Collision theory and rates of reaction

Question 1

What is the collision theory?

Answer 1

• In order for a chemical reaction to occur between two or more particles, they must collide
• Not all collisions result in a chemical reaction

Question 2

What two factors must exist for a collision to result in a chemical reaction?

Answer 2

• Reactant particles must collide with the correct orientation, must collide head on
• Reactant particles must collide with sufficient energy (activation energy, Ea)

Question 3

What happens when the factors are not met?

Answer 3

• The particles will bounce apart without reacting, known as unsuccessful collision
• Collisions can also be unsuccessful when the particles do not have equal or more than the activation energy needed in the reaction

Question 4

What is activation energy (Ea) and why is it needed?

Answer 4

• The minimum amount of energy that colliding particles must have for a chemical reaction to occur
• This is needed to overcome the energy barrier between the reactants and products
• Ea does not change with temperature

Question 5

How do you calculate the activation energy of a energy level profile? How does the activation energy vary between reactions?

Answer 5

• The difference in energy between the reactant and the transition state (peak in the profile)
• In faster reactions, there is a lower activation energy, in slower reactions there is a higher activation energy

Question 6

What is the transition state?

Answer 6

• The highest energy state on a reaction coordinate, the point at which new bonds are formed at the same time as old bonds are broken

Question 7

What is the Maxwell-Boltzmann distribution?

Answer 7

• In an ideal gas, the kinetic energy of the molecule is spread over a range of values (many different kinetic energies)
• The total area beneath the curve is equal to the total number of particles in the sample
• The particles in a certain region below the curve have a wide range of kinetic energies, the most probable value is where the curve is highest
• Essentially, at the same temperature, all gases have the same average kinetic energy

Question 8

What is the trend with temperature of the Maxwell-Boltzmann distribution?

Answer 8

• Temperature increase –> particles gain kinetic energy –> curve flattens out
• Area under each curve is the same
• Most probable value of kinetic energy is the highest point

Question 9

How is the Maxwell-Boltzmann useful with catalysts?

Answer 9

• It helps understand how changes in temperature and a catalyst will affect the rate of a reaction

Question 10

How does the Maxwell-Boltzmann distribution show gases in relation to the molecular speed?

Answer 10

• Comparing different types of gases, at the same temperature, lighter gaseous molecules, on average move faster than heavier gaseous molecules
• The molecular speed depends on the molar mass

Question 11

What is the rate of reaction?

Answer 11

• A measure of the speed of a reaction

- The change in concentration of a reactant or product per unit of time

Question 12

How do you calculate the rate of reaction using the decrease in concentration of the reactant?

Answer 12

Rate = decrease in reactant concentration / change in time

- Is always positive

Question 13

How do you calculate the rate of reaction using the increase in concentration of the product?

Answer 13

Rate = increase in product concentration / change in time

Question 14

What is the instantaneous rate of reaction?

Answer 14

• The rate of reaction at any particular point in time

Question 15

How is the instantaneous rate of reaction determined?

Answer 15

• Seen graphically from a change in reactant/product concentration against time
• On a graph the instantaneous rate at any given time is equal to the gradient of the line at that point in time
• Drawing a tangent to the curve at that time and calculating the gradient = ΔY/ΔX
• The steeper the gradient the faster the rate of reaction

Question 16

How is the average rate of reaction calculated?

Answer 16

• Dividing the change in concentration of a reactant/product by the time taken for the change to occur
• It is the rate of reaction averaged over a certain time period (straight line on the graph), same gradient calculation

Question 17

What factors can affect the rates of reaction?

Answer 17

• Temperature
• Concentration
• Pressure
• Surface area

Question 18

How does temperature affect the rate of reaction?

Answer 18

• Increased temperature makes particles gain kinetic energy, they have a higher velocity
• Frequency of collisions increases
• A greater proportion of reactant particles will collide with energy equal or greater than the Ea

Question 19

How is the affect of increased temperature represented on a Maxwell-Boltzmann distribution?

Answer 19

• Using a Maxwell-Boltzmann distribution, only the particles in the area underneath the flat part of the curve, have sufficient energy to react
• Increased temperature causes the peak of distribution curve shifts to the right, meaning at higher temperatures, the average kinetic energy increases
• Also the curve flattens so the total area under the curve remains constant
• The area under the curve increases to the right of the activation energy value

Question 20

How does concentration affect the rate of reaction?

Answer 20

• Increasing concentration of a solution increases rate of reaction because of the increased frequency of collisions between the substances
• Increased collisions results in a greater probability of successful collisions between the reactant particles, increase rate of reaction

Question 21

How does pressure affect the rate of reaction?

Answer 21

• Increasing the pressure of gases means more particles in a given volume, increasing the concentration
• Increase in the frequency of collisions between reactant particles and increase rate of reaction

Question 22

How does surface area affect the rate of reaction?

Answer 22

• Breaking up a solid, increases SA:V ratio causing increase in rate, more space to react on
• Smaller particles have larger surface area

Question 23

How can you measure a change in concentration during an experiment?

Answer 23

• Monitoring the change in mass of the product or reactant, volume or any colour changes too

Question 24

What are catalysts?

Answer 24

• They allow reactions to take place at lower temperatures and pressures
• They essentially increase the rate of chemical reactions
• Used in many industrial processes, they have a positive effect on the economics and safety of these processes

Question 25

How do catalysts increase the rate of reactions?

Answer 25

• They provide an alternative reaction pathway that has a lower activation energy than the uncatalysed pathway
• However the original activation energy for the reaction remains unchanged
• The catalyst remains chemically unchanged at the end of the reaction
• Be able to draw it on Maxwell-Boltzmann curve and in energy profiles