Chapter 10: Reaction Rates and Chemical Equilibrium – Study Guide

Reaction Rates and Chemical Equilibrium

Key Concepts and Definitions

This chapter focuses on the factors that affect the rates of chemical reactions and the concept of chemical equilibrium. Understanding these principles is essential for predicting how reactions proceed and how they can be controlled in laboratory and real-world settings.

• Activation Energy: The minimum energy required for a chemical reaction to occur.

• Catalyst: A substance that increases the rate of a reaction by lowering the activation energy, without being consumed in the reaction.

• Chemical Equilibrium: The state in which the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products remain constant over time.

• Collision Theory of Reactions: Explains how chemical reactions occur and why reaction rates differ for different reactions; reactions happen when particles collide with sufficient energy and proper orientation.

• Le Chatelier's Principle: If a system at equilibrium is disturbed, the system will shift in a direction that counteracts the disturbance to restore equilibrium.

• Rate of Reaction: The speed at which reactants are converted to products, often measured as change in concentration per unit time.

• Reversible Reaction: A reaction that can proceed in both the forward and reverse directions.

• Equilibrium Constant (K): A numerical value that expresses the ratio of product concentrations to reactant concentrations at equilibrium.

• Forward Reaction: The reaction in which reactants are converted to products.

• Reverse Reaction: The reaction in which products are converted back to reactants.

Factors Affecting Reaction Rate

The rate of a chemical reaction can be influenced by several factors:

• Temperature: Increasing temperature generally increases reaction rate by providing more energy for collisions.

• Concentration: Higher concentration of reactants leads to more frequent collisions and a faster reaction rate.

• Catalysts: Catalysts lower the activation energy, increasing the rate without being consumed.

Example: If a reaction is performed at a higher temperature, the molecules move faster, collide more often, and the reaction rate increases.

Chemical Equilibrium

When a reversible reaction occurs, it can reach a state where the rates of the forward and reverse reactions are equal. At this point, the concentrations of reactants and products remain constant, though both reactions continue to occur.

• Equilibrium Expression: For a general reaction $aA + bB \rightleftharpoons cC + dD$, the equilibrium constant is given by:

$K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

• Le Chatelier's Principle: If the equilibrium is disturbed by changing concentration, temperature, or pressure, the system will shift to counteract the change.

Example: If more reactant is added to a system at equilibrium, the system will shift toward the products to re-establish equilibrium.

Writing Forward and Reverse Reactions

For a given equilibrium reaction, both the forward and reverse reactions can be written:

• Example: $N_2O_4 (g) \rightleftharpoons 2NO_2 (g)$

• Forward: $N_2O_4 (g) \rightarrow 2NO_2 (g)$

• Reverse: $2NO_2 (g) \rightarrow N_2O_4 (g)$

Applying Le Chatelier's Principle

Consider the equilibrium:

$CO_2 (g) + H_2O (l) \rightleftharpoons H_2CO_3 (aq)$

• Adding CO2: Shifts equilibrium toward H2CO3 (product).

• Removing H2CO3: Shifts equilibrium toward product formation.

• Adding H2O: Shifts equilibrium toward product formation.

• Removing CO2: Shifts equilibrium toward reactants.

Sample Problem

Given the reaction:

$N_2 (g) + 3H_2 (g) \rightleftharpoons 2NH_3 (g)$

• If NH3 is removed, the equilibrium will shift to produce more NH3.

• If H2 is added, the equilibrium will shift toward the products (NH3).

Equilibrium Constant (K)

The equilibrium constant quantifies the position of equilibrium. A large K value means products are favored; a small K value means reactants are favored.

$K = \frac{[\text{products}]}{[\text{reactants}]}$

Summary Table: Effects of Changes on Equilibrium

Change	Direction of Shift	Effect on Products
Add reactant	Toward products	Increases
Remove reactant	Toward reactants	Decreases
Add product	Toward reactants	Decreases
Remove product	Toward products	Increases
Increase temperature (endothermic)	Toward products	Increases
Decrease temperature (endothermic)	Toward reactants	Decreases

Additional info:

• Equilibrium does not mean equal concentrations of reactants and products, but equal rates of forward and reverse reactions.

• Links to online resources and videos can provide further examples and explanations of reaction rates and equilibrium.