QCM : Chemical Equilibrium Mastery — 9 questions

Questions et réponses du QCM

1. What is the primary role of the equilibrium constant (Kc) in a reversible chemical reaction?

It measures the speed of the reaction.
It determines the temperature at which the reaction occurs.
It indicates the position of equilibrium, showing whether products or reactants are favored.
It controls the amount of catalyst needed for the reaction.

It indicates the position of equilibrium, showing whether products or reactants are favored.

Explication

The equilibrium constant (Kc) quantifies the ratio of concentrations of products to reactants at equilibrium, thus indicating the position of equilibrium and whether the reaction favors products or reactants at a given temperature.

2. What is the primary characteristic of a reversible reaction?

It proceeds only in the forward direction.
It proceeds in both forward and backward directions, reaching a dynamic equilibrium.
It never reaches equilibrium and goes to completion.
It occurs only at high temperatures.

It proceeds in both forward and backward directions, reaching a dynamic equilibrium.

Explication

Reversible reactions proceed in both directions and reach a dynamic equilibrium where forward and backward rates are equal, unlike irreversible reactions which proceed only forward.

3. What is the precise relation between the equilibrium constants Kp and Kc for gaseous reactions?

Kp = Kc + (RT)^Δn
Kp = Kc / (RT)^Δn
Kp = Kc (RT)^Δn
Kp = Kc - (RT)^Δn

Kp = Kc (RT)^Δn

Explication

The correct relation between Kp and Kc for gaseous reactions is Kp = Kc (RT)^Δn, where Δn is the change in moles of gas between products and reactants. This formula accounts for the difference in pressure and concentration-based equilibrium constants, as explicitly stated in the content.

4. Which of the following best describes the equilibrium constant, Kc?

It depends on initial concentrations.
It varies with temperature and represents the ratio of concentrations at equilibrium.
It is always greater than 1 for stable reactions.
It is affected by catalysts.

It varies with temperature and represents the ratio of concentrations at equilibrium.

Explication

Kc depends on temperature and provides a ratio of product to reactant concentrations at equilibrium, and is unaffected by initial concentrations or catalysts.

5. What is a reversible reaction?

A reaction that occurs only at high temperatures and is irreversible.
A reaction that proceeds only in one direction and cannot be reversed.
A reaction that proceeds in both forward and backward directions and reaches a dynamic equilibrium.
A reaction that does not reach equilibrium and is always incomplete.

A reaction that proceeds in both forward and backward directions and reaches a dynamic equilibrium.

Explication

A reversible reaction is one that proceeds in both forward and backward directions, allowing the system to reach a dynamic equilibrium where concentrations of reactants and products remain constant over time.

6. Under what condition can equilibrium be established in a chemical reaction?

In an open system allowing reactants and products to escape.
In a closed system where no reactants or products escape.
Only when the reaction is irreversible.
When temperature is constantly changing.

In a closed system where no reactants or products escape.

Explication

Equilibrium requires a closed system where no reactants or products escape, allowing the forward and reverse reactions to balance.

7. What effect does a catalyst have on a reversible reaction at equilibrium?

It shifts the equilibrium position.
It speeds up the attainment of equilibrium but does not change the equilibrium position.
It decreases the value of Kc.
It only affects irreversible reactions.

It speeds up the attainment of equilibrium but does not change the equilibrium position.

Explication

Catalysts increase the rate at which equilibrium is reached but do not alter the equilibrium position or the value of Kc.

8. How is the relation between Kp and Kc expressed when Δn = 0?

Kp is equal to Kc.
Kp is the square of Kc.
Kp is proportional to temperature.
Kp is zero.

Kp is equal to Kc.

Explication

When Δn = 0, the relation simplifies to Kp = Kc, meaning the partial pressure and concentration ratios are directly equal.

9. Which statement accurately describes the nature of the equilibrium state?

The concentrations of reactants and products constantly change.
The concentrations of reactants and products remain constant over time while reactions continue.
Reactions stop completely at equilibrium.
Only the forward reaction occurs.

The concentrations of reactants and products remain constant over time while reactions continue.

Explication

At equilibrium, the forward and reverse reactions continue to occur, but the concentrations of reactants and products remain constant, indicating a dynamic balance.

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Reversible Reaction — definition?

Reaction proceeding in both directions, reaching equilibrium.

Reversible Reaction — definition?

Reaction proceeding in both directions at equilibrium.

Equilibrium State — role?

Constant concentrations with equal forward and reverse rates.

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