BackChemical Kinetics: Rates of Chemical Reactions
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Chemical Kinetics
Introduction to Chemical Kinetics
Chemical kinetics is the branch of chemistry that studies the rates (speeds) of chemical reactions and the factors that affect these rates. Understanding kinetics allows chemists to control reaction speeds and gain insight into reaction mechanisms at the molecular level.
Chemical kinetics: Study and measurement of reaction rates.
Reaction rate: The speed at which a chemical reaction occurs.
Main factors affecting reaction rates:
Reactant concentration
Temperature
Presence of catalysts
Defining Rate
A rate expresses how much a quantity changes over a given period of time. In chemistry, the rate of a reaction refers to the change in concentration of reactants or products per unit time.
Examples of rates:
Speed of a car: miles per hour
Wages: dollars per hour
Pulse: heartbeats per minute
Reaction Rate
Defining Reaction Rate
The rate of a chemical reaction is defined by the change in concentration of a reactant or product over time. For reactants, the rate is negative (since their concentration decreases); for products, it is positive (since their concentration increases).
General definition:
For reactant A:
For product B:
Negative sign for reactants: Indicates decrease in concentration.
Example: Rate for a Simple Reaction
For the reaction :
Rate of a Chemical Reaction: Stoichiometry
For reactions with stoichiometric coefficients, the rate must account for these coefficients to accurately reflect the change in concentration per unit time.
Example reaction:
Rates for each species:
Coefficient needed because of stoichiometry: The change in [HI] is divided by 2, its coefficient in the balanced equation.
General Rate Expression and Stoichiometry
For a general reaction:
The rate can be written as:
Visualizing Reaction Rate
Reactions can proceed at different rates, as illustrated by molecular diagrams showing the progress of reactants converting to products over time. Fast reactions reach completion quickly, while slow reactions take longer.
Fast rate: Large fraction of molecules react in a short time.
Slow rate: Small fraction of molecules react over a longer period.
Summary Table: Rate Expressions for Reactants and Products
Species | Rate Expression |
|---|---|
Reactant A | |
Reactant B | |
Product C | |
Product D |
Key Concepts
Reaction rate is a measure of how quickly reactants are converted to products.
Rates are determined by measuring concentration changes over time.
Stoichiometric coefficients must be considered when expressing rates for each species in a reaction.
Example Application
For the reaction , the rate can be expressed as:
