Question

What will be the sign on ΔS for the following reaction, and why? 2 Mg (s) + O₂ (g) → 2 MgO (s)

a. Positive, because entropy increases in the formation of MgO.
b. Negative, because entropy decreases in the formation of MgO.
c. Zero, because there is no change in entropy.
d. Cannot be determined without additional information.

Answer 1

The sign on ∆S for the reaction 2 Mg (s) + O₂ (g) → 2 MgO (s) is negative because the reaction results in a more ordered solid phase and a decrease in the number of gaseous molecules.

Step-by-step explanation:

The sign of the entropy change (∆S) for the reaction 2 Mg (s) + O₂ (g) → 2 MgO (s) would be negative. This is because the reaction involves going from one molecule of gaseous oxygen and solid magnesium to magnesium oxide, which is a solid. According to entropy principles, a decrease in the number of gaseous molecules and the formation of a more ordered solid phase leads to a decrease in entropy, or a negative ∆S. Similar analogies can be drawn from reactions where a gas is produced leading to an increase in entropy, and reactions where gaseous reactants lead to fewer gaseous products or a solid, resulting in a decrease in entropy.