= Solution
Using bonds broken minus bonds formed, the enthalpies per mole of elementary events are
$$
\begin{array}{c|r}
\text{step}&\Delta H_i/\mathrm{kJ\,mol^{-1}}\\\hline
1&+242\\
2&436-431=+5\\
3&242-431=-189\\
4&-242
\end{array}
$$
Under the <steady-state approximation in chemical kinetics>, initiation and termination have equal event rates, so their <enthalpy> contributions cancel. The two propagation steps also have equal flux, and their combined <enthalpy> is $5-189=\boxed{-184\,\mathrm{kJ\,mol^{-1}}}$ per overall reaction producing two moles of HCl. \b[Step 3 dominates the <heat> release]; the small endothermic contribution of step 2 only slightly offsets it.
An insulated vessel cannot remove this released energy as <heat>, so its <temperature> rises as reaction proceeds. In a rigid vessel the precise energy balance uses reaction <internal energy> and constant-volume <heat> capacities, whereas a constant-pressure adiabatic balance uses <enthalpy>; the strong exothermic conclusion is the same. Warming changes the <rate constants>, and activated initiation and propagation can accelerate, producing positive thermal feedback and potentially a rapid <temperature> rise. A numerical final <temperature> or a claim of an inevitable explosion is not justified without composition, <heat> capacities and heat-loss information. The initially assumed 298-K <rate constants> cannot remain fixed throughout an appreciable adiabatic <temperature> rise.
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