JAMB - Chemistry (2006 - No. 43)
If the heat of combustion of hydrogen is -285.8kJ,
What is the heat of formation of water?
What is the heat of formation of water?
+571.6 kJ
-571.6 kJ
-285.8 kJ
+285.8 kJ
설명
Heat of combustion (ΔH°c) is the energy released when a substance is burnt in oxygen.
The heat of formation (ΔH°f) is the energy released or absorbed when a substance is formed from its constituent elements.
The chemical equation is H\(_2\) + 1/2 O\(_2\) → H\(_2\)O
Since the heat of formation of H\(_2\) is zero (ΔHf(H\(_2\) ) = 0 kJ/mol), and the heat of combustion of H\(_2\) is -285.8 kJ/mol, we can use the following relationship:
ΔHc(H\(_2\) ) = ΔHf( H\(_2\)O) - ΔHf(H\(_2\) )
Rearrange to solve for ΔHf( H\(_2\)O):
ΔHf( H\(_2\)O) = ΔHc(H\(_2\) ) + ΔHf(H\(_2\) )
= -285.8 kJ/mol + 0 kJ/mol
= -285.8 kJ/mol