9. Thermodynamics

Question 1

What is thermodynamics?

Answer 1

The study of energy and its effects on a system (open/ closed system)

Question 2

What is specific heat capacity, c?

Answer 2

The amount of energy required to raise the temperature of a 1kg substance by 1k
△E=mc△ θ

Question 3

Why do electrical conductors have low specific heat capacities?

Answer 3

Electrons gain more energy and then can move faster to transfer energy

Question 4

What’s the specific heat capacity for water?

Answer 4

4200 Jkg^-1K^-1
different to ice. ( 2200 Jkg^-1K^-1) as in a different structure

Question 5

What’s the definition of a solid turning into a gas?

Answer 5

Sublimation (subliming)

Question 6

What is specific latent heat?

Answer 6

The thermal energy required to change the state of a 1kg substance, without temperature change

△E=L△m
where L= specific latent heat
where △m=mass of substance changing substance (mass undergoing the phase)

Question 7

What are the two types of latent heat?

Answer 7

• Specific latent heat of fusion (melting)
• Specific latent heat of vaporisation (boiling)

Question 8

What are the specific latent heats of water?

Answer 8

• Specific latent heat of fusion = 330 kJ kg-1
• Specific latent heat of vaporisation = 2.26 MJ kg-1

Question 9

Why does it take so much more energy to turn water to gas than ice to liquid?

Answer 9

This is because of intermolecular forces
- When ice melts: energy is required to just increase the molecular separation until molecules can flow freely over each other
- When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between them, hence this requires much more energy

Question 10

What’s internal energy, U?

Answer 10

The energy stored within substances
- the hotter the object, the more internal energy

Question 11

What are the 2 components of internal energy?

Answer 11

Kinetic energy
- the speed of the molecules posses kinetic energy when moving
- gives the material its temperature
Potential energy
- the energy stored within the particles due to intermolecular forces

Question 12

When is potential energy higher?

Answer 12

In gases
This is because the separation is greater, so the forces are less, leading to greater potential energy as more energy is required to keep them separated like this

Question 13

How is internal energy distributed between particles?

Answer 13

Randomly.
Particles have different speeds and separations

Question 14

What phase of matter has the highest internal energy?

Answer 14

Gases have the highest
Solids have the lowest

Question 15

What does internal energy determined by?

Answer 15

• Temperature
• The random motion of molecules
• The phase of matter
• Intermolecular forces between the particles

Question 16

What’s the thermodynamic (kelvin) scale?

Answer 16

0K=-273.15 °C
Measures from absolute zero
goes up same rate as Celsius

Question 17

What’s absolute zero?

Answer 17

The temperature where molecules in a substance have zero kinetic energy
Not even the coldest planet can reach under absolute zero

Question 18

What does the kinetic theory of gases model?

Answer 18

the thermodynamic behaviour of gases by linking microscopic properties (mass and speed) of particles to macroscopic (pressure and volume)

Question 19

What does the kinetic theory of gases assume?

Answer 19

• Molecules of gas behave as identical, hard, perfectly elastic spheres
• The volume of the molecules is negligible compared to the volume of the container
• The time of a collision is negligible compared to the time between collisions
• There are no forces of attraction or repulsion between the molecules
• The molecules are in continuous random motion

Question 20

What is root mean square (rms)

Answer 20

Square all values
Find mean of the squared values
square root mean

Used as particles move in positive and negative velocities, so makes all of them positive to find actual speed.
rms is higher than normal mean however

Question 21

What is the kinetic theory of gases equation?

Answer 21

Imagine a cube, with a molecule moving horizontally side to side. If we find the pressure exerted by it, we can link macroscopic quantities to microscopic about the gas.

Step 1:
find momentum of molecule
△p=-mc-(+mc)
△p=-2mc

Step 2:
find time between collisions
speed (c) = distance (2L) /time
time = 2L/c

Step 3:
Find force of molecule exerted on wall
F=ma
F=△p/△t
F=mc^2/L

Step 4:
Find the pressure exerted by all molecules
p=F/A (L^2)
p=mc^3/L^3 (for one molecule)
p=Nm<c^2>/L^3

Step 5:
consider 3 dimensions
<c(x)^2>=1/3<c^2>
also L^3=V
so…
pV=(1/3) x Nm<c^2>

Question 22

How can we link the kinetic theory of gases equation with density?

Answer 22

Density, ρ = mass/volume
mass=Nm
ρ=Nm/V
so…
p=(1/3) x ρ<c^2>

Question 23

What’s an ideal gas?

Answer 23

A theoretical gas where particle interactions are ignored and have perfectly elastic collisions
no intermolecular forces act between the molecules, so ideal gases have no potential energy.

Question 24

What’s the molar gas constant, R

Answer 24

For any ideal gas, increasing 1 moles worth of substance by 1K increases its energy by 8.31J every time
R=8.31JK^-1mol^-1

Question 25

What’s Avogadro’s constant?

Answer 25

The number of particles per mole.
So moles don’t show the weight of a substance, only how a quantity of particles
N(A)=6.02x10^23 mol^-1

Question 26

What’s Boltzmann’s constant?

Answer 26

Relates the amount of energy added by an increase of 1K for any ideal gas, for only one particle.
k=Molar gas constant/ Avagadros constant
So it cancels the amount of moles so it’s in particles instead of moles.
k=R/N(A)
k=1.38x10^-23 JK^-1

Question 27

What is Boyles Law?

Answer 27

Considering the temp is constant in an ideal gas…
p∝1/V
So if volume decreases, pressure rises
P(1)V(1)=P(2)V(2)
Gives a constant for that specific gas in a container

Question 28

What is Charles Law?

Answer 28

Considering the pressure is constant in an ideal gas…
V∝T
Meaning for a fixed mass, increasing the temperature expands the volume as the pressure can expand, due to particles hitting walls more with more energy.
V(1)/T(1)=V(2)/T(2)

Question 29

What is the Pressure Law?

Answer 29

Considering the volume is constant in an ideal gas…
P∝T
As if you can’t increase the volume, the pressure is still expanding, so it just increases
P(1)/T(1)=P(2)/T(2)

Question 30

What is the ideal gas equation?

Answer 30

pV=NkT
NkT finds out the total amount of energy within a system
It is a constant which is why it equals the constant pV as that is also the constant of energy in the system.
So pV represents the work done by the gas or liquid expanding

Question 31

What’s the Average Molecular Kinetic Energy equation?

Answer 31

E(k)=(3/2) x kT
Derives from (1/3) x Nm<c^2>=NkT

Question 32

What’s black body?

Answer 32

• A theoretical object that absorbs and emits all radiation that falls upon it.
• Emit a spectrum of thermal radiation in the form of EM waves
• A perfect absorber/ emitter

Question 33

What decides what EM waves are emitted by black body radiation

Answer 33

Temperature.
The higher the temperature, the more thermal radiation so a higher intensity so the curve shifts up.
The Lower the wavelength of EM wave, the higher the energy levels, so the higher the temp, the greater the peak will be to the left.

Question 34

What’s the closest thing to a black body?

Answer 34

A star.
But black objects absorb a lot so emit a lot, so they are good, but nowhere near perfect.