AP Chemistry Quiz: Introduction To Le Chateliers Principle

What this quiz covers

This quiz focuses on Introduction To Le Chateliers Principle, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The equilibrium shifts toward reactants.
2. The equilibrium does not shift.
3. The equilibrium shifts in both directions equally.
4. The equilibrium shifts toward products. (correct answer)
5. The equilibrium shift cannot be predicted without $K_p$.

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.

Question 2

1. Shift toward reactants (correct answer)
2. Shift toward products and reactants equally
3. No shift occurs
4. No shift occurs because removing a reactant cannot affect equilibrium
5. Shift toward products

Explanation: This question tests Le Châtelier's principle. When I₂(g) is removed from the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), the concentration of a reactant decreases, disrupting the equilibrium. According to Le Châtelier's principle, the system will shift to counteract this stress by producing more I₂, which means the equilibrium shifts toward the reactants (left). This shift converts some HI back into H₂ and I₂, replacing some of the removed reactant. A common misconception is that removing a reactant cannot affect equilibrium (choice E), but removing any component definitely causes a shift. To solve Le Châtelier problems, identify the stress (removing a reactant), then predict the shift that counteracts it (producing more of that reactant by shifting left).

Question 3

1. No shift occurs because adding ions only affects conductivity
2. Shift toward reactants
3. Shift toward products (correct answer)
4. No shift occurs
5. Shift toward products and reactants equally

Explanation: This question tests Le Châtelier's principle. When SCN⁻(aq) is added to the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), the concentration of a reactant increases, disrupting the equilibrium. According to Le Châtelier's principle, the system will shift to counteract this stress by consuming the added SCN⁻, which means the equilibrium shifts toward the products (right) to form more FeSCN²⁺ complex. This shift reduces the concentration of the added SCN⁻ and partially restores equilibrium. A common misconception is that adding ions only affects conductivity (choice E), but adding reactant ions definitely shifts the equilibrium position. To solve Le Châtelier problems, identify the stress (adding a reactant), then predict the shift that reduces that stress (consuming the reactant by shifting right).

Question 4

1. The equilibrium shifts in both directions equally.
2. The equilibrium shifts toward products.
3. The equilibrium does not shift.
4. The equilibrium shifts toward reactants. (correct answer)
5. The equilibrium shift cannot be predicted without an equilibrium expression.

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 SCN⁻(aq) decreases the concentration of a reactant, disrupting the equilibrium by making Q larger than K. To counteract this, the system shifts toward the reactants, producing more SCN⁻ and Fe³⁺ by decomposing some FeSCN²⁺. This shift re-establishes equilibrium by replenishing the removed SCN⁻. A tempting distractor is choice B, which suggests the equilibrium shifts toward products, based on the misconception that removing a reactant drives complex formation without considering dissociation. Always identify the stress first, then predict the shift that reduces its effect.

Question 5

1. No shift occurs
2. Shifts toward reactants (correct answer)
3. Shifts toward products
4. Shifts toward reactants
5. No shift occurs

Explanation: This question tests Le Châtelier's principle. When volume decreases for PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), the pressure increases, stressing the system. The equilibrium shifts toward the side with fewer moles of gas: 1 mole on the left (PCl₅) versus 2 moles on the right (PCl₃ + Cl₂). Therefore, the equilibrium shifts toward reactants (left) to reduce the number of gas particles and decrease pressure. A common error is assuming all decomposition reactions favor products under pressure, but the system always shifts toward fewer gas moles. For volume decreases (pressure increases), count total gas moles and predict a shift toward the side with fewer moles.

Question 6

1. The equilibrium shifts in both directions equally.
2. The equilibrium does not shift.
3. The equilibrium shift cannot be predicted without $K_c$.
4. The equilibrium shifts toward products. (correct answer)
5. The equilibrium shifts toward reactants.

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 additional O₂(g) increases the concentration of a reactant, disrupting the equilibrium by making Q smaller than K. To counteract this, the system shifts toward the products, consuming some of the added O₂ by producing more NO₂. This shift re-establishes equilibrium by reducing the excess O₂ concentration. A tempting distractor is choice C, which suggests the equilibrium shifts toward reactants, based on the misconception that adding a reactant reverses the reaction without forward compensation. Always identify the stress first, then predict the shift that reduces its effect.

Question 7

1. The equilibrium shifts toward products. (correct answer)
2. The equilibrium shifts toward reactants.
3. The equilibrium shifts toward neither side.
4. The equilibrium shifts toward reactants and products equally.
5. The equilibrium shifts in the direction that increases the total moles of gas.

Explanation: This question tests Le Châtelier's principle. The equilibrium 2NO(g) + O₂(g) ⇌ 2NO₂(g) has 3 moles of gas on the left and 2 moles on the right. When the volume decreases (pressure increases), the system shifts to counteract by reducing the total moles of gas to decrease pressure, which means shifting toward the products (right) where there are fewer moles. A common misconception is thinking that pressure changes affect all equilibria equally, but the shift depends on the difference in gas moles between sides. To solve pressure problems, count moles of gas on each side and remember that increased pressure favors the side with fewer moles.

Question 8

1. The equilibrium shifts toward products.
2. The equilibrium shifts toward reactants.
3. The equilibrium shifts toward neither side. (correct answer)
4. The equilibrium shifts toward reactants and products equally.
5. The equilibrium shifts in the direction that decreases the amount of solid $\text{AgCl}(s)$.

Explanation: This question tests Le Châtelier's principle. The equilibrium AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) involves a pure solid reactant, and adding more solid AgCl does not change its concentration because pure solids have constant concentration. Since the concentration of reactants doesn't actually change, there is no stress on the equilibrium, and the system shifts toward neither side. A common misconception is thinking that adding any substance must cause a shift, but pure solids and liquids don't affect equilibrium position. To identify whether a stress affects equilibrium, check if the added/removed substance can change concentration—pure solids and liquids cannot.

