# Thermochemistry with Equation Stoichiometry

## Key Questions

• #### Answer:

Consider an exothermic combustion reaction. The amount of heat may be treated as a stoichiometric product, precisely dependent on the amount of hydrocarbon combusted.

#### Explanation:

Methane combustion drives our civilization to a very great degree:

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O , \Delta H = - 890$ $k J$ $m o {l}^{-} 1$.

The quoted enthalpy of combustion is per mole of reaction as written. You don't have to know these; you do have to know how to balance the equation. Because this energy is associated with the combustion of 1 mol of methane, I could also treat the evolved energy as a reagent or product in the reaction:

i.e. $C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O + 890$ $k J$.

It would be on the reactant side if the reaction was endothermic. In other words the minus sign denotes evolution of heat. Alternatively, $890$ $k J$ of heat are evolved from the above reaction, so I am treating energy as a product just as carbon dioxide and water are (as indeed it is; of course the heat is a consequence of the formation of water and carbon dioxide bonds). If less than 16 g (1 mol) methane are combusted, the heat evolved will diminish stoichiometrically. Does this address your question?