Introduction to Le Chatelier's Principle
Help Questions
AP Chemistry › Introduction to Le Chatelier's Principle
A student has a reaction mixture at equilibrium in a closed flask: $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$. The student then removes some $\text{HI}(g)$ from the flask. As the system responds to this stress to re-establish equilibrium, in which direction does the equilibrium shift?
The equilibrium shifts toward reactants.
The equilibrium shift cannot be predicted without $Q$.
The equilibrium shifts toward products.
The equilibrium does not shift.
The equilibrium shifts in both directions equally.
Explanation
Le Châtelier's principle states that a system at equilibrium will respond to a stress by shifting to minimize the effect of that stress. Removing some HI(g) decreases the concentration of a product, disrupting the equilibrium by making Q smaller than K. To counteract this, the system shifts toward the products, producing more HI by converting H₂ and I₂. This shift re-establishes equilibrium by replenishing the removed HI. A tempting distractor is choice C, which suggests the equilibrium shifts toward reactants, based on the misconception that removing a product reduces the forward reaction rate without increasing it to compensate. Always identify the stress first, then predict the shift that reduces its effect.
A closed container holds the gaseous equilibrium system $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$. After equilibrium is established, the volume of the container is decreased by moving a piston inward. As the system responds to this stress to re-establish equilibrium, in which direction does the equilibrium shift?
The equilibrium does not shift.
The equilibrium shift cannot be predicted without $K_p$.
The equilibrium shifts in both directions equally.
The equilibrium shifts toward reactants.
The equilibrium shifts toward products.
Explanation
Le Châtelier's principle states that a system at equilibrium will respond to a stress by shifting to minimize the effect of that stress. Decreasing the volume increases the pressure, disrupting the equilibrium since there are more gas moles on the reactant side (3) than the product side (2). To counteract this, the system shifts toward the products, reducing the number of gas molecules and thus lowering the pressure. This shift re-establishes equilibrium by favoring the side with fewer moles. A tempting distractor is choice A, which suggests the equilibrium shifts toward reactants, based on the misconception that pressure changes always favor the side with more moles without considering the counteraction. Always identify the stress first, then predict the shift that reduces its effect.
A piston-cylinder apparatus contains an equilibrium mixture for the reaction $\text{PCl}_5(g) \rightleftharpoons \text{PCl}_3(g) + \text{Cl}_2(g)$. Once equilibrium is established, the volume is increased by pulling the piston outward. As the system responds to this stress to re-establish equilibrium, in which direction does the equilibrium shift?
The equilibrium shifts toward reactants.
The equilibrium does not shift.
The equilibrium shifts in both directions equally.
The equilibrium shifts toward products.
The equilibrium shift cannot be predicted without $Q$.
Explanation
Le Châtelier's principle states that a system at equilibrium will respond to a stress by shifting to minimize the effect of that stress. Increasing the volume decreases the pressure, disrupting the equilibrium since there are fewer gas moles on the reactant side (1) than the product side (2). To counteract this, the system shifts toward the products, increasing the number of gas molecules and thus raising the pressure. This shift re-establishes equilibrium by favoring the side with more moles. A tempting distractor is choice B, which suggests the equilibrium shifts toward reactants, based on the misconception that volume increases always favor fewer moles without considering the pressure relief. Always identify the stress first, then predict the shift that reduces its effect.
A reaction mixture is at equilibrium for the process $\text{AgCl}(s) \rightleftharpoons \text{Ag}^+(aq) + \text{Cl}^-(aq)$. The student then adds solid $\text{AgCl}(s)$ to the container without changing the volume of solution. As the system responds to this stress to re-establish equilibrium, in which direction does the equilibrium shift?
The equilibrium shifts toward products.
The equilibrium shift cannot be predicted without $K_{sp}$.
The equilibrium shifts toward reactants.
The equilibrium does not shift.
The equilibrium shifts in both directions equally.
Explanation
Le Châtelier's principle states that a system at equilibrium will respond to a stress by shifting to minimize the effect of that stress. Adding solid AgCl(s) introduces more of a pure solid, but solids do not appear in the equilibrium expression, so their concentration is constant and does not disrupt the equilibrium. Therefore, the system does not need to shift to counteract this addition, as the activities of solids remain unity. This maintains the equilibrium without any change in the position. A tempting distractor is choice A, which suggests the equilibrium shifts toward reactants, based on the misconception that adding solids affects equilibrium like solutes. Always identify the stress first, then predict the shift that reduces its effect.
A container holds an equilibrium mixture: $\mathrm{Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons FeSCN^{2+}(aq)}$. Some $\mathrm{Fe^{3+}(aq)}$ is removed from the mixture. In response to this stress, how will the system shift to re-establish equilibrium?
Shift toward reactants
No shift
Shift toward reactants
Shift toward products
Shift toward products
Explanation
This question tests Le Châtelier's principle. The equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) is disturbed by removing some Fe³⁺(aq), which decreases the concentration of a reactant. When a reactant is removed from an equilibrium system, the equilibrium shifts to replace what was taken away by favoring the reverse reaction. This shift toward reactants will dissociate some FeSCN²⁺ back into Fe³⁺ and SCN⁻ to partially restore the Fe³⁺ concentration. A common misconception is thinking that removing a species always causes the reaction to proceed away from it, but the system actually shifts to replace what was removed. To apply Le Châtelier correctly, identify the stress (removed Fe³⁺ = decreased reactant) and predict the shift that opposes this change (make more reactant by shifting left).
