AP Chem Unit 5 Review - Kinetics in 10 Minutes!

Overview of Chemical Kinetics

• Kinetics studies the rates at which chemical reactions occur.

• Four main factors influencing reaction rates:

  • Concentration of reactants

  • Temperature

  • Particle size of reactants

  • Presence of catalysts

Determining Reaction Rates

• Relative rates of reactants and products can be found by analyzing the coefficients in a balanced equation.

  • Example: For the reaction 2 NO + O2 → 2 NO2:

    • Rate of disappearance of NO = Rate of appearance of NO2

    • Rate of disappearance of O2 will be half that of NO.

Rate Law and Calculation

• Reaction rate often expressed in units of concentration over time (e.g., moles/L/s).

• Rate law format:

  • Rate = k × [ClO2]^x × [OH^-]^y

  • k = rate constant; x and y = orders of reaction based on experimental data.

• Example calculations demonstrate how changes in concentration affect reaction rate:

  • ClO2: Tripling concentration leads to a rate increase by a factor of 9 (order is 2).

  • OH⁻: Tripling leads to a rate increase by a factor of 3 (order is 1).

• Overall reaction order is the sum of individual orders (total order = 3).

Graphical Determination of Reaction Order

• Graphs can help determine reaction order:

  • Concentration vs. Time: Zero order (straight line).

  • Natural log [A] vs. Time: First order (straight line).

  • 1/[A] vs. Time: Second order (straight line).

  • Slope of straight line graphs is equal to the rate constant.

Integrated Rate Laws and Half-Life

• Integrated rate laws connect rate constant, initial concentration, time, and final concentration for each order:

  • Zero order: [A] = [A]₀ - kt

  • First order: ln[A] = ln[A]₀ - kt

  • Second order: 1/[A] = 1/[A]₀ + kt

• For first-order reactions, half-life is given by

  • t₁/₂ = 0.693 / k.

Multistep Reactions and Rate Laws

• Multistep reactions consist of individual steps, each with a rate law:

  • E.g. Rate = k₁ × [NO] × [Cl2] for one step.

  • The slowest step determines the rate of the overall reaction.

• Reaction intermediates are formed temporarily and do not appear in the final balanced equation but can affect rate laws.

• Catalysts speed up reactions without being consumed:

  • They lower activation energy and often increase effective collisions.

Energy Profiles of Reactions

• Energy graphs show the progress of reactions from reactants to products:

  • Activation energy: Difference in energy from reactants to peak (transition state).

  • Change in enthalpy: Difference in energy between reactants and products determines exothermic or endothermic nature.

  • Arrhenius Equation connects temperature and activation energy to rate constants:

    • When plotted, slope provides insights into activation energy.

• Chemical kinetics involves understanding how reaction rates are affected by various factors, calculating rates using experimental data, and analyzing reaction mechanisms and energy profiles.

• Important concepts include rate laws, integration of rates with respect to concentration over time, and the impact of catalysts and multistep reactions on overall kinetics.

The collision model in chemical kinetics is a theory that explains how chemical reactions occur through the collisions of reactant molecules. According to this model, for a reaction to take place, the reacting molecules must collide with sufficient energy and proper orientation. Here are some key points about the collision model:

1. Collision Frequency: The number of collisions between molecules per unit time affects the reaction rate. Higher concentrations of reactants result in more frequent collisions.

2. Activation Energy: Not all collisions lead to a reaction; only those with energy equal to or greater than the activation energy (the minimum energy required to initiate a reaction) will result in a reaction.

3. Orientation: Molecules must also collide in the correct orientation for a reaction to occur. This means that the specific way in which the reactants come together can significantly affect whether or not they will react.

4. Temperature Effect: Increasing the temperature typically increases the kinetic energy of the molecules, leading to more frequent and more energetic collisions, which in turn raises the reaction rate.

5. Catalysts: Catalysts can enhance the reaction rate by providing an alternative pathway for the reaction with a lower activation energy, thus increasing the likelihood of effective collisions.

Overall, the collision model helps explain why certain factors, such as concentration, temperature, and the presence of catalysts, have a significant influence on the rates of chemical reactions.