If the heat of combustion of hydrogen is -285.8kJ, what is the of formation of water?

 If the heat of combustion of hydrogen is -285.8kJ, what is the of formation of water?

A. -285.8 kJ

B. -571.6kJ

C. +571.6kJ

D. +285.8kJ

The heat of combustion of hydrogen refers to the enthalpy change when 1 mole of hydrogen burns completely in oxygen to form water:

H2(g) + {1/2}\{O2}(g) 一> H2O (l)

This reaction is exactly the same as the standard enthalpy of formation of water, since water is being formed from its elements in their standard states.

Therefore, the enthalpy (heat) of formation of water is:

-285.8  kJ

✅ Correct answer: A. −285.8 kJ

Exam tip (WAEC/NECO):
Heat of formation = heat change when 1 mole of a compound is formed from its elements in their standard states.

PQN: 22