3.1.5 Kinetics

Collision Theory

  • Particles constantly move in random directions

  • Molecules must collide in order for them to react together

Activation Energy = The minimum amount of energy needed for a reaction to start/occur

In order for a reaction to occur there must be:

  • Particles must collide

  • With Energy greater than the activation energy (E>Ea)

Rate of Reaction

The rate can only be influenced by:

  • Temperature

  • Concentration

  • Pressure (gases)

  • Catalysts

  • Surface area (solid reactants - as surface area increases the rate of reaction increases)

Temperature Increase = More particles with energy greater than activation energy and increases the frequency of successful collisions

Concentration increase = Increase in number of particles per unit volume and increases the frequency of successful collisions

Pressure Increase = Increases the number of particles per unit volume and increases frequency of successful collisions

Catalyst added = Lowers activation energy so more particles have energy greater than the activation energy and increases the frequency of successful collisions

Surface area increased = Increased number of reactant particles available and increases the frequency of successful collisions

3.1.5.2 Maxwell-Boltzmann distribution of molecular energies

  • Particles in liquid/solution/gas have a range of kinetic energies

  • The range of energies of particles in a gas is represented by the Maxwell-Boltzmann distribution

The distribution shows that only a small fraction of particles can collide with enough energy to start a reaction/break bonds - overcome activation energy and form products

3.1.5.3 Effect of temperature on reaction rate

Increasing the temperature:

  • At high temp the rate of reaction increases because

    • The proportion of particles with energy greater than the activation energy increases

    • So more frequent successful collisions

Decreasing temperature:

  • At low temp/when the temp decreases the rate of reaction decreases because

    • The proportion of particles with energy greater than the activation energy decreases

    • So less frequent successful collisions

3.1.5.4 Effect of concentration & pressure

  • Increase in concentration (or pressure in gases) increases the rate of reaction because:

    • The number of particles per unit volume increases

    • So more frequent successful collisions

  • The rate of reaction doubles when concentration doubles because:

    • The number of particles per unit volume doubles

    • So the number of frequent successful collisions also doubles

  • The Emp and Ea would stay the same

3.1.5.5 Catalysts

Catalyst - A substance that increases the rate of reaction and is not used up or changed

A catalyst provides an alternative reaction pathway that has a lower activation energy

A catalyst increases the rate of reaction because:

  • The activation energy decreases

  • So more of the particles have a collision energy greater than the activation energy

  • So more frequent successful collisions