Why does entropy tend to decrease in combustion reactions?

Entropy tends to decrease in combustion reactions because they result in fewer gas molecules than the reactants.

In a combustion reaction, a substance reacts with oxygen to produce heat and light energy. This process often results in the formation of fewer gas molecules than were present in the reactants. According to the second law of thermodynamics, the entropy of a system tends to increase. However, in the case of combustion reactions, the opposite is often true. This is because entropy, which is a measure of the disorder or randomness of a system, decreases when the number of gas molecules decreases.

Consider the combustion of propane (C3H8) in oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). The balanced chemical equation for this reaction is C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g). Here, we start with six gas molecules (one of propane and five of oxygen) and end with seven gas molecules (three of carbon dioxide and four of water). Although the number of gas molecules has increased, the overall entropy has decreased. This is because the reactants are more disordered (higher entropy) than the products.

The decrease in entropy in combustion reactions can also be attributed to the release of energy. When a substance burns, it releases energy in the form of heat and light. This energy release makes the products of the reaction more stable and less random than the reactants, leading to a decrease in entropy.

In summary, while the second law of thermodynamics states that entropy tends to increase in a system, combustion reactions often result in a decrease in entropy. This is due to the formation of fewer gas molecules and the release of energy, both of which lead to a more ordered and less random system.