Reaction Rates Lecture Notes

Reaction Rates

Learning Outcomes

• By the end of this lecture, you should be able to:
  • Understand what happens in a chemical reaction.
  • Understand the meaning of effective collision.
  • Understand the concept of activation energy.
  • Determine the rate of a reaction.

How Do Reactions Occur?

• A chemical reaction involves the making and breaking of bonds.
• Key Criteria for a Reaction to Occur:
  1. Atoms or molecules must collide.
  2. There must be enough energy to break bonds.
  3. Molecules must be oriented correctly.
  4. The frequency of collision must be high enough.
• If these criteria are met, an effective collision will occur, leading to a reaction.

Collision Theory and Activation Energy

• For reactions to take place, atoms or molecules need to collide.
• The energy of a collision is affected by:
  • The relative speeds of the colliding particles.
  • The angle at which they approach each other.
• Activation Energy (Ea): The minimum energy required to make or break bonds during a reaction.

Factors Affecting Reaction Rate

• The reaction rate depends on several factors, including:
  • Concentrations of reactants.
  • Temperature of the system.
  • Orientation of molecules during collision.
• Collision Model:
  • Molecules must collide for a reaction to occur.

Activation Energy and Reaction Rate

• Reactions can be classified as either exothermic or endothermic.
• If the activation energy is high, the reaction proceeds slowly (inversely proportional relationship).
• Energy Profile Diagram: Describes energy levels of reactants, products, and the transition state.

The Concept of Reaction Rate

• Definition: The rate of a chemical reaction is the change in concentration of a reactant or product per unit of time.
• Average Rate Formula: ext{Rate of reaction} = \frac{\Delta c}{\Delta t}
  • Where ( \Delta c ) is the change in concentration and ( \Delta t ) is the time interval.
• Instantaneous Rate: The rate at a specific instant in time.

Concentration-Time Profile

• A concentration-time profile can illustrate how the concentrations of reactants and products change over time.
• Example Reaction:( A \rightarrow B )
  • Changes in concentration can be graphically represented to determine reaction rates at various times.

Instantaneous Rate Calculation

• Instantaneous rates can be calculated as:
  • Rate of Consumption of A:
    \text{Rate}{A} = \frac{[A]{t2} - [A]_{t1}}{t2 - t1}
  • Rate of Formation of B:
    \text{Rate}{B} = \frac{[B]{t2} - [B]_{t1}}{t2 - t1}

Rate of Reaction Across Different Species

• The rate of reaction should yield the same value regardless of which reactant or product is being monitored in the reaction:
  aA + bB \rightarrow cC + dD
• Rate Expression:
  \text{Rate of reaction} = -\frac{1}{a} \frac{d[A]}{dt} = -\frac{1}{b} \frac{d[B]}{dt} = \frac{1}{c} \frac{d[C]}{dt} = \frac{1}{d} \frac{d[D]}{dt}

Concept Check Example

• Scenario: The conversion of chloromethane to iodomethane results in 16.45 mol/L of iodomethane formed in 2.5 hours.
• Rate of Reaction Calculation:
  • Convert hours to minutes and calculate:
  • Which of the following is the correct rate of reaction in (mol/L)/min?
  • a. 3.54 (mol/L)/min
  • b. 0.0591 (mol/L)/min
  • c. 0.11 (mol/L)/min
  • d. 6.58 (mol/L)/min
  • e. 0.027 (mol/L)/min

Summary

• Reactions require effective collisions to occur, which are influenced by various factors like kinetics.
• The activation energy is essential for reactions to proceed.
• Reaction rates depend on the number of effective collisions per second and the energy available during those collisions.
• Reactions can either be exothermic or endothermic depending on energy transformations.