Examples of Temperature-Dependent Spontaneous Reactions

📚 Quick Study Guide

• 🌡️ Temperature plays a crucial role in determining the spontaneity of a reaction.
• ⚛️ Gibbs Free Energy ($G$) dictates spontaneity: $\Delta G = \Delta H - T\Delta S$, where $H$ is enthalpy, $T$ is temperature, and $S$ is entropy.
• 🔥 A reaction is spontaneous (favored) when $\Delta G < 0$.
• 🧊 At low temperatures, enthalpy ($\Delta H$) is more dominant.
• ☀️ At high temperatures, entropy ($\Delta S$) is more dominant.
• 🔑 Reactions can be:
  • Exothermic ($\Delta H < 0$) and have increasing entropy ($\Delta S > 0$): Spontaneous at all temperatures.
  • Endothermic ($\Delta H > 0$) and have decreasing entropy ($\Delta S < 0$): Non-spontaneous at all temperatures.
  • Exothermic ($\Delta H < 0$) and have decreasing entropy ($\Delta S < 0$): Spontaneous at low temperatures.
  • Endothermic ($\Delta H > 0$) and have increasing entropy ($\Delta S > 0$): Spontaneous at high temperatures.

🧪 Practice Quiz

1. Which of the following conditions favors the spontaneity of a reaction at high temperatures?

   1. $\Delta H < 0$, $\Delta S < 0$
   2. $\Delta H > 0$, $\Delta S < 0$
   3. $\Delta H < 0$, $\Delta S > 0$
   4. $\Delta H > 0$, $\Delta S > 0$

2. For a reaction where $\Delta H = +100 \text{ kJ}$ and $\Delta S = +200 \text{ J/K}$, at what temperature will the reaction become spontaneous?

   1. 200 K
   2. 500 K
   3. 600 K
   4. 700 K

3. The melting of ice at temperatures above 0°C is an example of a temperature-dependent spontaneous reaction. Which statement is true?

   1. Melting is spontaneous only below 0°C.
   2. Melting is spontaneous at all temperatures.
   3. Melting is spontaneous only above 0°C.
   4. Melting is never spontaneous.

4. Consider a reaction with $\Delta H = -50 \text{ kJ}$ and $\Delta S = -100 \text{ J/K}$. Under what temperature conditions is this reaction spontaneous?

   1. Only at high temperatures
   2. Only at low temperatures
   3. At all temperatures
   4. Never spontaneous

5. Which of the following reactions is most likely to be spontaneous at all temperatures?

   1. $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$, $\Delta H < 0$
   2. $\text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{O}(g)$, $\Delta H > 0$
   3. $\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$, $\Delta H < 0$
   4. $\text{2NH}_3(g) \rightarrow \text{N}_2(g) + 3\text{H}_2(g)$, $\Delta H > 0$

6. What is the Gibbs Free Energy change for a reaction at equilibrium?

   1. $\Delta G > 0$
   2. $\Delta G < 0$
   3. $\Delta G = 0$
   4. $\Delta G = \infty$

7. For the reaction $A(g) \rightarrow 2B(g)$, the entropy increases. If the reaction is endothermic, under what conditions will it be spontaneous?

   1. Low temperatures
   2. High temperatures
   3. All temperatures
   4. Never spontaneous