Thermal

Question 1

Name one of the criteria for an ideal gas.
HINT - Volume

Answer 1

Molecules of the ideal gas must have negligible volume.

Question 2

Name one of the criteria for an ideal gas.
HINT - Collisions

Answer 2

Collisions between particles of an ideal gas must be perfectly elastic.

Question 3

Name one of the criteria for an ideal gas.
HINT - Motion

Answer 3

Particles of an ideal gas must move rapidly with random direction.

Question 4

Name one of the criteria for an ideal gas.
HINT - IM Forces

Answer 4

No intermolecular forces may act between particles of an ideal gas EXCEPT during collisions

Question 5

Name one of the criteria for an ideal gas.
HINT - Time

Answer 5

The time of collisions between molecules is negligible compared to the time elapsed between them

Question 6

What are the two representations of the ideal gas equation?

Answer 6

PV=nRT for moles - where R is the molar gas constant
PV=NkT for molecules - where k is the Boltzmann constant

Question 7

What is the definition of an ideal gas?

Answer 7

Gases which obey the equation PV=nRT and follow all ideal gas laws at all temperatures, pressures and volumes.

Question 8

When do common gases typically follow the ideal gas laws?

Answer 8

At room temperature.

Question 9

Why do most gases stop acting as ideal gases under high pressure/low temperature conditions?

Answer 9

• At high pressures gas molecules occupy larger volumes of their container and intermolecular forces between to act between them
• At low temperatures gases condense and behave as liquids, hence intermolecular forces act between molecules to a greater degree
• Both of these result in motion being impacted to an appreciable degree due to intermolecular forces

Question 10

Give TWO examples of gases which do not obey the ideal gas laws.

Answer 10

Water vapour and carbon dioxide.

Question 11

State Boyle’s Law.

Answer 11

Pressure is inversely proportional to the volume of a gas.

Question 12

State Gay-Lussac’s Pressure Law.

Answer 12

Pressure is directly proportional to temperature of an ideal gas.

Question 13

State Charles’ Law.

Answer 13

The volume of a gas is directly proportional to its temperature.

Question 14

Describe a solid in terms of the kinetic model.

Answer 14

• Least KE: vibrate about fixed position
• Most -ve E/S: strong forces of attraction; regular close arrangement

Question 15

Describe a liquid in terms of the kinetic model.

Answer 15

• More KE: able to slide past one another
• Less -ve E/S: weaker forces of attraction; lose regular arrangement

Question 16

Describe a gas in terms of the kinetic model.

Answer 16

• Most KE: move rapidly with Boltzmann-defined speed and random direction
• Zero E/S: negligible forces of attraction; no arrangement and collides with walls of container causing pressure (force/area due to dp)
• Link to ideal gases!!

Question 17

Why must all particles have energy above absolute zero?

Answer 17

When we increase temperature, we increase the mean kinetic energy of particles
This means that internal energy U increases
Electrostatic potential energy also adds to this due to the separation/attraction between molecules at all temperatures

Question 18

How do we calculate total internal energy (U)?

Answer 18

Sum of kinetic and electrostatic potential energies.

Question 19

What type of mathematical distribution do gas particles follow?

Answer 19

The Maxwell-Boltzmann distribution.

Question 20

Describe a Maxwell-Boltzmann distribution.

Answer 20

• The energy of a given particle can lie within a range of values
• As temperature increases, so does range
• Most probable energy level increases, but no. of particles at this level falls to maintain constant area beneath the graph

Question 21

How is kinetic energy related to temperature in basic terms?

Answer 21

Kinetic energy is proportional to absolute temperature (Kelvin).

Question 22

Why do we only quote standard temperature and pressure in the context of gases?

Answer 22

Changes in temperature/pressure have negligible effect on solids and liquids due to their density.

Question 23

State the conditions for STP.

Answer 23

273 Kelvin and 100kPa pressure.

Question 24

What principle explains the random motion of particles in a fluid?

Answer 24

Brownian motion.

Question 25

Explain the motion of particles in terms of Brownian motion.

Answer 25

Small particles moving at fast speeds in random directions collide frequently with larger particles, transferring their momentum in a given direction. Thus larger particles move with random linear direction.

Question 26

Why is it that small molecules are able to cause Brownian motion, despite their size?

Answer 26

They carry lots of momentum which is transferred to larger molecules during collisions.

