the reaction equation is already balanced. when we look at the equation, the bonds which break are C-O bond and H-Cl bond. The bonds which form are C-Cl bond and O-H bond. Hence, ΔHrxn = ΣΔH breaking - ΣΔH making = (ΔH(C-O) + ΔH(H-Cl)) - (ΔH(C-Cl) + ΔH(O-H)) = ((358 + 427) - (339 + 467) kJ/mol = -21 kJ/mol