AP Chemistry · Topic 7.10
Reaction Quotient and Le Chatelier's Principle Practice
Part of Equilibrium.(TRA-8.B)
Practice questions
17
Sample questions
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Sample 1difficulty 2/5
For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) at constant T, the volume is suddenly halved.
Which way does the equilibrium shift?
- A
Toward more moles of gas (right).
- Bcheck_circle
Toward fewer moles of gas (left, toward PCl₅).
- C
No shift, K does not change.
- D
K decreases.
Why
Compression favors the side with fewer moles of gas. Reactants: 1 mol; products: 2 mol; equilibrium shifts left.
- A
Sample 2difficulty 2/5
Adding more reactant to an equilibrium causes
- A
Shift toward reactants
- B
No change
- Ccheck_circle
Shift toward products
- D
K increases
Why
Disturbance increases reactants; system shifts to consume them (forward).
- A
Sample 3difficulty 2/5
A student adds the following stresses one at a time to separate samples of an equilibrium mixture of [Co(H2O)6]^2+ (pink) <-> [CoCl4]^2- (blue) + 6 H2O (delta H > 0 forward) and records color change: Trial A: heat the solution -> turns blue Trial B: add concentrated HCl -> turns blue Trial C: add AgNO3 -> turns pink Trial D: add water -> turns pink
Why does adding water cause the solution to turn pink (Trial D)?
- Acheck_circle
Diluting Cl- (the reactant) shifts equilibrium back toward [Co(H2O)6]^2+
- B
Water removes heat from the reaction
- C
Water is a product, so adding water shifts forward
- D
AgNO3 forms with water
Why
Adding solvent reduces ion concentrations; the equilibrium shifts toward the side with more dissolved ions and less water on the right. Net effect: shift toward pink [Co(H2O)6]^2+.
- A
Sample 4difficulty 2/5
Removing a product from equilibrium causes
- Acheck_circle
Shift toward products
- B
K decreases
- C
No shift
- D
Shift toward reactants
Why
System replenishes the missing product (forward shift).
- A
Sample 5difficulty 3/5
For an endothermic reaction, increasing T causes
- Acheck_circle
Shift right; K increases
- B
No change
- C
Eₐ doubles
- D
Shift left; K decreases
Why
Treat heat as a reactant; adding heat shifts forward and raises K.
- A