High School

ΔH_rxn for the reaction below is -186 kJ. What is the value of ΔH_f for HCl(g)?

\[ \text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g) \]

A. -93 kJ/mol
B. -186 kJ/mol
C. 93 kJ/mol
D. 186 kJ/mol

Answer :

Final answer:

Correct option: a) -93 kJ/mol. The standard molar enthalpy of formation (ΔHf) for HCl(g) is – 93 kJ/mol, which is determined by dividing the overall reaction enthalpy ΔHrxn of – 186 kJ by the 2 moles of HCl formed in the reaction.

Explanation:

The value of the standard molar enthalpy of formation (ΔHf) of HCl(g) can be determined from the reaction:

H2(g) + Cl2(g) → 2HCl(g)

The given ΔHrxn (enthalpy change of the reaction) for this exothermic reaction is −186 kJ. This energy change corresponds to the formation of 2 moles of HCl(g), thus each mole of HCl formed releases half of the energy provided by the reaction.

ΔHf per mole of HCl(g) = −186 kJ / 2 moles

= −93 kJ/mol

This is close to the value cited in Appendix G, which provides a standard molar enthalpy of formation for HCl(g) of −92.307 kJ/mol. Therefore, the correct answer is a) −93 kJ/mol.