Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontanous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T.
(a) N₂(g) + 3 F₂(g) → 2 NF₃₍g) ΔH° = -249 kJ; ΔS° = -278 J/K
(b) N₂(g) + 3 Cl₂(g) → 2 NCl₃(g) ΔH° = 460 kJ; ΔS° = -275 J/K

검증된 단계별 안내

Identify the Gibbs free energy equation: ΔG° = ΔH° - TΔS°. This equation helps determine the spontaneity of a reaction.

For reaction (a), N2(g) + 3 F2(g) → 2 NF3(g), note that ΔH° = -249 kJ and ΔS° = -278 J/K. Convert ΔS° to kJ by dividing by 1000, resulting in ΔS° = -0.278 kJ/K.

Substitute the values into the Gibbs free energy equation for reaction (a): ΔG° = -249 kJ - T(-0.278 kJ/K). Analyze the sign of ΔG° at different temperatures to determine spontaneity.

For reaction (b), N2(g) + 3 Cl2(g) → 2 NCl3(g), note that ΔH° = 460 kJ and ΔS° = -275 J/K. Convert ΔS° to kJ by dividing by 1000, resulting in ΔS° = -0.275 kJ/K.

Substitute the values into the Gibbs free energy equation for reaction (b): ΔG° = 460 kJ - T(-0.275 kJ/K). Analyze the sign of ΔG° at different temperatures to determine spontaneity.

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주요 개념

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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 ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. A negative ΔG indicates a spontaneous reaction, while a positive ΔG suggests non-spontaneity.

Enthalpy and Entropy

Enthalpy (ΔH) is a measure of the total heat content of a system, while entropy (ΔS) quantifies the degree of disorder or randomness in a system. In chemical reactions, ΔH can be either positive (endothermic) or negative (exothermic), and ΔS can also be positive or negative depending on the change in disorder. The interplay between these two factors determines the spontaneity of a reaction across different temperature ranges.

Temperature Dependence of Spontaneity

The spontaneity of a reaction can depend on temperature due to the relationship between enthalpy and entropy. Reactions that are exothermic (negative ΔH) and have positive entropy (positive ΔS) are spontaneous at all temperatures. Conversely, reactions that are endothermic with negative entropy are non-spontaneous at all temperatures. The temperature can shift the balance for reactions that are spontaneous at low or high temperatures, depending on the signs of ΔH and ΔS.