By Oxana Fox, Updated Aug 30, 2022

Hydrochloric acid (HCl) reacts readily with zinc (Zn) to produce zinc chloride (ZnCl₂) and hydrogen gas (H₂). This reaction is exothermic; the released heat is quantified by the reaction enthalpy.

Step 1 – Write the balanced equation

Zn + 2 HCl → ZnCl₂ + H₂

Step 2 – Gather standard enthalpies of formation

Standard enthalpies (ΔH°ₓf) are listed in kilojoules per mole (kJ mol⁻¹). For this reaction they are:

• Zn (0 kJ mol⁻¹)
• HCl (−167.2 kJ mol⁻¹)
• ZnCl₂ (−415.1 kJ mol⁻¹)
• H₂ (0 kJ mol⁻¹)

Enthalpies for elemental forms such as Zn or H₂ are zero by definition.

Step 3 – Calculate the total enthalpy of the reactants

ΣΔH°ₓf (reactants) = 0 + 2(−167.2) = −334.3 kJ mol⁻¹

Step 4 – Calculate the total enthalpy of the products

ΣΔH°ₓf (products) = −415.1 + 0 = −415.1 kJ mol⁻¹

Step 5 – Determine the reaction enthalpy

ΔH°rxn = ΣΔH°ₓf (products) − ΣΔH°ₓf (reactants) = −415.1 − (−334.3) = −80.8 kJ mol⁻¹

The negative sign indicates that the reaction releases approximately 81 kJ of heat per mole of zinc consumed.

For more detailed thermodynamic data, see the NIST Chemistry WebBook.