[3.1.5] Kinetics

Question 1

What is activation energy?

Answer 1

Activation energy is defined as the minimum energy which particles need to collide to start a reaction.

Question 2

What is the Maxwell-Blotzmann distribution?

Answer 2

The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature.

Question 3

What should the y-axis and the x-axis of a Maxwell-Boltzmann distribution curve be labelled as?

Answer 3

• Y-AXIS = Fraction of molecules with energy.
• X-AXIS = Energy.

Question 4

Label this diagram of a Maxwell-Boltzmann distribution curve.

Answer 4

Question 5

What is the effect of increasing temperature on the Maxwell-Boltzmann distribution curve?

Answer 5

• As the temperature increases, the distribution shifts towards having more molecules with higher energies (i.e. shifts left).
  
  • Both the most probable energy (Eₘₚ) and mean energy shift to higher energy values.
• The total area under the curve should remain constant because the total number of particles is constant.
• At higher temperatures, the molecules have a wider range of energies than at lower temperatures.

Question 6

What is the effect of decreasing temperature on the Maxwell-Boltzmann distribution curve?

Answer 6

• As the temperature decreases, the distribution shifts towards having more molecules with lower energies (i.e. shifts right).
  
  • Both the most probable energy (Eₘₚ) and mean energy shift to lower energy values.
• The total area under the curve should remain constant because the total number of particles is constant.
• At lower temperatures, the molecules have a smaller range of energies than at higher temperatures.

Question 7

How can a reaction go to completion if few particles have energy greater than the activation energy?

Answer 7

Particles can gain energy through collisions.

Question 8

Define the term rate of reaction.

Answer 8

• The rate of reaction is defined as the change in concentration of a substance in unit time.
• The usual unit is mol dm⁻³ s⁻¹.

Question 9

How can we calculate the rate of reaction from a graph?

Answer 9

• Reaction rates can be calculated from graphs of concentration of reactants or products.
• This can be done by drawing a tangent to the curve at the time you wish to know the rate of reaction and calculating the gradient of the tangent.
  
  • Gradient = change in y ÷ change in x

Question 10

What is the effect of increasing temperature on rate of reaction?

Answer 10

• At higher temperatures, the kinetic energy of the particles increases.
• The particles collide more frequently and more often with energy greater than the activation energy.
• Therefore, there is a higher proportion of successful collisions which results in an increased rate of reaction.

Question 11

Use the Maxwell-Boltzmann distribution to explain why a small temperature increase can lead to a large increase in rate.

Answer 11

• As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy.
• So the frequency of successful collisions increases and thus the rate of reaction increases.

Question 12

What is the effect of increasing concentration and pressure on rate of reaction?

Answer 12

• At higher concentrations and pressures, there are more particles per unit volume.
• So the particles collide with a greater frequency and there will be more particles with energy greater than the activation energy so higher frequency of effective collisions.
• Therefore, rate of reaction increases.

Question 13

What is the effect of increasing concentration on the Maxwell-Boltzmann distribution curve?

Answer 13

• If concentration increases, the shape of the energy distribution curves does not change (i.e. the peak is at the same energy) so the Eₘₚ and mean energy do not change.
• The curves will be higher and the area under the curves will be greater because there are more particles.

Question 14

If a question mentions doubling concentration, what should you do?

Answer 14

• Double the number of particles per unit volume.
• Double the frequency of effective collisions.

Question 15

What is the effect of increasing surface area on reaction rate?

Answer 15

• Increasing surface area will cause successful collisions to occur more frequently between the reactant particles.
• This increases the rate of reaction.

Question 16

What is a catalyst? How do they work?

Answer 16

• Catalysts increase reaction rates without getting used up.
• They do this by providing an alternative route or mechanism with a lower activation energy.

Question 17

Use a Maxwell-Boltzmann distribution to help explain how a catalyst increases the rate of reaction.

Answer 17

• If the activation energy is lower, more particles will have energy greater than the activation energy.
• So there will be a higher frequency of successful collisions.
• Therefore, the rate of reaction increases.

Question 18

Use this diagram and annotate it to explain how you would compare these rate curves.

Answer 18