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:extRateofreaction=ΔcΔtext{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:
      Rate<em>A=[A]</em>t2−[A]t1t2−t1\text{Rate}<em>{A} = \frac{[A]</em>{t2} - [A]_{t1}}{t2 - t1}
    • Rate of Formation of B:
      Rate<em>B=[B]</em>t2−[B]t1t2−t1\text{Rate}<em>{B} = \frac{[B]</em>{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→cC+dDaA + bB \rightarrow cC + dD
  • Rate Expression:
    Rate of reaction=−1ad[A]dt=−1bd[B]dt=1cd[C]dt=1dd[D]dt\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.