Chemical Reaction Kinetics and Catalysis Principles

Reaction Rate Equations and Orders

The rate equation expresses the reaction rate (q) at a given instant (n) as a function of the concentrations at that moment. The constant, k, that appears in the equation is called the rate constant. The exponents, x and y, are the partial orders of the reaction with respect to the concentrations of each substance. Their sum (x + y) is called the overall reaction order.

The half-life (t_½) is the time it takes for a certain reactant's concentration to reduce to half.

Reaction Orders: Zero, First, Second, and Third

• Zero-order reactions
• First-order reactions
• Second-order reactions
• Third-order reactions

Theories of Chemical Reactions

Collision Theory: Molecular Interactions

Lewis proposed that chemical reactions occur simply as a result of collisions between molecules, atoms, or ions of the reactants. The reaction rate depends on two factors:

• The frequency with which two molecules collide in a given volume. This can be calculated from the kinetic theory of gases.
• The effectiveness of the collision. For an effective collision, it must lead to the breaking of some bonds and the formation of others.

Activation Energy and Arrhenius Equation

In an effective collision, molecules, atoms, or ions of the reactants must have enough kinetic energy to break the necessary bonds. This is known as the Activation Energy (E_a). The collision must also occur with the appropriate orientation.

Arrhenius Equation: k = Ae^-Ea/RT

Transition State Theory: Activated Complex

Eyring proposed that when reactive molecules collide, they form an aggregate – not just a simple reunion of molecules, but a single, unstable entity. This aggregate, called the activated complex or transition state, is very unstable. The activation energy represents the energy necessary for this complex to form.

k = Ae^-Ea/RT

In an energy diagram, the enthalpy of the reactants can be represented. The activation enthalpy is always positive, which is necessary for reactants to transform into products.

Catalysts: Modifying Reaction Rates

The speed of certain reactions can be significantly altered by the addition of small quantities of substances known as catalysts. Catalysts are substances that:

• Alter the speed of a reaction.
• Act in very small quantities.
• Are not chemically consumed or permanently changed.
• Alter the reaction mechanism by changing the type of activated complex and the activation energy required to reach it.

Positive and Negative Catalysis

According to their effect, catalysts can be:

• Positive: Increase reaction speed.
• Negative: Reduce reaction speed.

Types and Applications of Catalysts

Catalysis can be of two types:

Homogeneous and Heterogeneous Catalysis

• Homogeneous Catalysis: Occurs when the catalyst is in the same phase as the reactants. For example, if the reaction involves gases, the catalyst must also be a gas.
• Heterogeneous Catalysis: Occurs when the catalyst and reactants are in different phases. One common problem is catalyst poisoning or inactivation.

Enzyme Catalysis: Biological Accelerators

Enzyme catalysis is considered an intermediate type between homogeneous and heterogeneous catalysis. Enzymes are among the most effective and specialized catalysts.