Question

H2O(g) → H2 + O2(g) Predict whether the reaction above would have an increase or a decrease in entropy.

Answer 1

Answer: The reaction $H_2O(g)\rightarrow H_2(g)+O_2(g)$ would have increase in entropy.

Explanation: Entropy is the measure of degree of randomness or disorder of a system. Thus a system with less disorder has less entropy and a system with more disorder has more entropy. The balanced chemical equation will be:

$2H_2O(g)\rightarrow 2H_2(g)+O_2(g)$

The given reaction leads to formation of 3 moles of gaseous products from 2 mole of gaseous reactant. Thus the given reaction leads to increase in moles and thus increase in disorder and increase in entropy.

Answer 2

Since the reaction has one mole of reactants versus two moles of products, the reaction would have a decrease in entropy. One mole of gas exerts less pressure than two moles of gas, and therefore one mole of gas has more entropy than two moles of gas.

Hope this helps