Consider the following reactions : <br> (a)H+(aq)+OH−(aq)=H2O(l),ΔH=−X1kJmol−1 <br> (b)H2(g)+12O2(g)=H2O(l),ΔH=−X2kJmol−1 <br> (c)CO2(g)+H2(g)=CO(g)+H2O(l)−X3kJmol−1 <br> (d)C2H2(g)+52O2(g)=2CO2(g)+H2O(l)+X4kJmol−1 <br> Enthalpy of formation of H2O(l) is: