WJEC AS Chemistry Unit 2.2 - Rates of Reaction

Collision Theory

• For an effective collision to occur, the particles collide with;

  • The minimum amount of energy (greater than or equal to Ae)

  • Correct orientation

• Any factor which increases the frequency of effective collisions increases reaction rate

Calculating rates

Change in x/time

Calculating rate from a graph

Gradient

Factors that affect the rate of reaction

• Concentration

• Temperature

• Particle size

• Catalysts

• Light

How concentration affects the rate

• Increasing conc/pressure increases the rate of reaction

• More molecules in a given volume. Distances between molecules are reduced so there is an increase in the number of collisions per unit time. Greater chance that the number of effective collisions increases, the rate of reaction increases

How temperature increases the rate of reaction

• increasing temp increases the rate of reaction

• Increase in KE of the molecules and so they will move faster. This means that more molecules will have enough energy to react on collision. Reaction rate will increase

How particle size affects rate

• reducing size increases the SA of the molecules

• Molecules closer together

• Increase in number of collisions per unit time

• Reaction rate will increase

How catalysts affect rate

Increases the rate of chemical reaction without undergoing a permanent change itself

How light affects rate

Some reactions are much more vigorous when carried out in bright light

Activation energy

The minimum energy required to start a reaction by breaking of bonds

Boltzmann energy distribution curve

Boltzmann energy distribution curve at a higher temperature

• curve moves the right towards higher energies

• Curve is flatter

• At the higher temperature, more molecules have energy greater than or equal to Eact so the frequency of effective collisions increases and so does the rate

Catalysts

• Provide an alternative route with a lower activation energy

• Substance that increases the rate of a chemical reaction without being used up in the process

Energy profile diagram with catalyst

Boltzmann energy distribution curve with a catalyst

Types of catalyst

• Homogenous

• Heterogeneous

Homogenous catalysts

Catalysts in the same state as the reactants

Heterogenous catalysts

Catalyst is in a different state from the reactants

Enzymes

• Biological catalysts

Methods of studying rates of reaction

• Change in gas volume

• Change in gas pressure

• Change in mass

• Colorimeter

Colorimetry to study rates

• used when one of the reactants or products in a reaction is coloured, meaning there is a change in colour as the reaction proceeds

• The intensity of the colour at any time is related to the concentration of the coloured substance