Question

Which of the enthalpies of combustion in Table 5.2 the table are also standard enthalpies of formation?

Short answer

The combustion reaction of carbon, hydrogen, magnesium, and sulfur can be considered as a formation reaction for respective compounds: carbon dioxide, water, magnesium oxide, and sulfur dioxide.

Step-by-step solution

Enthalpy of formation

Enthalpy of formation: The enthalpy changes when 1 mole of a substance is formed from its constituent elements in their standard states. A pure element in its standard state has a standard enthalpy of formation of zero.

To check whether a chemical reaction is suitable to calculate the enthalpy of the formation of a reactant, we have to keep two things in mind:

• Firstly, there should be one reactant in which we are interested to calculate the formation of enthalpy on the product side. If the condition is not true in the chemical reaction, we can’t calculate the enthalpy of formation from the chemical reaction of that substance.

• Secondly, reactants present in the chemical reaction should be in their elemental form.

Enthalpy of formation of carbon dioxide

If we observe in Table 5.2, reactions from methane onwards have multiple products, so this reaction can’t be considered for calculating the enthalpy of formation.

For the combustion of carbon monoxide, it is not in elemental form, so the reaction can’t be considered for the enthalpy of formation of carbon dioxide.

Classification

To check the above combustion reaction, we have to know the exact elemental form.

Here is a list:

\({{\rm{O}}_{\rm{2}}}{\rm{(g), C}}{{\rm{l}}_{\rm{2}}}{\rm{(g), }}{{\rm{H}}_{\rm{2}}}{\rm{(g), }}{{\rm{N}}_{\rm{2}}}{\rm{(g), }}{{\rm{F}}_{\rm{2}}}{\rm{(g), B}}{{\rm{r}}_{\rm{2}}}{\rm{(l), }}{{\rm{I}}_{\rm{2}}}{\rm{(s)}}\)

Other than these, most of the elements should be present in solid-state.

The rest of the reactions can be considered as formation reactions and the enthalpy of combustion will be the enthalpy of formation for products.