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). 

6.2.2. & 6.2.3. I can define activation energy and describe collision theory using three factors which affect the rate of reaction

Definition of activation energy	Minimum value of kinetic energy which particles must have before they're able to react.
Three factors that affect the rate of reaction	1. Collision frequency
	2. Number of particles with a greater kinetic energy than the activation energy
	3. Collision geometry or orientation

6.2.2.   Define the term activation energy

Activation energy is defined as the minimum value of kinetic energy which particles must have before they're able to react.

6.2.3.    Describe the collision theory

The rate of reaction will depend on the frequency of collisions which occur between particles possessing both:

• Values of kinetic energy greater than the activation energy
• Appropriate collision geometry

When reactants are laced together, the kinetic energy that the particles possess causes them to collide with each other. The energy of these collisions results in some bonds between the reactants being broken and new bonds being formed. 

The rate of the reaction will depend on the number of collisions between particles which are successful - which lead to the formation of products. Not all collisions will be successful and there are two reasons for this: energy of collision and geometry of collision. 

1. Energy Of Collision

The particles must have a certain minimum value for their kinetic energy in order for a collision to lead to a reaction. This energy is necessary to overcome repulsion between molecules and to break some bonds in the reactants before they can react. 

When this energy is supplied, the reactants achieve the transitions state from which products can form. The energy required represents an energy barrier for the reaction and is known as the activation energy.

The value of activation energy varies greatly from one reaction to another and the magnitude of this value plays an important part in determining the overall rate of reaction. The rate of the reaction depends on the proportion of particles that have values of kinetic energy greater than the activation energy.

2. Geometry Of Collision

Because collisions between particles are random, they're likely to occur with the particles in many different orientations. This can be crucial in determining whether or not the collisions will be successful and therefore what proportion of collisions will lead to a reaction.

6.2.1. I can describe kinetic theory

Temperature in Kelvins is proportional to the average kinetic energy of the particles in a substance. 

Particles in a substance move randomly as a result of the kinetic theory they possess. Because of the random nature of these movements and collisions, not all particles in a substance have the same value of kinetic energy. The average of these values is taken and is related directly to its absolute temperature. 

Increasing temperature therefore means an increase in the average kinetic energy of the particles of a substance. As a substance is supplied with extra energy through heating it, the average kinetic energy of the particles as well as the temperature is raised. 

Using the kinetic theory apparatus we were able to see how particles, represented by the ball bearings, move randomly as a result of the kinetic theory they possess. To increase the voltage would be like increasing the temperature of the substance, this resulted in an evident increase in the average kinetic energy of the ball bearings. 

6.1.3 Analyze data from rate experiments

Rates Of Reaction: Reaction Of Calcium Carbonate With HCl

Experiment 1

Volume of HCl = 25 cm³

^{Mass of Calcium Carbonate = 1 g}

^{Experiment 2}

Mass of Calcium Carbonate = 1g