Question

Given the following information below, use Hess’s Law to calculate the enthalpy of formation for sodium oxide:

Na (s)     +      HCl (l)  à    NaCl (aq) + ½ H₂ (g)                ∆H_Rx = -393.1 kJ/mol

Na₂O (s)     +     2 HCl (l)  à 2 NaCl (aq)   + H₂O              ∆H_Rx = -675.2 kJ/mol

H₂ (g)       +      ½ O₂ (g)  à    H₂O (g)                              ∆H_Rx = -288.1 kJ/mol

2 Na (s)       +       ½ O₂ (g)   à   Na₂O (s)                          ∆H_Rx =    __________ kJ/mol

Calculated Heat of Reaction is....?

(Put your answer in 4 significant figures)

Answer 1

Lets number the reaction as 1, 2, 3, 4 from top to bottom

required reaction should be written in terms of other reaction

This is Hess Law

required reaction can be written as:

reaction 4 = +2 * (reaction 1) -1 * (reaction 2) +1 * (reaction 3)

So, deltaHo rxn for required reaction will be:

deltaHo rxn = +2 * deltaHo rxn(reaction 1) -1 * deltaHo rxn(reaction 2) +1 * deltaHo rxn(reaction 3)

= +2 * (-393.1) -1 * (-675.2) +1 * (-288.1)

= -399.1 KJ/mol

Answer: -399.1 KJ/mol