Thermodynamics helps us predict if a reaction will occur spontaneously. Enthalpy ($H$) represents the heat content of a system, and a negative change in enthalpy ($\Delta H < 0$) generally favors spontaneity (exothermic reactions release heat). Entropy ($S$) measures the disorder or randomness of a system; a positive change in entropy ($\Delta S > 0$) also favors spontaneity (increased disorder). Gibbs Free Energy ($G$) combines enthalpy and entropy to predict spontaneity at a given temperature.
The Gibbs Free Energy equation is: $\Delta G = \Delta H - T\Delta S$. If $\Delta G < 0$, the reaction is spontaneous (or thermodynamically favorable). If $\Delta G > 0$, the reaction is non-spontaneous. If $\Delta G = 0$, the reaction is at equilibrium. Temperature ($T$) is in Kelvin.
Match the terms with their definitions:
| Term | Definition |
|---|---|
| 1. Enthalpy | A. Measure of disorder |
| 2. Entropy | B. Energy available to do work |
| 3. Gibbs Free Energy | C. Heat content of a system |
| 4. Spontaneous Process | D. A process that occurs without external energy input |
| 5. Equilibrium | E. State where forward and reverse reaction rates are equal |
Answers: 1-C, 2-A, 3-B, 4-D, 5-E
A reaction is considered __________ if it releases heat, meaning its $\Delta H$ is __________. A reaction is favored by an __________ in entropy, meaning its $\Delta S$ is __________. The Gibbs Free Energy equation, $\Delta G = \Delta H - T\Delta S$, allows us to determine if a reaction is __________ at a specific temperature. A negative $\Delta G$ indicates a __________ reaction.
Answers: exothermic, negative, increase, positive, spontaneous, spontaneous
Consider a reaction where both $\Delta H$ and $\Delta S$ are positive. Explain how temperature affects the spontaneity of this reaction and why.
Answer: When both $\Delta H$ and $\Delta S$ are positive, the spontaneity of the reaction depends on the temperature. At low temperatures, the $T\Delta S$ term is small, so $\Delta G$ is dominated by the positive $\Delta H$, making the reaction non-spontaneous. However, as the temperature increases, the $T\Delta S$ term becomes larger and can eventually exceed $\Delta H$. At sufficiently high temperatures, $\Delta G$ becomes negative, and the reaction becomes spontaneous. Therefore, this reaction is spontaneous only at high temperatures because the increase in entropy (disorder) outweighs the endothermic nature of the reaction at higher temperatures.
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