# Compare Enthalpy Change of Formation of CO and CO2

Let's take a look at the following question:

Given enthalpy change of formation of

Carbon monoxide = -110 kJ mol-1
Carbon dioxide = -393 kJ mol-1

We want to determine whether:

1. CO or CO2 is more stable?
2. Enthalpy change of combustion of C or CO is more exothermic?

We can use the energy level diagram to compare stability of species and magnitude of enthalpy changes.

Enthalpy change of formation is defined as heat change when 1 mole of compound is formed from its constituent elements in standard states.

For carbon monoxide:

C(s) + 1/2 O2(g) → CO(g)

For carbon dioxide:

C(s) + O2(g) → CO2(g)

Let's draw out the energy level diagram using the enthalpy change of formations of CO and CO2.

Since both reactions are exothermic, the arrows will be pointing down.

Enthalpy change of formation of CO2 is more exothermic than that for CO, so the arrow for formation of CO2 is longer.

1. Which is more stable? CO or CO2?

Comparing stability is simply comparing enthalpy of CO and CO2.

The lower the enthalpy, the less heat content and the more stable the substance.

CO has higher enthalpy, higher heat content, hence less stable.

CO2 has lower enthalpy, lower heat content, hence more stable.

2. Which enthalpy change of combustion is more exothermic? C or CO?

Enthalpy change of combustion is defined as heat change when 1 mole of substance is burnt in excess oxygen.

For carbon:

C(s) + O2(g) → CO2(g)

Notice this reaction is exactly the same as that for enthalpy change of formation of CO2.

Hence enthalpy change of combustion of C = enthalpy change of formation of CO2 = -393 kJ mol-1

For carbon monoxide:

CO(g) + 1/2 O2(g) → CO2(g)

Both of these terms can be represented in the energy level diagram drawn earlier.

We can compare magnitude of enthalpy change terms by comparing length of the arrows.

The longer the arrow, the bigger the magnitude of enthalpy change.

Enthalpy change of combustion of C has a longer arrow, magnitude is greater, hence more exothermic.

Enthalpy change of combustion of CO has a shorter arrow, magnitude is smaller, hence less exothermic.

Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore

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