Consider the following endothermic reaction of gaseous AB3 molecules with A2 molecules (LO 9.16, 9.17). Identify the true statement about the spontaneity of the reaction. (a) The reaction is likely to be spontaneous at high temperatures. (b) The reaction is likely to be spontaneous at low temperatures. (c) The reaction is always spontaneous. (d) The reaction is never spontaneous.

Verified step by step guidance

1. Step 1: Understand that the reaction is endothermic, meaning it absorbs heat from the surroundings. This implies that the enthalpy change (ΔH) is positive.
2. Step 2: Recall that the spontaneity of a reaction is determined by the Gibbs free energy change (ΔG), which is given by the equation ΔG = ΔH - TΔS, where T is the temperature and ΔS is the change in entropy.
3. Step 3: Consider the effect of temperature on ΔG. For an endothermic reaction (positive ΔH), increasing the temperature can make the TΔS term larger, potentially making ΔG negative if ΔS is positive.
4. Step 4: Analyze the entropy change (ΔS). If the reaction results in an increase in disorder (positive ΔS), then higher temperatures will favor spontaneity because the TΔS term will become more significant.
5. Step 5: Conclude that for an endothermic reaction with a positive ΔS, the reaction is more likely to be spontaneous at high temperatures, making option (a) the true statement.

Key Concepts

Endothermic Reactions

Endothermic reactions absorb heat from their surroundings, resulting in a decrease in temperature of the environment. The enthalpy change (ΔH) for these reactions is positive, indicating that energy is required for the reaction to proceed. Understanding this concept is crucial for evaluating the conditions under which such reactions may occur spontaneously.

Gibbs Free Energy

Gibbs Free Energy (G) is a thermodynamic potential that helps predict the spontaneity of a reaction at constant temperature and pressure. The change in Gibbs Free Energy (ΔG) is calculated using the equation ΔG = ΔH - TΔS, where ΔS is the change in entropy. A reaction is spontaneous when ΔG is negative, which can occur under certain temperature conditions for endothermic reactions.

Entropy and Temperature

Entropy (S) is a measure of the disorder or randomness in a system. For endothermic reactions, an increase in temperature can enhance the entropy term (TΔS) in the Gibbs Free Energy equation, potentially making ΔG negative. Thus, understanding the relationship between temperature, entropy, and spontaneity is essential for determining when an endothermic reaction may occur spontaneously.