Chapter 7: Rate of Reaction

7.1 Determining Rate of Reaction

Definition of Rate of Reaction

The Rate of Reaction is defined as the change in the quantity of a reactant or product per unit time.

$$\text{Rate of Reaction} = \frac{\text{Change in quantity of reactant or product}}{\text{Time taken}}$$

Types of Rate of Reaction

• Average Rate of Reaction: The average value of the reaction rate over a specific time interval.
• Instantaneous Rate of Reaction: The actual rate of reaction at a specific point or given time. It is determined by calculating the gradient of the tangent to the curve on a quantity-against-time graph at that instant ($t$).

Observable Changes for Measuring Reaction Rate

• Formation of gas (measured using a gas syringe or water displacement).
• Decrease in the mass of reactants over time.
• Formation of a precipitate (e.g., sulfur precipitate obscuring a cross mark '$X$').
• Changes in pH value, temperature, or electrical conductivity.

7.2 Factors Affecting Rate of Reaction

There are five primary factors that influence the rate of a chemical reaction:

• Size of Reactants (Surface Area): Smaller solid reactant particles provide a larger total surface area exposed to collision, increasing the reaction rate.
• Concentration of Reactant: Higher concentration means more solute particles per unit volume, increasing the frequency of collisions.
• Temperature of Reaction: Higher temperatures increase the kinetic energy of particles, causing them to move faster and collide more frequently with energy exceeding the activation energy.
• Presence of a Catalyst: A catalyst provides an alternative pathway with lower activation energy ($E_a$), increasing the proportion of effective collisions without being chemically altered itself.
• Pressure (for Gaseous Reactants): Higher pressure increases the number of gas particles per unit volume.

7.3 Application of Factors Affecting Rate of Reaction in Daily Life and Industry

Haber Process (Production of Ammonia)

• Equation: $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$
• Temperature: $450^\circ\text{C} - 550^\circ\text{C}$
• Pressure: $200\text{ atm}$
• Catalyst: Finely divided iron ($\text{Fe}$)

Contact Process (Production of Sulfuric Acid)

• Key Reaction: $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$
• Temperature: $450^\circ\text{C}$
• Pressure: $1\text{ atm}$
• Catalyst: Vanadium(V) oxide ($\text{V}_2\text{O}_5$)

7.4 Collision Theory

Core Concepts

• Collision Theory: States that reactant particles must collide with each other for a reaction to occur.
• Effective Collision: A collision that leads to a chemical reaction. It occurs when particles collide with:
  1. Correct orientation
  2. Sufficient energy equal to or greater than the Activation Energy ($E_a$)
• Activation Energy ($E_a$): The minimum energy required by colliding reactant particles to start a chemical reaction.

Energy Profile Diagrams

Energy profile diagrams illustrate the activation energy ($E_a$) required for exothermic and endothermic reactions, as well as the effect of adding a catalyst to lower $E_a$.