(a) For each of the following reactions, predict the sign of ΔH° and ΔS° without doing any calculations. (i) 2 Mg(s) + O₂ (g) ⇌ 2 MgO(s) (ii) 2 KI(s) ⇌ 2 K(g) + I₂(g) (iii) Na₂(g) ⇌ 2 Na(g) (iv) 2 V₂O₅(s) ⇌ 4 V(s) + 5 O₂(g)

Verified step by step guidance

1. Examine the reaction (i) 2 Mg(s) + O_2(g) ⇌ 2 MgO(s): This is a combustion reaction where a solid and a gas form a solid. Typically, such reactions are exothermic, so ΔH° is likely negative. The reaction results in a decrease in the number of gas molecules, suggesting a decrease in disorder, so ΔS° is likely negative.
2. Consider the reaction (ii) 2 KI(s) ⇌ 2 K(g) + I_2(g): This reaction involves the sublimation of a solid to form gases. Sublimation is endothermic, so ΔH° is likely positive. The increase in the number of gas molecules indicates an increase in disorder, so ΔS° is likely positive.
3. Analyze the reaction (iii) Na_2(g) ⇌ 2 Na(g): This reaction involves the dissociation of a diatomic gas into two monatomic gases. The process of breaking bonds is endothermic, so ΔH° is likely positive. The increase in the number of gas particles suggests an increase in disorder, so ΔS° is likely positive.
4. Evaluate the reaction (iv) 2 V_2O_5(s) ⇌ 4 V(s) + 5 O_2(g): This decomposition reaction involves the formation of a gas from a solid, which is typically endothermic, so ΔH° is likely positive. The increase in the number of gas molecules indicates an increase in disorder, so ΔS° is likely positive.
5. Summarize the predictions: For reaction (i), ΔH° is negative and ΔS° is negative. For reaction (ii), ΔH° is positive and ΔS° is positive. For reaction (iii), ΔH° is positive and ΔS° is positive. For reaction (iv), ΔH° is positive and ΔS° is positive.

Key Concepts

Enthalpy (ΔH)

Enthalpy (ΔH) is a measure of the total heat content of a system at constant pressure. In chemical reactions, a negative ΔH indicates an exothermic reaction, where heat is released, while a positive ΔH indicates an endothermic reaction, where heat is absorbed. Understanding the nature of the reactants and products helps predict whether the reaction will release or absorb heat.

Entropy (ΔS)

Entropy (ΔS) is a measure of the disorder or randomness in a system. A positive ΔS indicates an increase in disorder, often occurring when gases are produced from solids or liquids, while a negative ΔS suggests a decrease in disorder. Evaluating the states of reactants and products in a reaction allows for predictions about changes in entropy.

Phase Changes and Reaction Direction

The direction of a chemical reaction and the associated changes in enthalpy and entropy can be influenced by the phases of the reactants and products. Reactions that produce gases from solids or liquids typically have positive ΔS, while reactions that form solids from gases may have negative ΔS. Recognizing these phase changes is crucial for predicting the signs of ΔH and ΔS in the given reactions.