Which way will the equilibrium shift, for the exothermic reaction below, if the temperature is decreased (and everything else is held constant)?

N2(g) + 3H2(g) 2NH3(g)

Question

Which way will the equilibrium shift, for the exothermic reaction below, if the temperature is decreased (and everything else is held constant)?

N2(g) + 3H2(g)  2NH3(g)

BlackTom AI · Accepted Answer

Exothermic reactions release heat, so treating heat as a product gives: N2(g) + 3H2(g) ⇌ 2NH3(g) + heat. Lowering temperature removes heat, and the equilibrium will shift to oppose that change.

1. Relevant knowledge points and concepts
- Le Châtelier’s principle: a system at equilibrium shifts to counteract an imposed change (temperature, pressure, concentration).
- For exothermic reactions, heat acts like a product; decreasing temperature favors the heat-producing (forward) direction.
- Equilibrium constant dependence on temperature (van ’t Hoff): d(ln K)/dT = ΔH°/(R T^2). For ΔH° < 0 (exothermic), ln K decreases with increasing T, so K increases when T decreases.

2. Step-by-step reasoning
- Identify reaction enthalpy sign: given as exothermic → ΔH° < 0.
- Decrease temperature means the system has less thermal energy (heat removed).
- By Le Châtelier, the equilibrium shifts to generate heat, i.e., toward the side that produces heat (the forward/exothermic side).

3. Intermediate/calculation steps (using van ’t Hoff qualitatively)
- d(ln K)/dT = ΔH°/(R T^2). With ΔH° < 0, the derivative is negative: as T increases, ln K decreases (K decreases). Conversely, as T decreases, K increases.
- An increased K means the ratio [products]/[reactants] at equilibrium rises, so product concentrations (NH3) increase.

4. Why the correct answer is correct
- Both Le Châtelier’s principle and the van ’t Hoff relationship predict a shift toward products (right) when temperature is lowered for an exothermic reaction, producing additional heat to counter the change.

5. Why the other options are incorrect
- Shifting left (more reactants) would consume heat; that would further reduce heat, not oppose the imposed decrease, so it contradicts Le Châtelier.
- “It will not shift” is incorrect because temperature changes always alter equilibrium positions unless ΔH° = 0 (not the case here).

6. Conclusion
The equilibrium shifts to the right (more products). This confirms the provided answer: To the right (more products).

Which way will the equilibrium shift, for the exothermic reaction below, if the temperature is decreased (and everything else is held constant)? N2(g) + 3H2(g) 2NH3(g)单项选择题

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