# Can you give examples of exothermic reactions?

Feb 12, 2017

An example of an exothermic reaction is provided by the combustion of a hydrocarbon:

#### Explanation:

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right)$

The extensively measured enthalpy change for this reaction is, $\Delta {H}_{f}^{\circ} = - 890.2 \cdot k J \cdot m o {l}^{-} 1$. This means that so much energy is released per mole of reaction as written.

Given the sign on $\Delta {H}_{f}^{\circ}$, we could write the reaction in the following terms:

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O + 890.2 \cdot k J$

Because the enthalpy difference represents the difference between bonds made (energy out) and bonds broken (energy in), we can thus use enthalpy as a virtual product in the reaction, with the appropriate 1:1 molar relationship with the methane that is combusted. Are you happy with this?