6.2.4. Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction

Factors affecting rate of reaction:

Temperature

Increasing the temperature increases the rate of all reactions. This is because temperature is a measure of the average kinetic energy of the particles and so a higher temperature represents an increase in their average kinetic energy. This means that a larger number of particles will have energies exceeding the activation energy. 

An increase in temperature results in:

• An increase in collision frequency
• More collisions involving particles with higher values of kinetic energy, specifically higher than the activation energy

An increase in the number of successful collisions and hence and increase in the rate of reaction. 

Many reactions double their reaction rate for every 10K increase in temperature. 

Changes in temperature affect collision theory but it should be understood that this is less significant than an increase in the number of particle with sufficient energy (Ea)

Concentration

Increasing the concentration of reactants increases the rate of reaction. This is because as concentration increases, the frequency of collisions between reactant particles increase, so that the frequency of successful collisions also increases. 

The effect of concentration can be seen by following the rate of a reaction as it progresses. As reactants are used up, their concentration falls and the rate of the reaction decreases, giving a typical rate curve. 

Changes in concentration affect the collision frequency only.

Particle Size

Decreasing the particle size increases the rate of reaction. This is because subdividing a large particle into smaller parts increases the total surface area and therefore allows more contact and a higher probability of collisions between reactants. 

In reactions involving solutions, stirring may help to decrease particle so and so increase the rate. 

Pressure

For reactions involving gases, increasing pressure increases the rate of reaction. This is is because the higher pressure compresses the gas, effectively increasing its concentration - this will increase the frequency of collisions.

The Harber process illustrates the effect of high pressure on reactions that involve gaseous reactants

Additional Notes:

• Light (EMR) can promote some reactions e.g. dye fading, photosynthesis, skin tanning, methane/chlorine explosion, skin cancer, photography, vitamin D in skin and hydrogen peroxide photodecomposition (it is kept in brown bottles)

Calcium Carbonate Chips & Hydrochloric Acid Reaction

Independent variable: The surface area of the marble/calcium carbonate chips used

Dependent variable: The volume (cm³) of carbon dioxide produced  

Control variables:

• Volume of HCl acid used
• The concentration of the HCl acid used
• Mass of calcium carbonate used 
• The volume of carbon dioxide produced was noted every 10 seconds 
• The temperature of the water and surroundings

The graph generally shows that the smaller the size of the calcium carbonate chips, the faster the rate of reaction. This is because with smaller calcium carbonate chips there is an increase in the total surface area, which allows more contact with the HCl particles and therefore a higher probability of collision between the two reactants. 

The smallest calcium carbonate chips shows an extremely fast production of carbon dioxide gas in a short amount of time, while chips #4 (the largest calcium carbonate chips) line on the graph has a much smaller gradient as it produces less gas in a greater amount of time. This correlates with the idea that the smaller the particle size, the faster the rate of reaction. However, the powdered calcium carbonate shows a much smaller gradient than expected and produces less gas in a longer amount of time when it was expected to have the fastest rate of reaction. This may be because the powder sticks together causing the pieces to act as one large clump of calcium carbonate, therefore decreasing the total surface area which allows less contact with the HCl particles. 

The marble chips also came from different sources and this could mean that there are different impurities mixed in with the marble. The level of these impurities aren't controlled and could account for the differences in the rate of reaction.

The gradient of the graph at any one point represents the rate of reaction, measured in centimeters cubed of carbon dioxide produced per second (cm³/sec).