Nitric acid, which is among the top 15 chemicals produced in the United States, was first prepared over 1200 years ago by heating naturally occurring sodium nitrate (called saltpeter) with sulfuric acid and collecting the vapors produced. Calculate ΔH°rxn for this reaction. ΔH°f [NaNO3 (s) ] = - 467.8 kJ/mol; ΔH°f [NaHSO4 (s) ] = - 1125.5 kJ/mol; ΔH°f [H2SO4 (l) = - 814.0 kJ/mol; ΔH°f [HNO3 (g) ] = - 135.1 kJ/mol NaNO3 (s) + H2SO4 (l) → NaHSO4 (s) + HNO3 (g)

21.2 kJ/mol

Explanation:

Let's consider the following reaction for the synthesis of nitric acid.

NaNO₃ (s) + H₂SO₄ (l) → NaHSO₄ (s) + HNO₃ (g)

We can calculate the standard enthalpy of the reaction (ΔH°rxn) using the following expression.

ΔH°rxn = ∑ np * ΔH°f (p) - ∑ nr * ΔH°f (r)

where,

n: moles

ΔH°f: standard enthalpy of formation

p: products

r: reactants

ΔH°rxn = 1 mol * ΔH°f (NaHSO₄) + 1 mol * ΔH°f (HNO₃) - 1 mol * ΔH°f (NaNO₃) - 1 mol * ΔH°f (H₂SO₄)

ΔH°rxn = 1 mol * (-1125.5 kJ/mol) + 1 mol * (-135.1 kJ/mol) - 1 mol * (-467.8 kJ/mol) - 1 mol * (-814.0 kJ/mol)

ΔH°rxn = 21.2 kJ/mol