Equilibrium constant Kp for homogeneous systems (A-level only) revision notes

Equilibrium Constant Kp for Homogeneous Systems

Introduction

The equilibrium constant, Kp, is a crucial concept in physical chemistry, particularly when dealing with gaseous reactions. It provides insight into the position of equilibrium in a chemical reaction involving gases. Understanding Kp involves several key concepts, including partial pressures, mole fractions, and the interpretation of Kp values.

Partial Pressure in a Gas Mixture

• Definition: Partial pressure is the pressure exerted by a single gas in a mixture of gases. It is a measure of the concentration of that gas in the mixture.
• Dalton's Law of Partial Pressures: According to this law, the total pressure of a gas mixture is equal to the sum of the partial pressures of each individual gas in the mixture.
• Formula:

P_total = P_1 + P_2 + P_3 + ... + P_n

where P_1, P_2, ..., P_n are the partial pressures of the individual gases.

Calculating Mole Fractions and Partial Pressures

• Mole Fraction: The mole fraction of a gas in a mixture is the ratio of the number of moles of that gas to the total number of moles of all gases in the mixture.
• Formula:

X_i = n_i / n_total

where X_i is the mole fraction of gas i, n_i is the number of moles of gas i, and n_total is the total number of moles of all gases.

• Calculating Partial Pressure: The partial pressure of a gas can be calculated using its mole fraction and the total pressure of the gas mixture.
• Formula:

P_i = X_i * P_total

where P_i is the partial pressure of gas i.

Constructing Kp Expressions for Gaseous Homogeneous Equilibria

• Kp Expression: The equilibrium constant Kp for a reaction at equilibrium can be expressed in terms of the partial pressures of the reactants and products.
• General Form: For a reaction

aA(g) + bB(g) ⇌ cC(g) + dD(g)

the Kp expression is given by:

Kp = (P_C^c * P_D^d) / (P_A^a * P_B^b)

where P_C, P_D, P_A, and P_B are the partial pressures of the gases at equilibrium.

Calculating Kp and Determining Appropriate Units

• Calculating Kp: To calculate Kp, substitute the equilibrium partial pressures into the Kp expression derived from the balanced equation.
• Units of Kp: The units of Kp depend on the reaction stoichiometry. For a general reaction, the units can be derived from the expression and will typically be in terms of pressure (e.g., atm, Pa).

Interpreting Kp Values in Terms of Equilibrium Position

• Kp > 1: Indicates that at equilibrium, the concentration of products is greater than that of reactants, favoring the formation of products.
• Kp < 1: Indicates that at equilibrium, the concentration of reactants is greater than that of products, favoring the formation of reactants.
• Kp = 1: Suggests that the concentrations of reactants and products are equal at equilibrium.

Key Terms

• Partial pressure
• Mole fraction
• Kp
• Equilibrium
• Gaseous systems
• Dalton's Law
• Homogeneous equilibria
• Reaction quotient
• Equilibrium position
• Concentration

Exam Tips

• Always define partial pressure and mole fraction clearly in your answers.
• Practice constructing Kp expressions from various chemical equations.
• Be familiar with the units of Kp and how to derive them from the Kp expression.
• Understand the significance of Kp values in predicting the direction of a reaction at equilibrium.
• Review examples of calculations involving Kp to reinforce your understanding.

Common Mistakes

• Confusing partial pressure with total pressure.
• Incorrectly applying the mole fraction formula.
• Forgetting to balance the chemical equation before constructing the Kp expression.
• Misinterpreting the significance of Kp values in relation to equilibrium position.
• Neglecting to include units when calculating Kp.

A-Level Chemistry focus

Use Equilibrium Constant Kp for Homogeneous Systems to connect the exact AQA A-Level Chemistry 7405 subtopic to calculation, mechanism, evidence, practical reasoning, or explanation depth. Avoid generic GCSE-level statements.

How to use this revision note

Start by naming the chemical idea, then identify the relevant equation, observation, mechanism, trend, or practical method. Where calculations are involved, show the formula, substitution, working, final answer, and unit.

Exam focus

Strong A-Level answers justify each step. They separate evidence from conclusion, mechanism from product, observation from interpretation, and mathematical working from the final statement.

Common mistake

Do not rely on a memorised phrase if the question asks for reasoning. Check the subtopic wording, use precise terminology, and make sure each conclusion follows from the data or chemical principle given.