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Reaction Rates (Chemical Kinetics)

Key Concepts

  • The rate of a chemical reaction is the speed with which reactants are converted to products.

  • Collision Theory is used to explain why chemical reactions occur at different rates.

  • Collision Theory states that in order for a reaction to proceed, the reactant particles must collide.
    The more collisions there are per unit of time, the faster the reaction will be.

  • In order for a reaction to proceed, the reactant particles must:

  1. collide with sufficient energy to break any bonds in the reactant particles.
    The activation energy is the minimum amount of energy the colliding reactant particles must have in order for products to form.

  2. be in an orientation favourable for breaking those bonds.

  • Factors that affect reaction rates:

  1. concentration of reactants
    Increasing the concentration of reactants in solution increases the number of reactant particles which increases the number of collisions so the reaction rate increases.
    Increasing the pressure of a gaseous reaction by adding more reactant gas particles increases the number of collisions so the reaction rate increases.
    Increasing the pressure of a gaseous reaction by reducing the volume of the reaction vessel increases the number of collisions so the reaction rate increases.

  2. temperature
    Increasing the temperature of a reaction increases the kinetic energy of the particles which increases the number of collisions so the reaction rate increases.
    Increasing the kinetic energy of reactant particles also means more of the reactant particles will have the minimum amount of energy required to form products (ie, activation energy) which leads to more successful collisions and therefore increases the reaction rate.
    Increasing the temperature will increase the reaction rates of both endothermic and exothermic reactions, it will also, by Le Chetalier's Principle, affect the equilibrium position.

  3. particle size
    Smaller reactant particles provide a greater surface area which increases the chances for particle collisions so the reaction rate increases.

  4. presence of a catalyst
    A catalyst lowers the activation energy for the reaction so more reactant particles will have the minimum amount of energy required to form products so the reaction rate increases.

  5. rate of stirring
    Stirring keeps reactant particles in motion increasing the chances of collision and increasing the rate of reaction.

  6. intensity of light affects some reactions
    Some reactions occur very slowly in the dark but much more quickly in light.
    eg, methane reacts very slowly with chlorine in the dark, but the rate of reaction is much faster in the presence of ultraviolet light.

Measuring Reaction Rates

How a reaction rate is measured depends on the nature of the reactants and products.

Some measurable quanitities are:
  • the volume of gas evolved per unit time

  • the mass of solid formed per unit time

  • the intensity of colour per unit time

  • the change in pH per unit time

  • the change in temperature per unit time
Plotting the points of quantity measured vs time produces a graph.
The initial slope of the graph provides you with the initial rate of reaction.
A fast reaction will have a steeper slope than a slower reaction.

Reaction rates slow down as the reaction approaches equilibrium.
As products are formed, there are fewer reactant particles to react which means there will be fewer successful collisions, so, the reaction rate decreases.

Example

Consider this reaction: Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Condition Affect on Rate Explanation
Concentration Increasing the concentration of HCl will increase the reation rate. More HCl particles means there will be more collisions between HCl and Zn.
Temperature Increasing temperature increases the reaction rate. HCl particles will gain more kinetic energy increasing the number of collisions with Zn atoms.
More Zn and HCl particles will have sufficient energy to react resulting in more successful collisions.
Particle Size Reducing the size of Zn particles will increase the rate of reaction. Reducing the size of the Zn particles increases the surface area available for reaction with HCl molecules resulting in more collisions.
Stirring Rate Increasing the stirring rate of this mixture will increase the reaction rate. Stirring will keep small Zn particles in suspension, increasing the surface area available for collisions, resulting in an increased reaction rate.


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