Consider the following reaction at equilibrium. What effect will reducing the pressure have on the system?

Question

Consider the following reaction at equilibrium. What effect will reducing the pressure have on the system?

BlackTom AI · Accepted Answer

Question restatement: The system is at equilibrium and the pressure is being reduced. You must assess how the position of equilibrium will respond to a decrease in pressure.

Option A: The reaction will shift to the right in the direction of products.
- Under Le Châtelier’s principle, reducing the pressure prompts the system to favor the side with more moles of gas in order to increase pressure. Unless the product side has more gaseous moles than the reactant side, this option is not supported. Since this choice asserts a shift to the right, it would only be correct if the product side had more gas moles. In general, this cannot be assumed, so this statement is not universally correct.

Option B: The reaction will shift to the left in the direction of reactants.
- If the left side (reactants) contains more moles of gas than the right side (products), then lowering the pressure will indeed favor the left, because the system tries to increase pressure by producing more gas. This aligns with Le Châtelier’s principle when the reactant side has the greater gas mole count. Therefore, this option can be correct given the typical scenario where the left has more gas moles.

Option C: The equilibrium constant will increase.
- The equilibrium constant K is a function of temperature, not pressure. If temperature remains constant, changing pressure does not change K. Therefore, this statement is incorrect.

Option D: The equilibrium constant will decrease.
- For the same reason as Option C, K does not change with pressure at constant temperature. This option is also incorrect.

Option E: No effect will be observed.
- A change in pressure typically shifts the position of equilibrium unless the system involves only liquids/solids with negligible gas-phase contributions or the reaction has the same number of moles of gas on both sides. In a general gaseous equilibrium, reducing pressure does cause a shift. Therefore, saying no effect is observed is not generally correct.

Summary of reasoning:
- The correct behavior under reduced pressure depends on the relative number of gas moles on each side; if the reactant side has more gas moles, the shift is to the left (toward reactants); if the product side has more gas moles, the shift is to the right. The provided correct choice corresponds to the scenario where the left side has more gas moles, so a shift to the left is expected in that case.
- The equilibrium constant K remains unchanged with pressure changes at constant temperature, so any option claiming a change in K is incorrect.
- A no-effect option is only plausible if there is no change in the gas-mole balance or if all species are not in the gas phase; in typical gaseous equilibria, there will be a visible shift when pressure is altered.

Consider the following reaction at equilibrium. What effect will reducing the pressure have on the system? 单项选择题

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