Learning Objectives

  • Understand the effects of surface area of solid reactants, temperature, catalysts, concentration and pressure on rate of reaction
  • Understand that the average energy of particles in a gas is proportional to its temperature in kelvin
  • Sketch the Maxwell–Boltzmann distribution and use it to explain the effect of a change in temperature on the rate of a reaction and how a catalyst speeds up a reaction

Main factors:

  • Concentrations of reactants (the number of colliding particles in unit volume)
  • Pressure for gas reactions (the number of colliding particles in unit volume)
  • Temperature
  • Surface area of solid reactants
  • Catalyst

CONCENTRATION or PRESSURE

The particles collide more often (the collision frequency is higher) with more particles in a certain volume and therefore there is greater chance of a successful collision (i.e. one that results in a reaction) occurring in a certain time.

For aqueous solutions:

For Gases:

For the reaction of calcium carbonate with hydrochloric acid:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

TEMPERATURE

As the temperature is increased,

  • average kinetic energy of particles increases,
  • average velocity of particles increases,
  • the number of reacting particles possess the necessary activation energy increases.
  • the rate of reaction increases.

SURFACE AREA OF SOLID REACTANT

Reactions generally only occur at the surface of a solid. Making a solid more finely crushed increases the surface area and therefore the number of particles exposed at the surface.

Then, there is a greater chance of the other reactant colliding with this particle on the surface and reaction occurring.

CATALYST

A catalyst

  • is a substance that increases the rate of reaction without itself being used up in the reaction.
  • acts by allowing the reaction to proceed by an alternative pathway of lower activation energy.
  • is often written above the reaction arrow and does not appear in the chemical equation because it does not change in the reaction.

When a catalyst is used in a particular reaction, the Maxwell–Boltzmann distribution curve also shows how a lower activation energy results in a faster reaction.

Collision Theory
Rate of Reaction