CAPE CHEMISTRY UNIT 1 MODULE 3
KINETICS & EQUILIBRIA
OBJECTIVES
- Explain the concepts associated with reaction rates.
- Design and carry out suitable experiments for studying the factors which affect rates of reactions.
- Construct trate equations of the form: Rate = k[A]nm limited to simple cases involving zero, first and second order reactions.
- Deduce the order of reaction from appropriate data.
- Interpret concentration against time and concentration against rate for zero and first oder reactions.
- Perform calculations from rate data.
- Perform simple calculations using half life data.
- Explain the effect of temperature and catalysts on the rate of reaction using Boltzmann distribution of energies (and of collision frequency).
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Concepts & Experiments Associated With Rates Of Reaction
Factors affecting the rate of a chemical reaction are:
- Temperature
- Concentration
- Surface Area
- Catalyst
- Light
The collision frequency depends upon:
- How closely the particles are crowded together, i.e. concentration or pressure.
- How large they are and,
- How fast they are moving which depends on their weight and the temperature.
The probability factor depends on the geometry of the particles and the kind of reaction that is taking place.
Energy factor depends upon temperature and the energy of activation, Eact.
The minimum energy needed to make a reaction take place is called the Activation Energy, Eact.
Concentration
An increase in concentration means there are more reactant molecules in a given volume. This may lead to more frequent collisions and to a faster rate of reaction.
When acid is added to sodium thiosulphate solution, a five colloidal solution of sulphur is formed. If the reacting mixture is placed over a white paper with a mark, the mark soon disappears after a certain time. With varying concentrations of either acid or thiosulphate the time will vary.
Catalysts
Catalyst alters the rate of a chemical reaction. They lower the activation energy for a given reaction ensuring that more are successful; enzymes are catalyst in biological systems. Many industries use different types of inorganic catalyst.
Light
Photosynthesis and photography both involve light sensitive reaction. Chlorophyll, the green pigment in leaves, absorbs radiation in the visible region of the electromagnetic spectrum and uses this energy to synthesize chemicals and provide food for the plant.
Enzymes – The Active Site
Lock & Key Theory
Enzymes are specific for substrates. The substrate must fit into the active site of the enzyme. This fit is referred to as the lock ad key hypothesis. The very precise fit is the reason why enzymes are so specific:
- Salivary amylase to starch (substrate).
- Pepsin to protein.
- Lipase to fat, etc.
The bonding between enzyme and substrate may involve electrostatic attraction, hydrogen bonding and van der Waals forces. The bond formed between the enzyme and the substrates weaken the bond in the substrate which is to be broken and therefore catalyze the reaction. Catalytic reaction is a surface phenomena.
Rates
Rate constant is defined as the proportionality constant between the rate of a reaction and the concentration of the species that appear in the rate law, raised to the approximate power.