Kinetics

Question 1

Activation energy (Ea)

Answer 1

The minimum energy needed for a reaction to start

Question 2

What must happen for a reaction to occur

Answer 2

Particles must collide With E≥Ea

Question 3

Why might a reaction occur very slowly?

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?

Answer 4

Molecules gain energy Due to collisions

Question 5

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

Answer 5

• Collisions
• Cause some molecules to slow down or lose energy

Question 6

What is the rate of reaction?

Answer 6

• The rate of change in concentration
• per unit of time

Unit: mol dm-3 s-1

Question 7

Explanations for rate increases:

Answer 7

Temperature increase:
More particles have E≥Ea
Increases frequency of successful collisions

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

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

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

Surface area increased:
Increased number of reactant particles made available
Increases frequency of successful collisions

Question 8

Maxwell–Boltzmann distribution

Answer 8

A- Line must start at the origin (0,0) as no particles can have zero energy
B- The peak of the curve shows most probable value for the energy of the particles (Emp)
C- The area under the curve shows the total number of particles
D- The area under the curve, but to the RIGHT HAND SIDE of Ea (E≥Ea) represents the particles with enough energy to react
E- The line levels out towards the x axis, but never touches it
F- The mean energy of particles is a line where the area under the curve on either side of the line is equal.

The distribution shows that only a small fraction of particles can collide with enough (E≥Ea) to start a reaction/break bonds

Question 9

Effect of increasing temperature on reaction rate

Answer 9

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

Question 10

Effect of decreasing temperature on reaction rate

Answer 10

At low temperature/when temperature decreases the rate of reaction decreases because:

• The proportion of particles with E≥Ea decreases
• so less frequent successful collisions

Question 11

Effect of concentration and pressure

Answer 11

An increase in concentration (or pressure in gases) increases the rate of reaction because:

• the number of particles per unit volume increases
• so more frequent successful collisions

The rate of reaction doubles when concentration doubles.
- the number of particles per unit volume doubles
- so the number of frequent successful collisions also doubles

Question 12

Catalysts

Answer 12

A catalyst is a substance that increases the rate of reaction but is not used up or changed

Works by providing an alternative reaction pathway that has a lower activation energy

Question 13

How do chemical reactions work?

Answer 13

• 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.

Question 14

Plotting concentrations of reactants and products on graphs.

Answer 14

A + 2B → C

• rate of reaction can be estimated from the gradient
• steep gradient shows the rate is fast
• shallow gradient shows the rate is slow.