3.2.2 Reaction Rates

Rate

Rate = change in concentration / time

Unit for rate of reaction

mol dm-3s-1

Requirement for particles to react

Collide with sufficient energy (activation energy) and the correct orientation

Collision reaction outcome

No, most collisions do not result in a reaction.

Factors affecting rate of reaction

Temperature, Pressure, Concentration, Surface area, Catalyst

Effect of increasing temperature on rate of reaction

Increasing temperature → increased rate of reaction due to a higher proportion of particles having energy greater than the activation energy, leading to more successful collisions per second.

Effect of increasing concentration/pressure on rate of reaction

Increased concentration/pressure → increased rate of reaction due to more particles in a given volume, resulting in more frequent successful collisions.

Variables to monitor for rate of reaction

Concentration of reactant or product, Gas volume of products, Mass of substances formed.

Calculating rate from concentration time graph

Draw a tangent and work out the gradient of the tangent using the equation Gradient = change in y / change in x.

Catalyst

A substance which increases the rate of reaction but is not used up in the reaction.

How catalysts work

They provide an alternate reaction pathway (with a lower activation energy), resulting in more particles having energy greater than activation energy, leading to more frequent successful collisions.

Homogeneous catalyst

A catalyst that is in the same phase as the reactants, e.g., liquid catalyst mixed with liquid reactants.

Heterogeneous catalyst

Catalyst used in the reaction is in a different phase to the reactants, e.g., gaseous reactants passed over solid catalyst.

Catalytic converters

They are present in vehicles to reduce toxic emission and prevent photochemical smog.

Activation energy

The minimum energy that particles must collide with for a reaction to occur.

Features of Boltzmann distribution

Area under the curve = total number of molecules; Area under the curve does not change when conditions alter; The curve starts at the origin; Curve does not touch or cross the energy axis; Only the molecules with energy greater than activation energy can react.

Axes in a Boltzmann distribution

X axis - energy; Y axis - number of molecules with a given energy.