UK K12 GCSE/A-Level: Year 13 A-Level Upper Sixth Chemistry - Equilibrium, Kc, Kp, Le Châtelier's Principle

Learning Objectives

By the end of this topic, students will be able to:

• Define equilibrium constant (Kc) and equilibrium pressure (Kp), and explain their significance in describing chemical equilibria.
• Calculate Kc and Kp from given data, and vice versa.
• Apply Le Chatelier's Principle to predict the effects of changes in concentration, pressure, and temperature on chemical equilibria.
• Explain the concept of equilibrium expression and its relationship to Kc and Kp.
• Analyze and interpret data from experiments involving chemical equilibria.

Core Concepts

The equilibrium constant (Kc) is a mathematical expression that describes the ratio of the concentrations of products to reactants at equilibrium. It is defined as:

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

where [C], [D], [A], and [B] are the concentrations of the products and reactants at equilibrium, and a, b, c, and d are their respective stoichiometric coefficients.

The equilibrium pressure (Kp) is a similar expression that describes the ratio of the partial pressures of products to reactants at equilibrium. It is defined as:

Kp = (Pc)^(c) (Pd)^(d) / (Pa)^(a) (Pb)^(b)

where Pc, Pd, Pa, and Pb are the partial pressures of the products and reactants at equilibrium.

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the equilibrium will shift in a direction that tends to counteract the change.

Worked Examples

Example 1

A reaction is represented by the equation:

2NO(g) + O2(g)-2NO2(g)

The equilibrium constant (Kc) is 0.5. If the concentration of NO is 0.2 M, calculate the concentration of NO2.

Solution

We can use the equilibrium expression to write:

Kc = [NO2]^2 / ([NO]^2 [O2])

Rearranging to solve for [NO2], we get:

[NO2] = sqrt(Kc [NO]^2 [O2])

Substituting the given values, we get:

[NO2] = sqrt(0.5 (0.2)^2 (1))

[NO2] = 0.2 M

Example 2

A reaction is represented by the equation:

N2(g) + 3H2(g)-2NH3(g)

The equilibrium pressure (Kp) is 10. If the partial pressure of N2 is 2 atm, calculate the partial pressure of NH3.

Solution

We can use the equilibrium expression to write:

Kp = (P NH3)^2 / (P N2 (P H2)^3)

Rearranging to solve for P NH3, we get:

P NH3 = sqrt(Kp P N2 (P H2)^3)

Substituting the given values, we get:

P NH3 = sqrt(10 (2) (1)^3)

P NH3 = 3.16 atm

Common Misconceptions

• Many students mistakenly believe that the equilibrium constant (Kc) is a measure of the rate of reaction. However, Kc is a measure of the ratio of concentrations of products to reactants at equilibrium.
• Some students may think that Le Chatelier's Principle only applies to changes in concentration. However, it also applies to changes in pressure and temperature.
• Students may also confuse the equilibrium constant (Kc) with the equilibrium pressure (Kp). While both are mathematical expressions that describe chemical equilibria, they have different units and are used in different contexts.

Exam Tips

• Make sure to read the question carefully and understand what is being asked.
• Use the equilibrium expression to write an equation that relates the concentrations of reactants and products.
• Apply Le Chatelier's Principle to predict the effects of changes in concentration, pressure, and temperature on chemical equilibria.
• Use the correct units for the equilibrium constant (Kc) and equilibrium pressure (Kp).
• Show all your working and units in your calculations.

MCQs

MCQ 1 [F]

What is the equilibrium constant (Kc) for the reaction:

2NO(g) + O2(g)-2NO2(g)

A) 0.5 B) 1.0 C) 2.0 D) 5.0

Correct answer: A) 0.5 Why the distractors fail: B) 1.0 is the equilibrium constant for a reaction at equilibrium, not the given reaction. C) 2.0 is the equilibrium constant for a reaction with a different stoichiometry. D) 5.0 is an arbitrary value that is not related to the given reaction.

MCQ 2 [H]

A reaction is represented by the equation:

N2(g) + 3H2(g)-2NH3(g)

The equilibrium pressure (Kp) is 10. If the partial pressure of N2 is 2 atm, calculate the partial pressure of NH3.

A) 1.0 atm B) 2.0 atm C) 3.16 atm D) 5.0 atm

Correct answer: C) 3.16 atm Why the distractors fail: A) 1.0 atm is a low value that is not consistent with the given equilibrium pressure. B) 2.0 atm is the partial pressure of N2, not NH3. D) 5.0 atm is an arbitrary value that is not related to the given reaction.

MCQ 3 [F]

What is the effect of increasing the concentration of a reactant on a chemical equilibrium?

A) The equilibrium will shift to the left. B) The equilibrium will shift to the right. C) The equilibrium will remain unchanged. D) The equilibrium will shift to the right and then to the left.

Correct answer: B) The equilibrium will shift to the right. Why the distractors fail: A) Increasing the concentration of a reactant will cause the equilibrium to shift to the right, not left. C) The equilibrium will not remain unchanged, it will shift to counteract the change in concentration. D) The equilibrium will not shift to the right and then to the left, it will only shift to the right.

MCQ 4 [H]

A reaction is represented by the equation:

CaCO3(s)-CaO(s) + CO2(g)

The equilibrium constant (Kc) is 10. If the concentration of CO2 is 0.1 M, calculate the concentration of CaO.

A) 0.01 M B) 0.1 M C) 1.0 M D) 10.0 M

Correct answer: A) 0.01 M Why the distractors fail: B) 0.1 M is the concentration of CO2, not CaO. C) 1.0 M is a high value that is not consistent with the given equilibrium constant. D) 10.0 M is an arbitrary value that is not related to the given reaction.

MCQ 5 [F]

What is the unit of the equilibrium constant (Kc)?

A) M B) atm C) L/mol D) K

Correct answer: C) L/mol Why the distractors fail: A) M is the unit of concentration, not equilibrium constant. B) atm is the unit of pressure, not equilibrium constant. D) K is the unit of temperature, not equilibrium constant.

Short-answer questions

1. Write the equilibrium expression for the reaction:

2NO(g) + O2(g)-2NO2(g)

1. Calculate the equilibrium constant (Kc) for the reaction:

N2(g) + 3H2(g)-2NH3(g)

given the partial pressures of N2 and H2 are 2 atm and 1 atm, respectively.

1. Explain the effect of increasing the temperature on a chemical equilibrium.

2. Write the equilibrium expression for the reaction:

CaCO3(s)-CaO(s) + CO2(g)

1. Calculate the concentration of CaO given the concentration of CO2 is 0.1 M and the equilibrium constant (Kc) is 10.