First slide

Hess law of constant summation

Question

ΔH^o for a reaction F₂ + 2HCl → 2HF + Cl₂ is given as -352.8 KJ. $∆ H_{f}^{0}$ for HF is –268.3KJ mol^-1, then $∆ H_{f}^{0}$ of HCl would be

Moderate

Solution

F₂ + 2HCl → 2HF + Cl₂ , ΔH^o =-352.8KJ
$\begin{array}{l} \Delta {H^o} = [2{\Delta _f}H(HF) + {\Delta _f}H(C{l_2})] - [{\Delta _f}H({F_2}) + 2{\Delta _f}H(HCl)]\\ - 352.8 = [2( - 268.3) + 0] - [0 + 2{\Delta _f}H(HCl)]\\ - 352.8 = - 536.6 - 2{\Delta _f}H(HCl) \end{array}$
${\Delta _f}H(HCl) = \frac{{ - 183.8}}{2} = - 91.9KJ/mole$

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