Chapter 17: Chemical Equilibrium

Equilibrium Constant (K_eq)

A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium.

Law of Mass Action

States that the equilibrium constant is determined by the concentrations of products raised to their coefficients divided by the concentrations of reactants raised to their coefficients.

Reaction Quotient (Q)

A ratio of product to reactant concentrations at any point during a reaction, used to predict the direction of the reaction.

Equilibrium Constant Expression

General Formula

$K_{\mathrm{eq}}=\frac{\mathrm{[products]}^\mathrm{coefficients}}{\mathrm{[reactants]}^\mathrm{coefficients}}$

Key Points

• K_eq depends only on the reaction and temperature.
• Solids and pure liquids are not included in the equilibrium expression.

Reaction Quotient (Q)

Definition

A measure of the relative concentrations of products and reactants at a given time.

Comparison with K_eq

• Q = K_eq: The system is at equilibrium.
• Q < K_eq: The reaction proceeds in the forward direction.
• Q > K_eq: The reaction proceeds in the reverse direction.

Factors Affecting K_eq

• Temperature: Changing the temperature alters K_eq since equilibrium depends on enthalpy and entropy changes.
• Concentration: Does not affect K_eq, but affects the reaction quotient Q.
• Pressure: Only affects K_eq if gases are involved and the reaction has unequal moles of gas on each side.

Applications of K_eq

• Predicting the extent of a reaction.
• Calculating equilibrium concentrations of reactants and products.
• Determining the direction of a reaction.

Questions for Students

1. What is the equilibrium constant, and how is it calculated?
2. Explain the law of mass action with an example.
3. Compare the reaction quotient (Q) and equilibrium constant (K_eq).
4. How does temperature affect the value of K_eq?
5. Why are solids and pure liquids excluded from equilibrium constant expressions?