Chapter 10: Reaction Rates and Chemical Equilibrium – Study Notes

Chapter 10: Reaction Rates and Chemical Equilibrium

Learning Objectives

• Define reaction rates and chemical equilibrium.

• Explain and apply Le Chatelier's Principle to chemical systems.

Section 10.1: Rates of Reactions

Introduction to Reaction Rates

Reaction rates describe how quickly reactants are converted into products in a chemical process. Everyday examples include the ripening of fruit, tarnishing of silver, and aging processes, each occurring at different rates.

• Reaction rate is the change in concentration of a reactant or product per unit time.

• Some reactions are fast (e.g., fruit ripening in days), while others are slow (e.g., aging over decades).

Defining Rate of Reaction

The rate (or speed) of a reaction is determined by measuring the amount of reactant used up or product formed in a certain period of time.

• Rate of reaction formula:

$\text{Rate of reaction} = \frac{\text{change in concentration of reactant or product}}{\text{change in time}}$

• Units are typically molarity per second ($\text{mol/L/s}$).

Factors Affecting Reaction Rate

Several factors influence how fast a chemical reaction occurs:

• Activation energy: Reactions with low activation energies proceed faster than those with high activation energies.

• Temperature: Increasing temperature generally increases reaction rate.

• Concentration: Higher concentration of reactants usually increases reaction rate.

• Catalyst: Adding a catalyst speeds up the reaction by lowering activation energy.

Section 10.2: Chemical Equilibrium

Introduction to Chemical Equilibrium

Most chemical reactions do not convert all reactants to products; instead, a reverse reaction occurs where products reform reactants. This leads to a dynamic state called chemical equilibrium.

• Reversible reaction: A reaction that proceeds in both forward and reverse directions.

• Example: $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g)$

Dynamic Equilibrium

At equilibrium, the concentrations of reactants and products remain constant because the rate of the forward reaction equals the rate of the reverse reaction.

• Both reactions continue, but there is no net change in concentrations.

• Before equilibrium, concentrations change; at equilibrium, they do not.

Section 10.3: Le Chatelier’s Principle

Le Chatelier’s Principle Explained

Le Chatelier’s Principle states that if a system at equilibrium is disturbed by a change in conditions (stress), the system will shift to counteract the disturbance and restore equilibrium.

• Stresses include changes in concentration, temperature, or pressure.

• The system responds by shifting the rates of forward or reverse reactions.

Effect of Concentration Changes

Changing the concentration of reactants or products shifts the equilibrium position:

Stress	Shift in Direction of
Increasing Reactant	Product (forward reaction)
Decreasing Reactant	Reactant (reverse reaction)
Removing Product	Product (forward reaction)

• Adding reactant increases the forward reaction rate.

• Removing reactant increases the reverse reaction rate.

• Removing product also increases the forward reaction rate.

Effect of a Catalyst on Equilibrium

Adding a catalyst lowers the activation energy, increasing the rates of both forward and reverse reactions equally. This allows equilibrium to be reached faster but does not change the equilibrium concentrations.

• Catalyst: Speeds up attainment of equilibrium, but does not affect the position of equilibrium.

Section 10.4: Chemistry Link to Health – Hemoglobin and Oxygen Transport

Equilibrium in Biological Systems

Oxygen transport in the body involves an equilibrium between hemoglobin (Hb), oxygen ($\text{O}_2$), and oxyhemoglobin (HbO$_2$):

$\text{Hb}(aq) + \text{O}_2(g) \rightleftharpoons \text{HbO}_2(aq)$

• High $\text{O}_2$ concentration in lungs shifts equilibrium toward oxyhemoglobin formation.

• Low $\text{O}_2$ concentration in tissues shifts equilibrium toward release of $\text{O}_2$ from oxyhemoglobin.

• Decrease in $\text{O}_2$ (e.g., at high altitude) shifts equilibrium toward reactants, reducing oxyhemoglobin and potentially causing hypoxia.

Summary Table: Factors Affecting Reaction Rate and Equilibrium

Factor	Effect on Rate	Effect on Equilibrium
Increase Temperature	Increases	May shift equilibrium (depends on reaction type)
Decrease Temperature	Decreases	May shift equilibrium (depends on reaction type)
Increase Reactant Concentration	Increases	Shifts toward products
Decrease Reactant Concentration	Decreases	Shifts toward reactants
Add Catalyst	Increases	No change in equilibrium position

Key Terms

• Reaction rate: Speed at which reactants are converted to products.

• Chemical equilibrium: State where forward and reverse reaction rates are equal.

• Le Chatelier’s Principle: System at equilibrium responds to stress by shifting to relieve the stress.

• Catalyst: Substance that increases reaction rate without being consumed.

• Activation energy: Minimum energy required for a reaction to occur.

Example Application

• For the reaction $2\text{CO}(g) + \text{O}_2(g) \rightarrow 2\text{CO}_2(g)$:

  • Increasing temperature: Increases rate

  • Removing $\text{O}_2$: Decreases rate

  • Adding catalyst: Increases rate

  • Lowering temperature: Decreases rate

Additional info: Biological examples, such as hemoglobin equilibrium, illustrate the importance of chemical equilibrium in health and physiology.