enthalpy formation

Question

A scientist measures the standard enthalpy change for the following reaction to be -29.6 kJ

I₂ + Cl₂ = 2ICl

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of I₂ is kJ/mol

Species (kJ/mol)

ICl 17.8

Cl₂ 0.0

Answer 1

You are given the ΔH_f for two of the species involved in the reaction so now all that is left is to find the remaining ΔH_f for I₂.

Let's see if I can walk you through it.

ΔH_rxn = ∑ΔH_f(products) - ∑ΔH_f(reactants)

ΔH_rxn = ΔH_f(2ICl) - ΔH_f(Cl₂) - ΔH_f(I₂)

-29.6 kJ = (2*17.8 kJ) - 0 - ΔH_f(I₂)

-29.6 kJ - 35.6 kJ = -ΔH_f(I₂)

ΔH_f(I₂) = 65.2 kJ/mol

Hope that helps!