Chapter 10 - Reaction rates and equilibrium

Equation for rate of reaction

Rate (mol/dm/s) = change in concentration (mol dm³) / time (s)

Factors affecting rate of reaction

• Concentration

• Pressure

• Temperature

• Catalyst

• Surface area of solid reactants

Define collision theory

Two reactants must collide with enough energy for a reaction to occur

Why are some collisions effective and some ineffective

• Particles have to collide with correct orientation

• Particles have to have sufficient energy to overcome activation energy barrier of the reaction

How does increasing concentration affect rate of reaction

When the concentration of a reactant is increased, the rate of reaction generally increases. An increase in concentration increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given period of time there will therefore be more effective collisions and an increased rate of reaction.

How does increasing pressure of a gas affect rate of reaction

When a gas is compressed into a smaller volume the pressure of a gas is increased and the rate of reaction increases. The concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time.

2 methods for monitoring the progress of a reaction

• Monitoring the removal of a reactant

• Following the formation of a product

Define catalyst

A substance that increases the rate of reaction, by providing an alternative reaction pathway of lower activation energy, without being used up.

Define homogenous catalyst

A homogenous catalyst has the same physical properties as the reactants. The catalyst reacts with the reactants to form an intermediate. The intermediate then breaks down to give the product and regenerates the catalyst.

Define heterogenous catalyst

A heterogenous catalyst has a different physical state to the reactants. Heterogenous catalysts are usually solids in contact with gaseous reactants or reactants in solution. Reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After the reaction, the products leave the surface of the catalyst by desorption.

3 features of Boltzmann distribution

• No molecules = No energy, curve starts at origin

• Area under curve is the total number of molecules

• There is no maximum energy for a molecule, hence curve does not meet the x-axis

How does the Boltzmann distribution change at a higher temperature

• More molecules have energy greater than/equal to the activation energy

• Greater proportion of collisions will lead to reaction so increased rate of reaction

• Collisions will be more frequent as molecules are moving faster, but increased energy of molecules is more important than increased frequency off collisions.