A closed container holds the equilibrium system $\mathrm{2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)}$. After equilibrium is reached, some $\mathrm{SO_3(g)}$ is removed from the container. In response to this stress, which direction will the system shift to re-establish equilibrium?
No shift occurs because equilibrium means concentrations cannot change
Shift toward reactants
Shift toward products and reactants equally
No shift occurs
Shift toward products
Explanation
This question tests Le Châtelier's principle. When SO₃(g) is removed from the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the concentration of a product decreases, disrupting the equilibrium. According to Le Châtelier's principle, the system will shift to counteract this stress by producing more SO₃, which means the equilibrium shifts toward the products (right). This shift replaces some of the removed SO₃ and partially restores equilibrium. A common misconception is that equilibrium means concentrations cannot change (choice E), but equilibrium is dynamic—the system can shift to new equilibrium concentrations when stressed. To solve Le Châtelier problems, identify the stress (removing a product), then predict the shift that counteracts it (producing more of that product by shifting right).
A closed container holds the equilibrium system $\mathrm{2NO_2(g) \rightleftharpoons N_2O_4(g)}$. After equilibrium is reached, the volume of the container is increased (by pulling a piston outward). In response to this stress, which direction will the system shift to re-establish equilibrium?
Shift toward products
Shift toward products and reactants equally
No shift occurs because pressure changes do not affect gas equilibria
No shift occurs
Shift toward reactants
Explanation
This question tests Le Châtelier's principle. When the volume increases for 2NO₂(g) ⇌ N₂O₄(g), the pressure decreases, which is the stress on the system. The left side has 2 moles of gas while the right side has 1 mole of gas. According to Le Châtelier's principle, the system shifts to counteract the pressure decrease by favoring the side with more gas molecules—shifting toward reactants (left). This shift increases the total number of gas particles and partially counteracts the pressure decrease. A common misconception is that pressure changes do not affect gas equilibria (choice E), but they definitely do when the number of gas moles differs on each side. To solve pressure/volume problems, count gas moles on each side, then predict the shift toward more moles when pressure decreases.
A closed system is at equilibrium: $\mathrm{2,SO_2(g) + O_2(g) \rightleftharpoons 2,SO_3(g)}$. The volume of the container is decreased (at constant temperature). In response to this stress, how will the system shift to re-establish equilibrium?
Shift toward products
Shift toward reactants
Shift toward reactants
No shift
No shift
Explanation
This question tests Le Châtelier's principle. The equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) has 3 moles of gas on the left and 2 moles on the right. When volume decreases, pressure increases, and the system responds by shifting toward the side with fewer gas molecules to reduce pressure. Since the products side has fewer moles of gas (2 vs 3), the equilibrium shifts toward products to counteract the pressure increase. A common misconception is that pressure changes affect all equilibria equally, but the shift depends on the difference in moles of gas between reactants and products. To solve pressure/volume problems, count gas moles on each side, then remember that decreased volume (increased pressure) favors the side with fewer gas molecules.
A reaction mixture is initially at equilibrium in a rigid container: $\mathrm{H_2(g) + I_2(g) \rightleftharpoons 2,HI(g)}$. A student removes some $\mathrm{HI(g)}$ from the container. In response to this stress, how will the system shift to re-establish equilibrium?
No shift
Shift toward reactants
Shift toward products
Shift toward reactants
Shift toward products
Explanation
This question tests Le Châtelier's principle. The equilibrium H₂(g) + I₂(g) ⇌ 2HI(g) is disturbed by removing some HI(g), which decreases the concentration of products. When a product is removed from an equilibrium system, the equilibrium shifts to replace what was taken away by favoring the forward reaction. This shift toward products will convert more H₂ and I₂ into HI to partially restore the HI concentration. A common misconception is thinking that removing a substance always causes a shift away from it, but actually the system shifts to replace what was removed. To apply Le Châtelier's principle correctly, identify the stress (removed HI = decreased product) and predict the shift that opposes this change (make more product by shifting right).
A closed flask contains the equilibrium system: $\mathrm{2,NO(g) + O_2(g) \rightleftharpoons 2,NO_2(g)}$. Some $\mathrm{O_2(g)}$ is removed from the flask. In response to this stress, how will the system shift to re-establish equilibrium?
Shift toward reactants
Shift toward products
Shift toward reactants
Shift toward products
No shift
Explanation
This question tests Le Châtelier's principle. The equilibrium 2NO(g) + O₂(g) ⇌ 2NO₂(g) is disturbed by removing some O₂(g), which decreases the concentration of a reactant. When a reactant is removed from an equilibrium system, the equilibrium shifts to replace what was taken away by favoring the reverse reaction. This shift toward reactants will convert some NO₂ back into NO and O₂ to partially restore the O₂ concentration. A common misconception is thinking that removing a reactant always drives the reaction forward, but the system actually shifts to replace what was removed. To apply Le Châtelier correctly, identify the stress (removed O₂ = decreased reactant) and predict the shift that opposes this change (make more reactant by shifting left).