Heat of Combustion and Heat of Formation

Heat of combustion is the amount of heat released when a substance undergoes complete combustion in the presence of oxygen. It is usually expressed in kJ/mol. On the other hand, heat of formation is the enthalpy change that accompanies the formation of one mole of a compound from its elements in their standard states.

Given Data:

Heat of combustion of carbon (C) = -393.5 kJ/mol
Heat of combustion of carbon monoxide (CO) = -283.5 kJ/mol

Calculation:

To find the heat of formation of carbon monoxide (CO), we need to use the concept of Hess's Law. According to Hess's Law, the overall enthalpy change of a reaction is independent of the pathway between the initial and final states.

We can start by writing the balanced chemical equation for the combustion of carbon to form carbon monoxide:

C + 1/2O2 → CO

Now, we can write the overall reaction using the heat of formation values:

C + 1/2O2 → CO (ΔH = ?)

We can break down the overall reaction into two steps:

1. Combustion of carbon to form carbon dioxide (CO2):

C + O2 → CO2 (ΔH = -393.5 kJ/mol)

2. Formation of carbon monoxide (CO) from carbon dioxide (CO2):

CO2 → CO (ΔH = ?)

According to Hess's Law, we can add the enthalpy changes of these two steps to find the overall enthalpy change:

ΔH = ΔH1 + ΔH2

ΔH = -393.5 kJ/mol + ΔH2

We rearrange the equation to solve for ΔH2:

ΔH2 = ΔH - ΔH1

ΔH2 = -283.5 kJ/mol - (-393.5 kJ/mol)

ΔH2 = 110 kJ/mol

Therefore, the heat of formation of carbon monoxide (CO) per mole is 110 kJ/mol.

Since the given options are in the form of positive values, we need to convert the heat of formation to a positive value:

Heat of formation of carbon monoxide (CO) = -110 kJ/mol

Therefore, the correct answer is option 'D' (-110.5 kJ/mol).