For the reaction 2 Hg(l) O2 → 2HgO(s), ∆H = -43 kcal/mol (-180 kJ/mol). Does entropy increase or decrease in this process? Explain.

Verified step by step guidance

1. Identify the states of matter for each substance in the reaction: Hg is a liquid, O2 is a gas, and HgO is a solid.
2. Consider the change in the number of moles of gas: The reaction starts with 1 mole of O2 gas and ends with no gas, indicating a decrease in the number of gas molecules.
3. Recall that entropy (S) is a measure of disorder or randomness in a system. Gases have higher entropy than liquids and solids due to their higher degree of freedom and movement.
4. Since the reaction involves the conversion of a gas (O2) to a solid (HgO), the system becomes more ordered, leading to a decrease in entropy.
5. Conclude that the entropy decreases in this process because the gaseous reactant is converted into a solid product, reducing the overall disorder of the system.

Key Concepts

Enthalpy (∆H)

Enthalpy is a thermodynamic property that reflects the total heat content of a system. In the given reaction, the negative value of ∆H indicates that the reaction is exothermic, meaning it releases heat. This release of energy can influence the entropy of the system, as it often leads to a decrease in disorder when heat is lost.

Entropy (S)

Entropy is a measure of the disorder or randomness in a system. In chemical reactions, an increase in entropy typically occurs when the number of gas molecules increases or when solids and liquids are converted into gases. Conversely, a decrease in entropy happens when a system becomes more ordered, such as when gaseous reactants form solid products.

Gibbs Free Energy (∆G)

Gibbs Free Energy combines enthalpy and entropy to determine the spontaneity of a reaction. The relationship is given by the equation ∆G = ∆H - T∆S, where T is the temperature in Kelvin. For a reaction to be spontaneous, ∆G must be negative, which can occur if the enthalpy change is negative and the entropy change is also negative, as in the case of the formation of solid HgO from liquid Hg and O2.