Kinetics

Question 1

What does the term activation energy mean? (1)

Answer 1

The minimum energy needed for a reaction to occur / start

Question 2

Why might a reaction occur very slowly? (1)

Answer 2

A small number of particles have E≥Ea

Question 3

Why do most collisions not cause a reaction? (1)

Answer 3

A small number of particles have E≥Ea

Question 4

If a reaction occurs very slowly because only a small number of particles have E≥Ea, why will all of the reactants eventually gain enough energy to react? (2)

Answer 4

Molecules gain energy
Due to collisions

Question 5

Why do some particles only have a very small amount of energy? (1)

Answer 5

Collisions
Cause some molecules to slow down or lose energy

Question 6

What is the rate of reaction? (2)

Answer 6

The rate of change in concentration
per unit of time

Question 7

What factors affect the rate of a reaction? (5)

Answer 7

Temperature
Concentration
Pressure (for gas based reactions)
Catalysts
Surface area

Question 8

How does an increase in temperature affect the rate of reaction? (2)

Answer 8

More particles have E≥Ea
Increases frequency of successful collisions

Question 9

How does an increase in concentration affect the rate of reaction? (2)

Answer 9

Increase in number of particles per unit volume
Increases frequency of successful collisions

Question 10

How does an increase in pressure affect the rate of reaction? (2)

Answer 10

Increase in number of particles per unit volume
Increases frequency of successful collisions

Question 11

How does an increase in surface area affect the rate of reaction? (2)

Answer 11

Increased number of reactant particles made available
Increases frequency of successful collisions

Question 12

How does a catalyst affect the rate of reaction? (2)

Answer 12

Lowers Ea
So more particles with E≥Ea
Increases frequency of successful collisions

Question 13

Why must the Maxwell-Boltzmann curve start at the origin? (1)

Answer 13

No particles can have zero energy

Question 14

What labels does the x-axis and y-axis of Maxwell-Boltzmann curve have? (2)

Answer 14

X-axis: Energy
Y-axis: number of particles/molecules with given energy

Question 15

What does the peak of the Maxwell-Boltzmann curve show? (1)

Answer 15

Most probable value for the energy of the particles (Emp)

Question 16

Why does the area under the Maxwell-Boltzmann curve show? (1)

Answer 16

Total number of particles

Question 17

Why does the area under the Maxwell-Boltzmann curve but to the right hand side of Ea show? (1)

Answer 17

Particles with enough energy to react

Question 18

Where is the ‘mean energy of particles’ on a Maxwell-Boltzmann curve? (1)

Answer 18

A line where the area under the curve on either side of the line is equal

Question 19

How to draw the curve for a higher temperature on a Maxwell-Boltzmann curve? (1)

Answer 19

The High T peak must be further to the right and lower
The curves must only cross once
High T curve is wider/broader
The size of the shaded area (Ea) is larger for High T

Question 20

Why does a small increase in temperature affect the rate of reaction? (1)

Answer 20

A small increase in temperature leads to a significantly large number of particles with the activation energy.

Question 21

How to draw the curve for a lower temperature on a Maxwell-Boltzmann curve? (1)

Answer 21

The Low T peak must be further to the left and higher
The curves must only cross once
Low T curve is narrower
The size of the shaded area is smaller for Low T

Question 22

How does the Maxwell-Boltzmann curve change at a higher concentration/pressure? (1)

Answer 22

The Emp and Ea would stay the same
The total area under the graph would increase
(Graph would have exact same shape but be above the first one)

Question 23

Why do the majority of chemical reactions start off with a fast rate of reaction and then slow down before the reaction stops? (3)

Answer 23

Initially there is a high concentration of reactants so a large frequency of successful collisions.
As the reactants start to get used up successful collisions become less frequent.
Once one or more reactants are used up the can be zero successful collisions