Reaction Rates Notes

Chapter 17: Reaction Rates
What You'll Learn
  • A model describing how chemical reactions occur as a result of collisions.
  • Comparison of the rates of chemical reactions under varying conditions.
Why It's Important
  • Understanding reaction rates is crucial for controlling chemical reactions effectively, such as in the space program where rockets are propelled by chemical reactions.
17.1 A Model for Reaction Rates
Objectives
  • Relate rates of chemical reactions to collisions between reacting particles.
Vocabulary
  • Reaction rate
  • Collision theory
  • Activated complex
  • Transition state
  • Activation energy
Expressing Reaction Rates
  • Chemists need to be specific about reaction rates.
  • Reaction rate of a chemical reaction is stated as the change in concentration of a reactant or product per unit time, expressed as mol/(L·s).
  • Reaction rates are determined experimentally.
The Collision Theory
  • Collision theory states that atoms, ions, and molecules must collide in order to react.
  • Explains why reactions occur and how their rates can be modified.
Orientation and the Activated Complex
  • Molecules must collide with favorable orientations for a reaction to occur.
  • Activated complex (transition state): a temporary, unstable arrangement of atoms that may form products or re-form reactants.
Activation Energy and Reaction
  • Activation energy (EaE_a): the minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction.
  • High EaE_a means a low reaction rate, and vice versa.
Energy Diagrams
  • Energy diagrams show the activation energy of a reaction.
  • Exothermic reactions lose energy.
  • Endothermic reactions require an input of energy.
The Influence of Spontaneity
  • Spontaneity (ΔG\Delta G) does not necessarily indicate reaction speed.
  • Spontaneity indicates the natural tendency for a reaction to proceed, but other factors affect the rate.
17.2 Factors Affecting Reaction Rates
Objectives
  • Identify factors that affect the rates of chemical reactions.
  • Explain the role of a catalyst.
Vocabulary
  • Catalyst
  • Inhibitor
  • Heterogeneous catalyst
  • Homogeneous catalyst
The Nature of Reactants
  • Some substances react more readily than others.
Concentration
  • Reactions speed up when the concentrations of reacting particles are increased.
  • Increasing the concentration of a reactant increases the collision frequency and, therefore, the reaction rate.
Surface Area
  • Greater surface area allows more collisions between reactants, increasing the reaction rate.
  • Pulverizing a substance increases its surface area.
Temperature
  • Increasing the temperature generally increases the reaction rate.
  • Increasing the temperature increases the average kinetic energy of particles, leading to more frequent and energetic collisions.
Catalysts
  • A catalyst increases the rate of a chemical reaction without being consumed in the reaction.
  • Enzymes are biological catalysts.
  • Catalysts lower the activation energy of a reaction.
Inhibitors
  • An inhibitor slows down, or inhibits, reaction rates.
  • Preservatives in food are inhibitors.
Heterogeneous and Homogeneous Catalysts
  • Heterogeneous catalyst: Exists in a different physical state than the reaction it catalyzes.
  • Homogeneous catalyst: Exists in the same physical state as the reaction it catalyzes.
Reaction Mechanisms
  • Most chemical reactions consist of a sequence of two or more simpler reactions.
  • Each step is an elementary step.
  • A complex reaction consists of two or more elementary steps.
  • Reaction mechanism: The complete sequence of elementary steps.
Rate-Determining Step
  • A complex reaction can proceed no faster than the slowest of its elementary steps (rate-determining step).