Physical 5: Kinetics

Question 1

What is collision theory?

Answer 1

Reactions can only occur when collisions take place between particles with sufficient energy

Question 2

What is activation energy?

Answer 2

The minimum energy needed to break bonds and therefore start a reaction

Question 3

What are the three necessary conditions for particles to react?

Answer 3

1) Collision
2) Activation energy
3) Correct orientation

Question 4

Describe the Maxwell-Boltzmann graph.

Answer 4

X-axis: Energy
Y-axis: Number of particles
The turning point of the graph represents the most probable energy (Emp)
Slightly to the right of the turning point is the average energy
Activation energy is further to the right of the average energy

Question 5

How does increasing temperature affect the Maxwell Boltzmann graph?

Answer 5

There are more particles with energy greater than the activation energy
So there is an increased frequency of successful collisions

Question 6

How does decreasing temperature affect the Maxwell Boltzmann graph?

Answer 6

There are less particles with energy greater than the activation energy
So the frequency of successful collisions decreases

Question 7

Why does the line never touch the x-axis on a Maxwell Boltzmann graph?

Answer 7

Because there are no particles have no energy

Question 8

What is the difference between most probable energy and average energy?

Answer 8

Most probable energy: The energy that the particles in the system are most likely to have
Average energy: An average of all particles in the system

Question 9

What are the 5 factors that affect rate of reaction?

Answer 9

1) Temperature
2) Concentration of solutions
3) Gas pressure
4) Surface area of solids
5) Presence of a catalyst

Question 10

How does increasing temperature affect rate of reaction?

Answer 10

Increasing temperature increases the kinetic energy of the particles
Meaning that more particles have the activation energy required to react
So increases the frequency of successful collisions

Question 11

How does increasing concentration of solution affect rate of reaction?

Answer 11

Increased concentration means there are more particles per volume
This increases the frequency of collisions
More collisions means there is a greater chance for a successful collision where the particles have sufficient activation energy

Question 12

How does increasing gas pressure affect rate of reaction?

Answer 12

Increasing gas pressure means there are more particles per volume
This increases the frequency of collisions
More collisions means there is a greater chance for a successful collision where the particles have sufficient activation energy

Question 13

How does increasing surface area of solids affect rate of reaction?

Answer 13

Increasing surface area means that there are more particles exposed on the surface
So more particles are available to collide
So the frequency of successful collisions increases

Question 14

How does the presence of a catalyst affect rate of reaction?

Answer 14

Adding a catalyst lowers the activation energy for a reaction
Meaning there are more particles that have the minimum activation energy required to react
So frequency of successful collisions increases

Question 15

What is a catalyst?

Answer 15

A substance that increases the rate of reaction without being used up by providing an alternative pathway with a lower activation energy

Question 16

Why don’t catalysts affect enthalpy change of reactions or position of equilibrium?

Answer 16

Because catalysts are not used up in the reaction
They increase the forward and reverse reactions in equilibrium equally, so don’t affect the overall position

Question 17

Why is it cheaper to use catalysts in industry as opposed to changing pressure or temperature?

Answer 17

Catalysts are not used up, so can be reused again and again
Increasing pressure and temperature can require expensive equipment

Question 18

Give examples of catalysts and the reactions they’re used in.

Answer 18

1) Haber process: iron catalyst, used to make fertilisers
2) Cracking hydrocarbons: Aluminium oxide and silicon dioxide zeolite catalyst, used to make petrol
3) Hydration of ethene: H+ catalyst, used to make ethanol

Question 19

What would happen to a Maxwell Boltzmann graph if you reduced the volume of the container?

Answer 19

There would be no change in number of molecules or activation energy
Graph would stay the same

Question 20

Why does the line on a Maxwell Boltzmann graph start at the origin?

Answer 20

Because there are no particles with no energy

Question 21

What would happen to a Maxwell Boltzmann graph if half of the molecules of the sample were removed?

Answer 21

The peak would be at the same energy
But the overall area of the graph should be half the original area

Question 22

Magnesium reacts with 30cm3 of 1moldm3 hydrochloric acid
The reaction is repeated using 20cm3 of 2moldm3 hydrochloric acid
How would the volume of hydrogen produced change?

Answer 22

2/3 of volume
Doubled concentration
2/3 x 3 = 2
2 x 2 = 4
Answer is B

Question 23

Describe the average kinetic energy of gas molecules.

Answer 23

Constant at a given temperature