Тест: Understanding Chemical Equilibrium — 10 въпроса

1. What does the equilibrium constant (Kc) indicate about a chemical reaction?

A. The energy change during the reaction

B. The temperature at which the reaction occurs

C. The speed at which the reaction occurs

D. The ratio of reactants to products at equilibrium

Answer: (D)

Обяснение

The equilibrium constant (Kc) represents the ratio of concentrations of products to reactants at equilibrium, each raised to their stoichiometric powers. It indicates the position of equilibrium and whether the reaction favors products or reactants.

2. What does a large value of the equilibrium constant, Kc (e.g., greater than 1000), indicate about the position of equilibrium in a reaction?

A. The reaction favors reactants at equilibrium.

B. The concentrations of reactants and products are equal.

C. The reaction favors products at equilibrium.

D. The reaction does not reach equilibrium.

Answer: (C), (D)

Обяснение

A large Kc value indicates that at equilibrium, the concentration of products is much higher than that of reactants, meaning the reaction favors products. When Kc is greater than 10^3, the equilibrium lies far to the right.

3. According to Le Châtelier’s principle, what happens when the temperature of an exothermic reaction is increased?

A. The reaction shifts toward more reactants

B. The reaction shifts toward more products

C. The reaction rate decreases

D. The reaction reaches equilibrium faster

Answer: (A)

Обяснение

Increasing temperature in an exothermic reaction adds heat, which Le Châtelier’s principle states will shift the equilibrium toward the reactants to counteract the disturbance, thus favoring the reverse reaction.

4. According to the revision sheet, who is associated with formulating the principle that describes how systems respond to disturbances in chemical equilibrium?

A. Albert Einstein

B. Le Châtelier

C. Marie Curie

D. Michael Faraday

Answer: (B)

Обяснение

Le Châtelier is the scientist associated with the principle that states any change in concentration, temperature, or pressure will cause the system to adjust to partially counteract the disturbance — known as Le Châtelier’s principle.

5. How does increasing pressure affect the equilibrium of a gaseous reaction?

A. It has no effect on the equilibrium position

B. It shifts the equilibrium toward more moles of gas

C. It only affects the reaction rate, not the equilibrium

D. It shifts the equilibrium toward fewer moles of gas

Answer: (D)

Обяснение

For gaseous reactions, increasing pressure favors the side with fewer moles of gas, shifting the equilibrium toward that side to minimize pressure change, according to Le Châtelier’s principle.

6. What is the effect of increasing temperature on an endothermic reaction at equilibrium, based on the key points from the revision sheet?

A. It shifts equilibrium to the left, favoring reactants.

B. It shifts equilibrium to the right, favoring products.

C. It does not affect the position of equilibrium.

D. It decreases the rate of reaction without shifting equilibrium.

Answer: (B)

Обяснение

For an endothermic reaction, increasing temperature supplies heat as a reactant, causing the equilibrium to shift to the right (favoring products). This is explained by Le Châtelier’s principle.

7. What is the primary role of a catalyst in a chemical reaction, as described in the revision sheet?

A. It shifts the equilibrium position toward products or reactants.

B. It lowers the activation energy, speeding up both forward and reverse reactions equally.

C. It changes the value of the equilibrium constant, Kc.

D. It consumes reactants, driving the reaction forward.

Answer: (B)

Обяснение

Catalysts work by lowering the activation energy, which speeds up both the forward and reverse reactions equally, without changing the position of equilibrium or the value of Kc.

8. Which of the following correctly describes the reaction quotient (Q) in relation to the equilibrium constant (Kc)?

A. Q always equals Kc at equilibrium.

B. Q is used only after the reaction reaches equilibrium.

C. Q is unrelated to the concentrations of reactants and products.

D. Q can only be calculated for gas-phase reactions.

Answer: (A)

Обяснение

Q is the current ratio of concentrations of products to reactants, like Kc, but evaluated at any moment. When Q equals Kc, the system is at equilibrium.

9. What does the expression for the equilibrium constant Kc look like for the reaction aA + bB ↔ cC + dD?

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

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

C. Kc = [A] + [B] − [C] − [D]

D. Kc = [A] / [B] + [C] / [D]

Answer: (B)

Обяснение

The equilibrium constant expression for this reaction is the ratio of the concentrations of products raised to their coefficients over those of reactants, which is [C]^c [D]^d / [A]^a [B]^b.

10. According to the revision sheet, which factor does not alter the position of equilibrium in a reversible reaction?

A. Changes in concentration of reactants or products.

B. Temperature change in an exothermic reaction.

C. Adding a catalyst.

D. Changing pressure in a gaseous reaction.

Answer: (C), (D)

Обяснение

Adding a catalyst speeds up both the forward and reverse reactions equally, speeding up equilibrium attainment but not shifting its position or changing Kc.