Questions 22-25 refer to the following information. N2O4(g) ⇄ 2 NO2(g) Kp = 3.0 at 70°C colorless br — Equilibrium Chemistry Question

STEPS:

1. Analyze the chemical equation and colors of the species:
* The equilibrium is represented by the equation:
$$\text{N}_2\text{O}_4(g) \rightleftharpoons 2\ \text{NO}_2(g)$$
* According to the given information, reactant $\text{N}_2\text{O}_4$ is colorless, and product $\text{NO}_2$ is brown.
2. Interpret the physical observation:
* When the equilibrium mixture is placed in an ice bath, the temperature of the system is lowered.
* The contents of the tube turn a lighter color, which indicates that the concentration of the brown gas ($\text{NO}_2$) has decreased, while the concentration of the colorless gas ($\text{N}_2\text{O}_4$) has increased.
* This means the decrease in temperature has caused the equilibrium to shift to the left (toward the reactants).
3. Apply Le Chatelier's Principle for temperature changes:
* Le Chatelier's Principle states that a system at equilibrium will shift to counteract any imposed change.
* When a system is cooled (heat is removed), the equilibrium will shift in the direction that produces heat (the exothermic direction) to raise the temperature back up.
* Since cooling the tube causes a shift to the left, the reverse reaction must be exothermic:
$$\text{N}_2\text{O}_4(g) \rightleftharpoons 2\ \text{NO}_2(g) + \text{heat}$$
4. Identify the thermodynamic nature of the forward reaction:
* If the reverse reaction is exothermic (releases heat), the forward reaction must be endothermic (absorbs heat, $\Delta H > 0$):
$$\text{heat} + \text{N}_2\text{O}_4(g) \rightleftharpoons 2\ \text{NO}_2(g)$$
* For this endothermic system, cooling (removing heat) pulls the equilibrium to the left, decreasing the brown $\text{NO}_2$ and making the tube look lighter.
5. Conclude:
* The observation that the mixture turns lighter in an ice bath is explained by the fact that the forward reaction is endothermic, confirming Option B is correct.

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WHY_OTHERS_WRONG:

• Option A is incorrect: If the forward reaction were exothermic, heat would be a product. Placing the mixture in an ice bath (removing heat) would shift the equilibrium to the right to produce more heat, which would increase the concentration of the brown $\text{NO}_2$ gas and make the mixture turn a darker brown rather than lighter.
• Options C and D are incorrect: The activation energy ($E_a$) is the minimum energy barrier that reactants must overcome to transform into products. It is an intrinsic barrier determined solely by the reaction pathway and the transition state. Changing the temperature of a system (such as placing it in an ice bath) shifts the Maxwell^-Boltzmann distribution of molecular kinetic energies, but it does not alter the activation energy barrier itself.