QCM : Understanding Chemical Equilibrium — 10 questions

Questions et réponses du QCM

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

The energy change during the reaction
The temperature at which the reaction occurs
The speed at which the reaction occurs
The ratio of reactants to products at equilibrium

The ratio of reactants to products at equilibrium

Explication

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?

The reaction favors reactants at equilibrium.
The concentrations of reactants and products are equal.
The reaction favors products at equilibrium.
The reaction does not reach equilibrium.

The reaction favors products at equilibrium.

Explication

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?

The reaction shifts toward more reactants
The reaction shifts toward more products
The reaction rate decreases
The reaction reaches equilibrium faster

The reaction shifts toward more reactants

Explication

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?

Albert Einstein
Le Châtelier
Marie Curie
Michael Faraday

Le Châtelier

Explication

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?

It has no effect on the equilibrium position
It shifts the equilibrium toward more moles of gas
It only affects the reaction rate, not the equilibrium
It shifts the equilibrium toward fewer moles of gas

It shifts the equilibrium toward fewer moles of gas

Explication

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?

It shifts equilibrium to the left, favoring reactants.
It shifts equilibrium to the right, favoring products.
It does not affect the position of equilibrium.
It decreases the rate of reaction without shifting equilibrium.

It shifts equilibrium to the right, favoring products.

Explication

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?

It shifts the equilibrium position toward products or reactants.
It lowers the activation energy, speeding up both forward and reverse reactions equally.
It changes the value of the equilibrium constant, Kc.
It consumes reactants, driving the reaction forward.

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

Explication

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)?

Q always equals Kc at equilibrium.
Q is used only after the reaction reaches equilibrium.
Q is unrelated to the concentrations of reactants and products.
Q can only be calculated for gas-phase reactions.

Q always equals Kc at equilibrium.

Explication

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?

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

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

Explication

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?

Changes in concentration of reactants or products.
Temperature change in an exothermic reaction.
Adding a catalyst.
Changing pressure in a gaseous reaction.

Adding a catalyst.

Explication

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

Révisez avec les flashcards

Mémorisez les réponses avec 10 flashcards sur Understanding Chemical Equilibrium.

Le Châtelier’s principle — effect?

System shifts to counteract disturbances to restore equilibrium.

Chemical equilibrium — definition?

State with constant concentrations over time.

Chemical equilibrium — definition?

State where reactant and product concentrations remain constant.

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