A-Level Chemistry OCR Notes

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Collision Theory

Particles in a gas or liquid are constantly moving and colliding with each other. However, not all collisions result in a reaction.
For a reaction to occur, two particles must collide with:
- Sufficient energy to overcome the activation energy
- The correct orientation

Maxwell-Boltzmann Distribution

The Maxwell-Boltzmann curve shows the distribution of molecular kinetic energies in a gas at a constant temperature
In the Boltzmann distribution:
- The area under the curve is equal to the total number of molecules in the system
- The curve starts at the origin – no molecules have zero energy
- Only molecules with an energy greater than the activation energy, Ea , can react
- The curve’s peak represents the most probable energy that a molecule will have
- The average energy is to the right of, and slightly greater than, the most probable energy

Rate of Reaction

The rate of reaction is the change in the concentration of a reactant or product in a given time.

Factors affecting the rate of reaction:
- Temperature- increasing the temperature increases the kinetic energy of the molecules, leading to more frequent successful collisions and an increase in the rate of reaction
- Pressure – increasing the pressure of a gaseous reaction increases the number of gaseous molecules in a given volume, so molecules are closer together. This leads to more frequent successful collisions and an increase in the rate of reaction
Concentration- increasing the concentration of an aqueous reactant increases the number of molecules in a given volume, so molecules are closer together. This leads to more frequent successful collisions and an increase in the rate of reaction
Catalysts- adding a catalyst provides an alternative pathway that has a lower activation energy, leading to more frequent successful collisions and an increase in the rate of reaction

Catalysts

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.
Catalysts work by reacting with the reactants to form an intermediate. Catalysts are then regenerated later in the reaction
Homogeneous catalysts are in the same phase as the reactants
Heterogenous catalysts are in a different phase to the reactants
Heterogeneous catalysts work by:
- Adsorption- the reactants forming weak bonds with the atoms on the surface of the catalyst, holding the reactants in the correct position for them to react.
- Desorption- the products detach from the atoms on the surface of the catalyst
- As the products detach more reactants can be adsorbed, and the process is repeated
The use of catalysts has economic, environmental and social benefits
- The demand for fossil fuels is lowered
- Emission of pollutants are lowered
- Reduced energy use means lower costs

The Effect of Temperature and Catalysts on The Maxwell-Boltzmann Distribution

At higher temperatures, the kinetic energy of the molecules increases, so the molecules move faster
A greater proportion of molecules have an energy greater than the activation energy, so more frequent successful collisions occur, increasing the rate of reaction.
The area under the curve remains the same as the number of molecules in the system remains the same

Catalysts lower the activation energy of a reaction by providing an alternative reaction route with a lower activation energy
A greater proportion of molecules have an energy greater than the new, lower activation energy, and more frequent successful collisions occur, increasing the rate of reaction.
The addition of the catalyst does not change the distribution of the molecular energies.

Calculating the Rate of Reaction

The rate of a reaction can be calculated by recording the amount of a reactant or a product at regular time intervals during a single reaction (continuous monitoring) or by determining the initial rate of several reactions and finding an average.
There are several methods of continuous monitoring
- Monitoring by gas collection – use a syringe to measure the volume of gas evolved
- Mass loss – set up the system on a balance so the decrease in mass of reactants can be measured
- Colorimetry – concentration change is monitored by measuring the amount of light absorbed by a sample
A graph can be plotted using the variable measured against time:
The gradient of the graph is equal to the rate i.e.
- y/x = concentration change/time = rate

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