Chapter 9 Kinetics

Five factors affect the rates of a chemical reaction:

1. Nature of reactants

2. The temperature

3. The concentration of reactants

4. The physical state of reactants

5. Catalysts

   

THE RATE EQUATION

• k is the rate constant—a constant for each chemical reaction at a given temperature. The exponents m and n, called the orders of reaction,

INTEGRATED RATE LAWS

• only cases in which instantaneous data are used in the rate expression have been shown. These expressions allow us to answer questions concerning the speed of the reaction at a particular moment, but not questions about how long it might take to use up a certain reactant, etc.
• If changes in the concentration of reactants or products over time are taken into account, as in the integrated rate laws, these questions can be answered.
• The Arrhenius equation is most commonly used to calculate the activation energy of a reaction.
• The Arrhenius equation has the form: k = Ae-Ea /RT where k is the rate constant, A is a term called the frequency factor that accounts for molecular orientation, e is the natural logarithm base, R is the universal gas constant 8.314 J mol K-1, T is the Kelvin temperature, and Ea is the activation energy,
• A + B → C
• C + B → D
• D → E + F

If you add together the three equations above, you will get the overall equation A + 2B →E + F. C and D are called reaction intermediates, chemical species that are produced and consumed during the reaction, but that do not appear in the overall reaction.

• Each individual reaction in the mechanism is called an elementary step or elementary reaction.
• One of the reaction steps is slower than the rest and is the rate-determining step.

ACTIVATION ENERGY

• T is the Kelvin temperature, and Ea is the activation energy, the minimum amount of energy that is needed to initiate or start a chemical reaction.

RATION MECHANISM

• Many reactions proceed from reactants to products through a sequence of reactions. This sequence of reactions is called the reaction mechanism.

HOMOGENEOUS CATALYST

• Homogeneous catalysts are catalysts that are in the same phase or state of matter as the reactants. They provide an alternative reaction pathway (a mechanism) with lower activation energy.
• The decomposition of hydrogen peroxide is a slow, one-step reaction, especially if the solution is kept cool and in a dark bottle:
• 2 H2O2 → 2 H2O + O2
• However, if ferric ions are added, the reaction speeds up tremendously. The proposed reaction sequence for this new reaction is:
• 2 Fe3^+ + H2O2 → 2 Fe2^+ + O2 + 2 H^+
• 2 Fe2^+ + H2O2 + 2 H^+ → 2 Fe3^+ + 2 H2O

HETEROGENEOUS CATALYST

• A heterogeneous catalyst is in a different phase or state of matter from the reactants.
• Most commonly, the catalyst is a solid and the reactants are liquids or gases.
• These catalysts lower the activation energy for the reaction by providing a surface for the reaction, and also by providing a better orientation of one reactant so its reactive site is more easily hit by the other reactant

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