Thermal

Question 1

What is 1 mole?

Answer 1

A collection of 6.02×10²³ molecules

(Avogadro’s constant)

Question 2

What is the molar mass of a substance?

Answer 2

The mass of each mole (every 6.02×10²³ molecules)

Eg for He each mole has a mass of 4g

Question 3

How do you calculate the molar mass of a compound eg NO₂

Answer 3

Add up the nucleon numbers

(14+16+16=46gmol^-1)

Question 4

How do you calculate the number of molecules in a substance?

Answer 4

N = n × N_A

(Number of molecules = moles × Avogadro’s constant)

Question 5

What is the molecular mass and how is it calculated?

Answer 5

The mass of each molecule of the substance

m = M/N

(molecular mass = total mass / number of molecules)

Question 6

How is the total mass of a substance calculated?

Answer 6

M = n × m_r

(Total mass = moles × molar mass)

Question 7

How do you convert a temperature from °C to K?

Answer 7

T(K) = T(°C) + 273

Question 8

Define absolute zero

Answer 8

The point at which an ideal gas exerts no pressure

(0K, -273°C, molecules have no kinetic energy)

Question 9

What is Boyle’s Law?

Answer 9

The pressure in a gas is inversely proportional to the volume it occupies

at a fixed temperature

and a fixed mass of gas

(P ∝ 1/V)

Question 10

What does the P-V graph look like for an ideal gas?

Answer 10

Question 11

How do you prove Boyle’s law graphically?

Answer 11

Plot a graph of P against 1/V

Should be a straight line passing through the origin

Question 12

What is Charles’ Law?

Answer 12

Volume a gas occupies is directly proportional to the temperature of the gas

at a fixed pressure

and a fixed mass of gas

(V ∝ T)

Question 13

How do you prove Charles’ law by graph?

Answer 13

Plot a graph of V against T

Should be a straight line passing through the origin

Question 14

For an ideal gas, what does a graph of V against T(°C) look like?

Answer 14

Note: x-intercept represents absolute zero

Question 15

What is the Pressure law?

Answer 15

The pressure of a gas is directly proportional to the temperature of the gas

at a fixed volume

and a fixed mass of gas

Question 16

How do you prove the pressure law graphically?

Answer 16

Plot a graph of P against T

Should be a straight line passing through the origin

Question 17

For an ideal gas, what does a graph of P against T(°C) look like?

Answer 17

Note: x-intercept is absolute zero

Question 18

What is the ideal gas relationship?

Answer 18

Question 19

When can you use the ideal gas relationship?

Answer 19

If the mass of the gas is constant

Question 20

How do you calculate the work done compressing or expanding a gas?

Answer 20

Calculate the area under the curve

Question 21

What is the general equation for pressure?

Answer 21

P = F / A

(Pressure = Force / Area)

Question 22

How does a gas exert a pressure on a container?

Answer 22

• The gas molecules collide with the container walls changing their momentum.
• This creates a force on the molecule and the wall
• Exerting a pressure

Question 23

What are the 5 conditions for an ideal gas?

Answer 23

1. Volume of the molecules must be much smaller than the volume of the gas itself
2. The intermolecular forces are negligible
3. The collision time of molecules with each other and the walls is much less than the time between them
4. The collisions are elastic (no loss in KE)
5. The molecules’ motion is random

Question 24

How does Brownian motion explain the random motion of smoke?

Answer 24

• Air molecules are moving randomly
• They collide with the smoke changing momentum and exerting a force on the smoke particles
• If at one moment there are more collisions on one side than the other
• The smoke particle has a resultant force so accelerates in that direction

Question 25

Explain Boyle’s Law using the molecular Kinetic Theory

Answer 25

• When volume of container is decreased
• More collisions per second
• So total momentum change bigger (▲p)
• So force exerted bigger
• So pressure bigger (From P = F/A)

Question 26

Explain Charles’ Law using the molecular kinetic theory

Answer 26

• When temperature is increased
• Volume increases to increase the distance travelled between collisions
• Molecules have greater kinetic energy but travel further so frequency stays same
• Change in momentum (▲p) stays constant
• So pressure is constant (P = F/A)

Question 27

Explain the Pressure law using the molecular kinetic theory

Answer 27

• As temperature increases
• The average kinetic energy of the molecules increases
• Increasing the number of collisions per second with container walls
• So greater change in momentum
• Greater force and pressure exerted (P = F/A)

Question 28

How would you use this equation to work out the density of a gas?

Answer 28

Question 29

How do you calculate c_rms from a list of speeds?

Answer 29

1. Square the speeds and add up
2. Take a mean of the squares
3. Square root the value

Question 30

How is c_ms calculated?

Answer 30

c_ms = (c_rms)²

Question 31

What are the units of c_ms?

Answer 31

[m²s^-2]

Question 32

What does the maxwell-boltzmann distribution tell us about gases?

Answer 32

Molecules have a range of kinetic energies.

So temperature of the gas is a measure of the average kinetic energy.

Question 33

For these equations how do you calculate the internal energy of the gas?

Answer 33

Multiply each by the number of molecules of the gas.

Question 34

How do two objects brought into contact reach thermal equilibrium?

Answer 34

• There is a net flow of thermal energy from the hotter object to the colder object
• Until both objects are at the same temperature
• And there is now no net flow of thermal energy

Question 35

Define specific heat capacity

Answer 35

The energy required to increase 1kg of a substance by 1K [Jkg^-1K^-1]

Question 36

When would you use this equation?

Answer 36

To calculate the mass flowing per kg of a fluid

Question 37

Why does the temperature of a substance changing state not increase?

Answer 37

The thermal energy is used to break some of the intermolecular bonds (solid → liquid) or the rest of the intermolecular bonds (liquid → gas)

Question 38

Define specific latent heat of fusion

Answer 38

The energy required to change the state of 1kg of a solid to a liquid at its melting point.

Question 39

Define specific latent heat of vaporization

Answer 39

The energy required to change the state of 1kg of a liquid to a gas at its boiling point.

Question 40

What is wrong with this?

Answer 40

Haven’t considered the change of states. Need to break it into 5 equations:

Question 41

For the Temperature graph of heating up a solid, how is the energy of the molecules changing at each stage?

Answer 41

Kinetic Energy changes when temperature changes

Potential Energy changes when it changes states (solid->liquid or liquid->gas )

Question 42

When heat melts a solid why doesn’t the kinetic energy of the molecules increase?

Answer 42

The heat energy increases the potential energy of the molecules (breaking some of the intermolecular bonds)

Question 43

When heat boils a liquid why doesn’t the kinetic energy of the molecules increase?

Answer 43

The heat energy increases the potential energy of the molecules (breaking the rest of the intermolecular bonds)

Question 44

What is internal energy?

Answer 44

KE + PE for a substance

Question 45

What is the internal energy for a gas?

Answer 45

The total KE of all molecules in gas

Question 46

Why is the internal energy for a gas = KE?

Answer 46

PE = 0 because all intemrolecular bonds broken for gas