Consider the following reactions:-

1) H+(aq) + OH-(aq) ----------> H2O(l);ΔH= -X1kJ/mol
2) H2(g) + 1/2 O2(g) ----------> H2O(l);ΔH = -X2 kJ/mol
3) CO2(g) + H2(g) ------------> CO(g) + H2O(l);ΔH = -X3kJ/mol
​4) C2H2(g) + 5/2 O2(g) ------------> 2CO(g) + H2O(l);ΔH = +X4kJ/mol

Enthalpy of formation of H2O(l) is:-
a) +X1 kJ/mol
b) +X2 kJ/mol
c) +X3 kJ/mol
d) +X4 kJ/mol

​ Kindly answer sir/mam.

Option 2  is the correct answer. Enthalpy of formation of water is defined as the heat energy absorbed or released when 1 mole of water is formed from its elements. Though water is formed in the other reactions, its not from its elements because,
The first reaction is neutralisation.
The third reaction is combination reaction
The fourth reaction is combustion reaction.

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