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 (E_a): the minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction.
High E_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 (\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).