# Chemical equilibrium concept questions and answers

1. What is chemical equilibrium?

Chemical equilibrium is a state in a chemical reaction where the rates of the forward and reverse reactions are equal. At equilibrium, the concentrations of reactants and products remain constant over time, but they are not necessarily equal.

2. What is the law of mass action?

The law of mass action states that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants, each raised to the power equal to its stoichiometric coefficient in the balanced chemical equation.

3. What is the equilibrium constant (Kc)?

The equilibrium constant (Kc) is a measure of the ratio of the concentrations of products to reactants at equilibrium, with each concentration raised to the power of its stoichiometric coefficient.

4. How is the equilibrium constant expressed for the reaction: aA + bB ⇌ cC + dD?

For the reaction aA + bB ⇌ cC + dD, the equilibrium constant is expressed as: Kc = ([C]c x [D]d) / ([A]a x [B]b), where [A], [B], [C], and [D] are the molar concentrations of the substances and a, b, c, d are their respective stoichiometric coefficients.

5. What does it mean when Kc is greater than 1?

If Kc is greater than 1, the equilibrium strongly favors the products. This means at equilibrium, the concentrations of products are greater than the concentrations of reactants.

6. What is the significance of Kp and how is it related to Kc?

Kp is the equilibrium constant for gas-phase reactions measured in terms of partial pressures. It is related to Kc by the equation: Kp = Kc(RT)Δn, where Δn is the difference in the stoichiometric coefficients of gaseous products and reactants, R is the universal gas constant, and T is the temperature in Kelvin.

7. What is Le Chatelier’s Principle?

Le Chatelier’s Principle states that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration of a component, the system will adjust its equilibrium position to counteract the effect of the disturbance.

8. How does a change in concentration affect the position of equilibrium according to Le Chatelier’s Principle?

According to Le Chatelier’s Principle, if the concentration of a reactant is increased, the system will shift to consume the added reactant by making more product. Conversely, if the concentration of a product is increased, the system will shift to consume the added product by making more reactant.

9. How does a change in temperature affect the position of equilibrium according to Le Chatelier’s Principle?

If a system at equilibrium is heated (for an endothermic reaction) or cooled (for an exothermic reaction), it will shift in the direction that absorbs the heat. Conversely, if the system is cooled (for an endothermic reaction) or heated (for an exothermic reaction), it will shift in the direction that releases heat.

10. How does a change in pressure affect the position of equilibrium in gaseous reactions?

According to Le Chatelier’s Principle, if the pressure is increased, the system will shift to the side with fewer moles of gas to reduce the pressure. Conversely, if the pressure is decreased, the system will shift to the side with more moles of gas to increase the pressure.

1. What is the reaction quotient (Qc)?

The reaction quotient (Qc) is calculated in the same way as the equilibrium constant, but the concentrations used are not necessarily equilibrium concentrations. It helps to predict the direction in which a reaction will proceed to reach equilibrium.

1. What is the relationship between Qc and Kc in predicting the direction of a reaction?

If Qc > Kc, the reaction will proceed in the reverse direction to reach equilibrium. If Qc < Kc, the reaction will proceed in the forward direction to reach equilibrium. If Qc = Kc, the system is already at equilibrium.

1. What is the role of a catalyst in a system at equilibrium?

A catalyst speeds up the rates of both the forward and reverse reactions equally. It helps the system reach equilibrium faster but does not change the position of equilibrium.

1. What is an example of an equilibrium constant expression for the reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)?

The equilibrium constant expression for this reaction is: Kc = [NH3]² / ([N2] x [H2]³), where the concentrations are those at equilibrium.

1. What is the difference between dynamic and static equilibrium?

In a static equilibrium, there is no movement or change in the system. In a dynamic equilibrium, there is continual motion and change, but it is balanced by an equal and opposite change so the system as a whole appears unchanging.

1. What is a homogeneous equilibrium?

A homogeneous equilibrium is a system where all reactants and products are in the same phase.

1. What is a heterogeneous equilibrium?

A heterogeneous equilibrium is a system where the reactants and products are in different phases.

1. What is meant by ‘partial pressure’ in the context of equilibrium in gas reactions?

In the context of equilibrium in gas reactions, the partial pressure is the contribution each gas makes to the total pressure of the system.

1. How are solids and liquids treated in equilibrium constant expressions?

In equilibrium constant expressions, the concentrations of pure solids and liquids are not included. This is because their concentrations (densities) are essentially constant.

1. What is the Van’t Hoff equation and its relevance to equilibrium?

The Van’t Hoff equation describes the temperature dependence of the equilibrium constant. It is given by ln(K2/K1) = -ΔH/R(1/T2 – 1/T1), where K1 and K2 are the equilibrium constants at temperatures T1 and T2 respectively, ΔH is the enthalpy change of the reaction, and R is the gas constant. It is useful for predicting how the position of equilibrium will change with temperature.