Hydrogen (H2) combines with chlorine (Cl2) to form hydrochloric acid (HCl): H2 + Cl2 → 2HCl. Using the chart below, what is the estimated enthalpy change for this reaction? (Assume no changes in pressure or volume.)

A. 494 kJ/mol
B. 247 kJ/mol
C. –58 kJ/mol
D. –184 kJ/mol

Explanation:

You calculate the energy required to break all the bonds in the reactants.

Then you subtract the energy needed to break all the bonds in the products.

                       H₂  +  Cl₂ ⟶ 2HCl

Bonds:          1H-H   1Cl-Cl   2H-Cl

D/kJ·mol⁻¹:     436    242       431

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