Question

The heat of the combustion of carbon and carbon monoxide is ${ -393.5 }$ and ${ -283.5 }{ kJmol }^{ -1 }$ respectively. The heat of formation (in kJ) of carbon monoxide per mole is:
A. ${ 676.5 }$
B. ${ -676.5 }$
C. ${ -110 }$
D. ${ 110.5 }$

Answer 1

Hint: Combustion means the burning of a substance. The heat of combustion is that the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The reaction is usually a hydrocarbon reacting with oxygen to make water, carbon dioxide, and heat.

Complete step by step solution:
The following reactions will take place and their heat of combustion are given below;
${ C+O }_{ 2 }{ \rightarrow CO }_{ 2 }{ ;\Delta H }_{ Comb. }{ =-393.5 }{ kJmol }^{ -1 }$..........(1)

${ CO+{ 1/2 }O }_{ 2 }{ \rightarrow CO }_{ 2 }{ ;\Delta H }_{ Comb. }{ =-283.5 }{ kJmol }^{ -1 }$............(2)

When we subtract equation (2) from equation (1), we get
${ C+{ 1/2 }O }_{ 2 }{ \rightarrow CO }$
= ${ -393.5 - (-283.5) }$
= ${ -393.5 + 283.5 }$
= ${ -110kJ }$.

Hence, the correct option is C.

Additional Information:
i) The heat of combustion depends on the number of a carbon atom. For isomers, differences in the heat of combustion are due to differences in the internal energy of the adducts.
ii) If intermolecular interaction (lattice energy etc.) can be neglected it reflects differences in the internal energy of the molecules.
iii) In the combustion process, all the carbon-hydrogen bonds are going to be broken and more the strength of the bonds lesser will be the heat of combustion. The stability of a compound is inversely proportional to the heat of combustion.

Note: The possibility to make a mistake is that you may choose option B. As we have to calculate the heat of formation of carbon monoxide, we have to subtract the equations and not add them.