Chemistry Reaction Rates

Chemical Reaction Rates

• A rate measures how much something changes within a specified amount of time.
• In chemistry, the rate of a chemical reaction (reaction rate) is expressed as the change in the amount of reactant or product per unit time.

Collision Theory

• Atoms, ions, and molecules react to form products when they collide if they have enough kinetic energy.
• The minimum energy required for colliding particles to react is called the activation energy.
• An activated complex (or transition state) is an unstable arrangement of atoms that forms briefly at the peak of the activation-energy barrier.

Factors Affecting Reaction Rates

• Temperature: raising the temperature usually increases the reaction rate, while lowering it slows it down.
• Concentration: increasing the concentration of reactants increases the frequency of collisions, leading to a higher reaction rate.
• Particle size: smaller particles have a greater surface area, increasing the reaction rate.
• Catalysts: substances that increase the reaction rate without being used up during the reaction, permitting reactions to proceed along a lower energy path.

Rate Laws

• The rate at which A forms B can be expressed as the change in A with time.
• The rate law is an expression for the rate of a reaction in terms of the concentration of reactants.
• The specific rate constant (k) in a rate law is large if products form quickly and small if they form slowly.

Reaction Orders

• A first-order reaction is one in which the rate is directly proportional to the concentration of only one reactant.
• The overall order of a reaction is the sum of the exponents for the individual reactants.

Reaction Mechanisms

• An elementary reaction is a reaction in which reactants are converted to products in a single step.
• The series of elementary reactions or steps that take place during a complex reaction is called a reaction mechanism.
• An intermediate is a product of one step in a reaction mechanism and a reactant in the next step.
• The slowest step determines the rate of the overall reaction.