Consider the following reactions:
(i) `H^(+)(aq)+OH^(-)(aq)rarrH_(2)O(l)` ,
`DeltaH==-X_(1)Kjmol^(-1)`
(ii) `H_(2)(g)+(1)/(2)O_(2)(g)rarrH_(2)O(l),DeltaH=-X_(2)Kjmol^(-1)`
(iii) `CO_(2)(g)+H_(2)(g)rarrCO(g)+H_(2)O(l)` ,
`DeltaH=-X_(3)KJmol^(-1)`
(iv) `C_(2)H_(2)(g)+(5)/(2)O_(2)(g)rarr2CO_(2)(g)+H_(2)O(l)` ,
`DeltaH=+X_(4)KJmol^(-1)`
Enthanlpy of formation of `H_(2)O(l)` is

Options

A. `-x_(2)KJmol^(-1)`
B. `+x_(3)KJmol^(-1)`
C. `-x_(4)KJmol^(-1)`
D. `+x_(1)KJmol^(-1)`

Answer

A

Explanation

The amount of heat evolved or absorbed during the formation of 1 mole of a compound from its constituent is known as heat of formation. So, the correct answer is
`H_(2)(g)+(1)/(2)O_(2)(g)rarrH_(2)O(l),DeltaH=-X_(2)Kjmol^(-1)`