)+O2(g)----->4CuO(s) delta Hxn=-292 kj/mol

Calculate the energy released as heat when 25.46 g of Cu2O(s) undergo oxidation at constant pressure.

Answer 1

1) Standard Enthalpy of various products are

Enthalpy of Mg(OH)2 = -924.54 KJ/mol

Enthalpy of HCl = -92.30 KJ/mol

Enthalpy of MgCl2 = -641.8 KJ/mol

Enthalpy of H2O = -241.8 KJ/mol

Enthalpy of Reaction = Enthalpy of MgCl2 + 2 * Enthalpy of water - Enthalpy of Mg(OH)2 - 2 * enthalpy of HCl

=> -641.8 + 2(-241.8) + 2(92.30) + 924.54

=> -16.26 KJ/mol

2)

Molar mass of Cu2O = mass of Cu * 2 + mass of oxygen

=> 2 * 63.5 + 16

=> 143 gm/mol

Number of moles of Cu2O = mass of Cu2O/molar mass = 25.46/143 = 0.1780 moles

Energy Released = 0.1780 moles * -(-292 KJ/moles) = 51.988 KJ