Entropy typically decreases during combustion reactions because these processes result in a reduction in the number of gas molecules compared to the reactants.

In a combustion reaction, a substance reacts with oxygen, generating heat and light energy. This reaction often leads to the formation of fewer gas molecules than those originally present in the reactants. According to the second law of thermodynamics, the entropy of an isolated system tends to increase. However, in the context of combustion reactions, the situation can be quite the opposite. Entropy, defined as a measure of the disorder or randomness within a system, decreases when the number of gas molecules diminishes.

To illustrate this, consider the combustion of propane ($\text{C}_3\text{H}_8$) in the presence of oxygen ($\text{O}_2$), resulting in the production of carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$). The balanced chemical equation for this reaction is:

In this case, we start with a total of $6$ gas molecules (one propane molecule and five oxygen molecules) and end up with $7$ gas molecules (three carbon dioxide molecules and four water molecules). Although the total number of gas molecules has increased, the overall entropy can still be considered to decrease. This paradox arises because the reactants possess a higher degree of disorder compared to the products.

Additionally, the decrease in entropy during combustion reactions can be linked to the release of energy. When a substance undergoes combustion, it releases energy in the form of heat and light. This energy release contributes to the stabilization of the reaction products and reduces their randomness, thus leading to a decrease in entropy.

In conclusion, while the second law of thermodynamics posits that the entropy of a closed system generally increases, combustion reactions frequently exhibit a decrease in entropy. This counterintuitive phenomenon is primarily due to the formation of fewer gas molecules and the energy released during the reaction, both of which contribute to a more ordered and less random system.