CHEM 104 Module 3: Chemical Equilibrium, Le Chatelier’s Principle, and Equilibrium Constants 2026/2027 UPDATE

CHEM 104 Module 3: Chemical Equilibrium, Le Chatelier’s Principle,
and Equilibrium Constants 2026/2027 UPDATE


1. What is the primary characteristic of a system at chemical equilibrium?

A. The rate of the forward reaction equals the rate of the reverse reaction.

B. The concentrations of reactants and products are equal.

C. The reaction has completely stopped.

D. The mass of reactants is converted entirely to products.

Answer: A
Rationale: At equilibrium, the forward and reverse reactions occur at the same rate,
meaning there is no net change in concentrations over time.

2. According to the Law of Mass Action, what is the equilibrium constant
expression for the reaction: aA + bB ⇌ cC + dD?

A. K = [A][B] / [C][D]

B. K = [C]^c [D]^d / [A]^a [B]^b

C. K = (a[A] + b[B]) / (c[C] + d[D])

D. K = [A]^a [B]^b / [C]^c [D]^d

Answer: B
Rationale: The equilibrium constant is defined as the product of the concentrations of the
products raised to their coefficients divided by the product of the reactants raised to their
coefficients.

,3. Which of the following substances are typically omitted from the equilibrium
constant expression?

A. Gases and aqueous solutions

B. Products only

C. Reactants only

D. Pure solids and pure liquids

Answer: D
Rationale: The activities of pure solids and pure liquids are constant (defined as 1) and do
not change with the progress of the reaction, so they are omitted.

4. If the equilibrium constant K is much greater than 1 (K >> 1), what does this
indicate about the reaction?

A. The reaction favors the reactants at equilibrium.

B. The reaction favors the products at equilibrium.

C. The reaction is very slow.

D. The reaction is at a very low temperature.

Answer: B
Rationale: A large K value indicates that at equilibrium, the concentration of products is
significantly higher than that of the reactants.

5. What happens to the direction of a reaction if the reaction quotient Q is less
than the equilibrium constant K (Q < K)?

A. The reaction is already at equilibrium.

B. The reaction shifts to the left (toward reactants).

C. The reaction shifts to the right (toward products).

D. The reaction stops.

Answer: C
Rationale: If Q < K, there are not enough products yet to reach equilibrium, so the reaction
proceeds in the forward direction.

, 6. Which of the following describes Le Chatelier’s Principle?

A. Energy cannot be created or destroyed.

B. If a stress is applied to a system at equilibrium, the system shifts to counteract the stress.

C. A system at equilibrium will remain at equilibrium regardless of external changes.

D. The total pressure of a gas mixture is the sum of partial pressures.

Answer: B
Rationale: Le Chatelier’s Principle states that a system will shift its equilibrium position to
offset changes in concentration, pressure, or temperature.

7. In the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), what is the effect of increasing the
pressure by decreasing the volume?

A. The equilibrium shifts toward the reactants.

B. There is no shift in equilibrium.

C. The equilibrium shifts toward the products.

D. The equilibrium constant K increases.

Answer: C
Rationale: Increasing pressure shifts the equilibrium toward the side with fewer moles of
gas. The product side has 2 moles, while the reactant side has 4 moles.

8. For an exothermic reaction, how does an increase in temperature affect the
equilibrium constant K?

A. K decreases.

B. K increases.

C. K remains the same.

D. K becomes zero.

Answer: A
Rationale: In an exothermic reaction, heat is a product. Increasing temperature shifts the
equilibrium to the left, decreasing the concentration of products and thus decreasing K.