## 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. 