Question 27

Give an example of Brownian motion in the world around us.

Answer 27

Smoke particles in the air.

Question 28

In what direction is thermal energy transferred?

Answer 28

Areas of high temperature to low temperature. This occurs until thermal equilibrium is achieved.

Question 29

Describe the zeroth law of thermodynamics.

Answer 29

If body A is in equilibrium with body B, which is also in equilibrium with body C, then bodies A and C will also be in equilibrium if brought into thermal contact.

Question 30

What two fixed endpoints define the absolute (Kelvin) thermodynamic scale?

Answer 30

The triple point of water (where it exists in all three states) and absolute zero where particles have negligible internal energy.

Question 31

What is the equation used to calculate the mean kinetic energy of an ideal gas?

Answer 31

Ek = 3kT/2. Note that since ideal gases are considered here, we can say that Ep ~ 0.

Question 32

How do we convert from Celsius to Kelvin?

Answer 32

0 degrees Celsius = +273.15 Kelvin

Question 33

Define specific heat capacity.

Answer 33

The energy required to raise 1kg of a given material by 1 degree Celsius.

Question 34

Define specific latent heat.

Answer 34

The energy required to change the state of 1kg of a substance without raising its' temperature.

Question 35

What is the equation for specific heat capacity?

Answer 35

E=m*c*dtheta

Question 36

What is the equation for specific latent heat?

Answer 36

E=mL

Question 37

How does specific heat capacity relate to the rate of heating?

Answer 37

For a given power supply, a given temperature rise will take longer for a higher specific heat capacity as more energy is needed to effect a given change.

Question 38

How does specific latent heat relate to the rate of phase change?

Answer 38

The higher the SLH, the longer a phase change takes for a given power supply.

Question 39

Why is more energy required to convert liquids to gases than solids to liquids?

Answer 39

- There are greater differences in internal energy stores - Intermoleculars bond must be fully broken from liquid - gas for molecules to move around fully unimpeded; meaning this phase changes requires more energy.

Question 40

Describe the method of mixtures.

Answer 40

1. Mix 2 substances of known mass and temperature 2. Measure temperature at thermal equilibrium 3. Equate m*c*dtheta for both materials

Question 41

How do we compete SHC calculations for mixtures of materials with varying heat capacities?

Answer 41

Calculate overall specific heat capacity as follows... (m1c1+m2c2...+mXcX)/(mTotal)=cAvg

Question 42

How do we determine root mean square speed?

Answer 42

Sum the squares of the velocities of all molecules, divide by the number of molecules, then find the square root of this value.

Question 43

Why do we use RMS speed for thermodynamic calculations?

Answer 43

This always yields positive results regardless of direction.

Question 44

What is the formula for gas pressure when using RMS speed?

Answer 44

pV=1/3Nmc^2 where c^2 = RMS speed

Question 45

How are kinetic energy and temperature linked?

Answer 45

Temperature is proportional to the mean kinetic energy of particles.

Question 46

How do we derive Ek=3/2kT?

Answer 46

pV=NkT and pV=1/3Nmc^2 kT = 1/3mc^2 3/2kT = 1/2mc^2, where the latter reflects a RMS interpretation of Ek=1/2mv^2

Question 47

What assumption can be made about the internal energy of an IDEAL gas?

Answer 47

Since electrostatic potential energy is equal to zero, internal energy U is equal to the kinetic energy of molecules.

Question 48

Why do gas molecules exert a pressure on the walls of a container?

Answer 48

- When they collide they rebound elastically - They undergo a momentum change equal to 2mv - Force = change in momentum wrt. time - So a force is exerted over a given area (the surface of the container) - Pressure = force/area

Question 49

What is temperature defined as?

Answer 49

A measure of the mean kinetic energies of all particles in a substance.

Question 50

Why is the electrostatic potential energy of a solid/liquid negative?

Answer 50

- Energy must be input to separate particles - This is achieved by supplying energy such that a phase change occurs (at MP/BP)

Question 51

Describe the energy changes of a substance when temperature is increasing.

Answer 51

- Internal energy U increases - KE increases - EP is constant

Question 52

Describe energy changes in a substance when it changes phase.

Answer 52

- Internal energy U increases - KE is constant - EP increases

Question 53

Why is temperature constant when phase changes?

Answer 53

The energy being supplied still increases internal energy stores; only it is used to increase EP not KE