Question

Ammonia (NH3(g), Delta.Hf = –46.19 kJ/mol) reacts with hydrogen chloride (HCl(g), Delta.Hf = –92.30 kJ/mol) to form ammonium chloride (NH4Cl(s), Delta.Hf = –314.4 kJ/mol) according to this equation:

NH3(g) + HCl(g) Right arrow. NH4Cl(s)

What is Delta.Hrxn for this reaction?

kJ

Answer 1

ΔHrxn = ΣΔHf(products) − ΣΔHf(reactants)

= [ΔHf(NH4Cl(s))] − [ΔHf(NH3(g)) + ΔHf(HCl(g))]
= (−314.4 kJ) − [ (−46.19 kJ) + (−92.30 kJ) ]
= −314.4 kJ − (−138.49 kJ)
= −175.91 kJ

So ΔHrxn ≈ −176.0 kJ per mole of NH3 + HCl → NH4Cl (exothermic).