8. Reaction Kinetics

Question 1

What is the rate of reaction?

Answer 1

• The rate at which reactants are used up or products are produced
• It is measured in mol dm⁻³ s⁻¹, as it measures concentration change

Question 2

What factors are required for a collision to be successful?

Answer 2

• Correct orientation (of either the atoms or the molecules reacting)
• Sufficient energy (activation energy)
• Correct reactants

The frequency of successful collisions per unit volume determines the rate of reaction

Question 3

How does temperature affect rate of reaction?

Answer 3

• Increasing temperature increases rate of reaction
• This is because particles have more kinetic energy so move faster
• This increases the frequency of collisions per unit volume as there is more movement so a higher likelihood of collision
• These collisions also have a higher probability of being successful as the energy of most particles will equal or exceed the activation energy

Question 4

How does pressure affect rate of reaction?

Answer 4

• If the reactants are gases, increasing pressure increases the rate of reaction
• This is because there are more particles per unit volume, so the frequency of collisions will be higher

The concept is the same for increasing the concentration of reactants, though concentration also applies to liquid and aqueous reactants

Question 5

How does the rate of a reaction change over time if all external factors are kept constant and how can the rate of such a reaction be obtained graphically?

Answer 5

• It decreases as the concentration of reactants decreases over time
• A tangent can be drawn on a curve showing concentration of reactants (or products) against time to find the rate of reaction
• The rate of reaction should be directly proportion to the concentration of products

Question 6

What does a Boltzmann distribution show?

Answer 6

• A curve showing molecular energy against number of molecules
• It usually shows how there are a few molecules at low energy levels, a few at high energy levels, and many in between
• Activation energy is usually marked on the curve to show how only a few particles have energy levels that exceed the activation energy

Question 7

How does an increase in temperature affect the Boltzmann distribution of a reaction mixture?

Answer 7

• It flattens and the peak shifts to the right
• This shows that the average energy of the particles has increased
• It also demonstrates how an increase in temperature increases the proportion of particles with energy levels that exceed the activation energy

Question 8

How does adding a catalyst change the Boltzmann distribution of a reaction mixture?

Answer 8

• There will be no change to the curve
• However, the activation energy will lower so the proportion of molecules past this threshold will increase

Question 9

What is the difference between a homogeneous and a heterogeneous catalyst?

Answer 9

• Homogeneous catalysts are in the same phase as the reactants
• Heterogeneous catalysts are in a different phase to the reactants (e.g. gaseous reactants and solid catalyst)

If a catalyst is in the same state as some of the reactants but a different state to others, it is considered heterogeneous

Question 10

How do catalysts lower the activation energy of a reaction?

Answer 10

• They can provide alternate reaction pathways that involve intermediaries that react with lower activation energies
• The catalyst itself (especially if it is homogeneous) may form a reactive intermediate, but will always be regenerated
• Heterogeneous catalysts (particularly ones in the solid phase) adsorb reactants and weaken their bonds, lowering the activation energy and forming products on the surface of the catalyst