AP Chemistry · Topic 7.9
Introduction to Le Chatelier's Principle Practice
Part of Equilibrium.(TRA-8.A)
Practice questions
13
Sample questions
5 of 13 — sign in to practice the rest with adaptive difficulty and mastery tracking.
Sample 1difficulty 1/5
A sealed glass tube containing the equilibrium 2 NO2(g, brown) <-> N2O4(g, colorless), delta H = -57.2 kJ/mol (forward), is placed in three water baths in succession: ice water, room-temperature water, and hot water. Observations: ice bath - tube becomes nearly colorless; room temp - light brown; hot bath - dark brown.
Which observation is most consistent with the forward reaction being exothermic?
- A
color is darker in cold water
- B
color is independent of temperature
- Ccheck_circle
color darkens upon heating, indicating shift toward NO2 (reactant) when T rises
- D
color lightens upon heating
Why
For an exothermic forward reaction, raising T shifts equilibrium toward reactants. Reactant NO2 is brown, so dark brown at high T is consistent.
- A
Sample 2difficulty 1/5
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
Trial A confirms that the forward reaction is:
- A
non-spontaneous at all temperatures
- B
at equilibrium with K = 1
- C
exothermic, since heating shifts toward reactants
- Dcheck_circle
endothermic, since heating shifts the equilibrium toward products (blue)
Why
For an endothermic forward reaction, increasing T shifts to the right (products). Solution turning blue indicates more [CoCl4]^2-.
- A
Sample 3difficulty 1/5
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
Which observation is best explained by Le Chatelier's principle removing Cl-?
- A
Trial A
- B
Trial B
- Ccheck_circle
Trial C
- D
Trial D
Why
AgNO3 precipitates AgCl, removing Cl-. The equilibrium shifts back toward [Co(H2O)6]^2+ (pink).
- A
Sample 4difficulty 2/5
The reaction 2 NO2(g) <-> N2O4(g) is at equilibrium when the container volume is suddenly decreased at the dashed line.
The plot is consistent with which response to a volume decrease?
- Acheck_circle
Shift to the side with fewer moles of gas (toward N2O4)
- B
Both species rise equally
- C
No shift, since K is constant
- D
Shift to the side with more moles of gas (toward NO2)
Why
Decreasing volume increases pressure; the system shifts toward fewer moles of gas. 2 mol NO2 -> 1 mol N2O4 reduces moles, so equilibrium shifts toward N2O4.
- A
Sample 5difficulty 2/5
What does the catalyst change?
- Acheck_circle
Lowers Ea (forward and reverse) but not K
- B
Increases K
- C
Changes dH
- D
Lowers Ea forward only and increases K
Why
A catalyst lowers activation energy for both forward and reverse paths equally; equilibrium is reached faster but K is unchanged.
- A