4.5 Matter Hot or Cold

Question 1

The average thermal energy per particle is very roughy equal to what?

Answer 1

kT

Question 2

What is the approximate thermal energy per particle at 1K?

Answer 2

1 x 10^-23 J

Question 3

What is the approximate thermal energy per mole at 300K?

Answer 3

2000 J mol^-1

Question 4

What is the approximate thermal energy per particle at 6000K (The temperature of the surface of the Sun) in eV?

Answer 4

0.5 eV

Question 5

Particles are held together by bonds, give the symbol for activation energy required to break these bonds.

Answer 5

ε

Question 6

What happens when kT is big enough, compared with ε?

Answer 6

Bonds are broken and matter comes apart. (Less specifically, “stuff happens”)

Question 7

What is activation energy?

Answer 7

The energy required for a process to start.

Question 8

Explain activation energy in terms of change of state.

Answer 8

Particles need enough energy to break attractive forces between them.

Question 9

Explain activation energy in terms of thermionic emission.

Answer 9

When you heat a conductor, electrons are released from its surface. These electrons require enough energy to escape from the attraction of the positive nuclei.

Question 10

Explain activation energy in terms of ionisation in a candle flame.

Answer 10

The molecules in the air need enough energy to split up into individual atoms and ions.

Question 11

Explain activation energy in terms of conduction in a semiconductor.

Answer 11

Semiconductors only conduct when there are electrons in the high energy state called the conduction band. Electrons need enough energy to jump from the ground state to this state.

Question 12

Explain activation energy in terms of viscous flow.

Answer 12

Particles need enough energy to overcome the attractive forces between them that causes the fluid to be viscous.

Question 13

What range of values of ε/kT give a fair rate of reaction.

Answer 13

15-30

Question 14

Why does such a small fraction of ε/kT still give a fair rate of reaction.

Answer 14

Very large numbers of particles collide each second, so some will end up with sufficient energy to react.

Question 15

If f represents the fraction of particles with E more energy than kT (they have an energy of kT + E) what fraction of particles have an energy of kT + 3E

Answer 15

f^3

Question 16

What does the Boltzmann factor tell you?

Answer 16

The ratio of the number of particles in energy state kT + ε compared with the energy state kT.

Question 17

What is the Boltzmann factor?

Answer 17

e^(-ε/kT)

Question 18

What is the Boltzmann factor when ε/kT = 15?

Answer 18

10^-7

Question 19

How often do gas particles collide?

Answer 19

Around 10^9 times every second.

Question 20

Describe the graph of Boltzmann factor plotted against temperature.

Answer 20

An s shared curved, starting at the origin with a point of inflection at ε=kT.

Question 21

What happens to the graph of Boltzmann factor against temperature when activation energy is higher?

Answer 21

The graph is stretched horizontally.

Question 22

The rate of reaction with activation energy ε is proportional to what?

Answer 22

The Boltzmann factor.