Enthalpy Changes

Question

The reaction of hydrogen bromide(g) with chlorine(g) to form hydrogen chloride(g) and bromine(g) proceeds as follows:

1. 2HBr(g) + Cl₂(g)2HCl(g) + Br₂(g)

When 31.2 g HBr(g) reacts with sufficient Cl₂(g), 15.6 kJ is evolved.

Calculate the value of _rH for the chemical equation given.

Answer 1

Convert 31.2 g HBr to moles:

31.2 g HBr x 1 mol HBr / 80.9 g = 0.386 mols HBr

This generates 15.6 kJ

So, on a per mol basis, we would have...

15.6 kJ / 0.386 mols = 40.14 kJ / mol HBr

Since the equation shows 2 mols HBr reacting, the ∆Hrxn as written is...

40.14 kJ / mol HBr x 2 mols HBr = 80.8 kJ