Question 9

1. The equilibrium shifts toward reactants.
2. The equilibrium shifts toward products.
3. The equilibrium shifts toward neither side. (correct answer)
4. The equilibrium shifts toward reactants and products equally.
5. The equilibrium shifts in the direction that increases the total moles of gas.

Explanation: This question tests Le Châtelier's principle. The equilibrium CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g) has 2 moles of gas on each side, so when the volume decreases (pressure increases), there is no favored direction based on mole numbers. Since both sides have equal moles of gas, the system cannot reduce pressure by shifting, so the equilibrium position remains unchanged and shifts toward neither side. A common misconception is thinking that any pressure change must cause a shift, but this only occurs when the mole numbers differ between reactants and products. To solve pressure/volume problems, count the moles of gas on each side first—if equal, no shift occurs.

Question 10

1. The equilibrium shifts toward reactants and products equally.
2. The equilibrium shifts toward neither side.
3. The equilibrium shifts toward reactants. (correct answer)
4. The equilibrium shifts toward products.
5. The equilibrium shifts in the direction that increases the amount of $\text{NO}_2(g)$.

Explanation: This question tests Le Châtelier's principle. The equilibrium N₂O₄(g) ⇌ 2NO₂(g) is disrupted when additional NO₂(g) is injected, increasing the concentration of a product. According to Le Châtelier's principle, the system will shift to counteract this stress by consuming the excess NO₂, which means the equilibrium shifts toward the reactants (left) to form more N₂O₄. A common misconception is thinking that adding more product always pushes the reaction forward, but the system actually shifts to reduce the added substance. To solve Le Châtelier problems, first identify what was changed (here, NO₂ was added), then predict the shift that opposes that change (consuming NO₂ by shifting left).

Question 11

1. No shift
2. Shift toward reactants
3. Shift toward products (correct answer)
4. Shift toward reactants
5. No shift

Explanation: This question tests Le Châtelier's principle. The equilibrium Ag⁺(aq) + Cl⁻(aq) ⇌ AgCl(s) is disturbed by adding a soluble chloride salt, which increases the concentration of Cl⁻ (a reactant). When a reactant concentration is increased, the system shifts to consume some of that excess reactant by favoring the forward reaction. This shift toward products will combine the added Cl⁻ with Ag⁺ to form more AgCl(s) precipitate, demonstrating the common ion effect in precipitation equilibria. A common misconception is thinking that solids affect equilibrium position, but only dissolved species (Ag⁺ and Cl⁻) appear in the equilibrium expression. To solve precipitation problems, identify which ion concentration changed (Cl⁻ increased = reactant increased) and predict the shift that uses it up (forward to form more solid).

Question 12

1. No shift occurs
2. Shifts toward products
3. Shifts toward products
4. No shift occurs
5. Shifts toward reactants (correct answer)

Explanation: This question tests Le Châtelier's principle. When CH₃COO⁻(aq) is added to the equilibrium CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq), the concentration of a product increases, creating stress. To counteract this stress, the system shifts toward reactants (left) to consume some of the added acetate ion. This shift combines CH₃COO⁻ with H⁺ to reform CH₃COOH, reducing both product concentrations. Students might think this is just about pH, but it's a straightforward application of Le Châtelier's principle to any product addition. When a product is added to any equilibrium, the system shifts toward reactants to consume the excess.

Question 13

1. No shift occurs
2. Shifts toward products (correct answer)
3. Shifts toward reactants
4. Shifts toward products
5. Shifts toward reactants

Explanation: This question tests Le Châtelier's principle. When HI(g) is removed from the equilibrium H₂(g) + I₂(g) ⇌ 2HI(g), the concentration of a product decreases, disrupting the equilibrium. To counteract this stress, the system shifts toward products (right) to replace some of the removed HI. This shift consumes H₂ and I₂ while producing more HI to partially restore its concentration. A common error is thinking that removing a product shifts the equilibrium toward reactants, but the system actually shifts to replace what was removed. When a substance is removed, the equilibrium always shifts toward the side containing that substance.

Question 14

1. The equilibrium does not shift.
2. The equilibrium shifts toward products. (correct answer)
3. The equilibrium shifts toward reactants.
4. The equilibrium shifts in both directions equally.
5. 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. 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.

Question 15

1. The equilibrium shifts in both directions equally.
2. The equilibrium shifts toward reactants.
3. The equilibrium does not shift.
4. The equilibrium shift cannot be predicted without $Q$.
5. The equilibrium shifts toward products. (correct answer)

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.

Question 16

1. The equilibrium shifts in both directions equally.
2. The equilibrium shift cannot be predicted without $K_{sp}$.
3. The equilibrium does not shift. (correct answer)
4. The equilibrium shifts toward reactants.
5. 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. 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.

Question 17

1. No shift
2. Shift toward reactants (correct answer)
3. Shift toward products
4. Shift toward reactants
5. 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).

Question 18

1. No shift occurs
2. Shift toward products (correct answer)
3. Shift toward reactants
4. Shift toward products and reactants equally
5. No shift occurs because equilibrium means concentrations cannot change

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).

Question 19

1. Shift toward products
2. Shift toward reactants (correct answer)
3. No shift occurs
4. Shift toward products and reactants equally
5. No shift occurs because pressure changes do not affect gas equilibria

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.

Question 20

1. Shift toward reactants
2. Shift toward products (correct answer)
3. No shift
4. Shift toward reactants
5. 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.