Understanding Chemical Equilibrium Concepts

At what time does the reaction reach equilibrium?

• t₁

• t₂

• t₃

• t₄

Answer explanation

The reaction reaches equilibrium at t₂ when the concentrations of reactants and products remain constant, indicating that the forward and reverse reaction rates are equal.

This endothermic reaction is at equilibrium.

If its temperature is increased (at constant volume and pressure), which way will the equilibrium shift?

• To products (right)

• To reactants (left)

Answer explanation

In an endothermic reaction, heat is absorbed. Increasing the temperature shifts the equilibrium to the right, favoring the formation of products, as the system tries to counteract the added heat.

How could we make this reaction happen more quickly?

• Decrease the concentration of the acid

• crush the chalk to increase the surface area

• Put the test tube in an ice bath

Answer explanation

Crushing the chalk increases its surface area, allowing for more contact with the acid, which speeds up the reaction. Decreasing acid concentration or using an ice bath would slow the reaction down.

According to Le Chatelier's Principle, what happens to the position of equilibrium when the concentration of a reactant is increased?

• The equilibrium shifts to the left.

• The equilibrium shifts to the right.

• The equilibrium remains unchanged.

• The equilibrium shifts randomly.

Answer explanation

According to Le Chatelier's Principle, increasing the concentration of a reactant causes the equilibrium to shift to the right, favoring the formation of products to counteract the change.

Which of the following factors does NOT affect the position of equilibrium in a chemical reaction?

• Temperature

• Pressure

• Catalyst

• Concentration

Answer explanation

A catalyst speeds up a reaction but does not change the position of equilibrium. In contrast, temperature, pressure, and concentration directly influence the equilibrium state of a chemical reaction.

In a dynamic equilibrium, which of the following statements is true?

• The concentrations of reactants and products are equal.

• The forward and reverse reactions occur at the same rate.

• The reaction has stopped completely.

• The concentration of products is always greater than that of reactants.

Answer explanation

In a dynamic equilibrium, the forward and reverse reactions occur at the same rate, leading to constant concentrations of reactants and products, but not necessarily equal concentrations.

What effect does increasing the temperature have on an exothermic reaction at equilibrium?

• The equilibrium shifts to the right.

• The equilibrium shifts to the left.

• The equilibrium remains unchanged.

• The reaction rate decreases.

Answer explanation

Increasing the temperature of an exothermic reaction shifts the equilibrium to the left, favoring the reactants. This is because heat is a product, and adding heat drives the reaction in the direction that absorbs heat.

• The equilibrium shifts to the right.

• The equilibrium shifts to the left.

• The equilibrium remains unchanged.

• The reaction stops.

Answer explanation

Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas. In this reaction, there are 4 moles of reactants (1 N2 + 3 H2) and 2 moles of products (2 NH3), so the equilibrium shifts to the right.

Which of the following best describes a system at dynamic equilibrium?

• The system is static with no movement of particles.

• The concentrations of reactants and products are constantly changing.

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

• The system is in a state of constant energy input.

Answer explanation

A system at dynamic equilibrium is characterized by the rate of the forward reaction equaling the rate of the reverse reaction, leading to constant concentrations of reactants and products, despite ongoing processes.

If a catalyst is added to a reaction at equilibrium, what is the effect on the equilibrium position?

• The equilibrium shifts to the right.

• The equilibrium shifts to the left.

• The equilibrium position remains unchanged.

• The reaction stops.

Answer explanation

A catalyst speeds up the rate of both the forward and reverse reactions equally, but it does not change the concentrations of reactants and products at equilibrium. Therefore, the equilibrium position remains unchanged.