QCM : Mastering Chemical Equilibrium Concepts — 9 questions

Questions et réponses du QCM

1. What is chemical equilibrium?

A condition where the forward and reverse reactions occur at the same rate, resulting in constant concentrations of reactants and products.
A state where the reaction has completely gone to completion, with no reactants left.
A state where the concentrations of reactants and products are constantly changing.
A situation where only the forward reaction occurs and the reverse reaction is negligible.

A condition where the forward and reverse reactions occur at the same rate, resulting in constant concentrations of reactants and products.

Explication

Chemical equilibrium is defined as a state in a reversible reaction where the forward and reverse reactions occur at equal rates, leading to constant concentrations of reactants and products over time. This does not mean concentrations are unchanging in a dynamic sense, but that they remain constant because the reactions balance each other.

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

Reactants and products are present in equal concentrations.
The forward and reverse reactions occur at identical rates.
No reactions are occurring.
The concentrations of reactants and products are constantly changing.

The forward and reverse reactions occur at identical rates.

Explication

At equilibrium, the forward and reverse reactions occur at the same rate, which keeps the concentrations of reactants and products constant over time. It's a dynamic balance, not a static one.

3. What is the precise definition of 'Dynamic Equilibrium' in a reversible chemical reaction?

A situation where only the forward reaction occurs, and the reverse reaction is negligible.
A condition where the forward and reverse reactions occur at the same rate, maintaining constant concentrations.
A state where the concentrations of reactants and products change continuously over time.
A state where the reaction has reached completion, with no reactants remaining.

A condition where the forward and reverse reactions occur at the same rate, maintaining constant concentrations.

Explication

The correct answer is that 'Dynamic Equilibrium' is a condition where the forward and reverse reactions occur at the same rate, resulting in constant concentrations of reactants and products over time, despite ongoing reactions. This is the fundamental definition of dynamic equilibrium in reversible reactions. The other options are incorrect: the first describes a non-equilibrium state with changing concentrations; the third describes a one-sided, irreversible process; and the fourth describes a reaction that has gone to completion, not equilibrium.

4. Which statement accurately describes the equilibrium constant, K?

It varies with temperature.
It always equals 1 at equilibrium.
It is unaffected by changes in concentration or pressure.
It measures the rate of the reaction at equilibrium.

It varies with temperature.

Explication

The equilibrium constant, K, is temperature-dependent and provides a ratio of concentrations at equilibrium, not a rate or static value.

5. What is the primary role of the equilibrium constant in a chemical reaction?

It determines the temperature at which the reaction occurs.
It indicates whether the reaction favors products or reactants at equilibrium.
It measures the speed at which equilibrium is reached.
It shows how catalysts affect the reaction rate.

It indicates whether the reaction favors products or reactants at equilibrium.

Explication

The equilibrium constant (K) is used to express the ratio of concentrations of products to reactants at equilibrium, indicating whether the reaction favors products (K > 1) or reactants (K < 1). It is a measure of the position of equilibrium, not the rate of reaction, temperature, or effects of catalysts.

6. According to Le Chatelier's Principle, what happens if you increase the temperature of an exothermic reaction at equilibrium?

Equilibrium shifts to produce more reactants.
Equilibrium shifts to produce more products.
There is no change in the equilibrium position.
The reaction rate decreases.

Equilibrium shifts to produce more reactants.

Explication

Increasing temperature in an exothermic reaction causes the system to shift toward reactants to counteract the addition of heat, according to Le Chatelier's Principle.

7. How does the addition of a catalyst affect the equilibrium of a reaction?

It shifts the position of equilibrium.
It increases the equilibrium constant, K.
It speeds up reaching equilibrium without changing the position.
It decreases the reaction rate.

It speeds up reaching equilibrium without changing the position.

Explication

Catalysts lower activation energy, speeding up the attainment of equilibrium but do not alter the equilibrium position or the value of K.

8. Which of the following industrial processes relies heavily on principles of chemical equilibrium to optimize yield?

Photosynthesis.
Haber process for ammonia synthesis.
Rusting of iron.
Photosensitive paper development.

Haber process for ammonia synthesis.

Explication

The Haber process is an industrial application that optimizes conditions based on Le Chatelier's Principle to maximize ammonia production.

9. What effect does decreasing pressure have on the equilibrium position of a gaseous reaction involving more moles of gas on the reactant side?

Shifts the equilibrium toward products.
Shifts the equilibrium toward reactants.
Has no effect on the equilibrium position.
Increases the value of the equilibrium constant, K.

Shifts the equilibrium toward reactants.

Explication

Decreasing pressure favors the side with more moles of gas, so in this case, the reaction shifts toward reactants.

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Chemical Equilibrium — definition?

A state where reactant and product concentrations remain constant.

Equilibrium — definition?

Balanced state with constant concentrations.

Dynamic Equilibrium — role?

Reactions occur in both directions at equal rates.

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