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Thermodynamics > Content up to MIDTERM > Flashcards

Content up to MIDTERM Flashcards

1
Q

What is a macrostate?

A

Describes a system at the macroscopic level

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2
Q

What is a microstate?

A

Describes a system at a microscopic level

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3
Q

What is the number of microstates required to provide a macrostate?

A

Avogadro’s Number 6 x 10^23

the number of molecules in one mole of a substance

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4
Q

What is a closed system?

A

Exchanges energy but not matter

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5
Q

What is an open system?

A

Exchanges energy and matter

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6
Q

What is an extensive variable?

A

Depend on the size of the system (internal energy and volume)

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7
Q

What is an intensive variable?

A

Are independent of the size of the system (T and P)

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8
Q

What is the Zeroth Law?

A

If two systems are in thermal equilibrium with a third system then they are in equilibrium with each other
There is a single property (temperature) that serves to indicate whether systems are in thermal equilibrium.

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9
Q

What does a reversible pr quasistatic process mean?

A

These idealised processes take place infinitely slowly in such a way that the system is at equilibrium at every stage

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10
Q

What is the First Law?

A

When a system undergoes a change of state, the sum of the different energy changes is independent of the manner of the transformation.
It depends only on the initial and final states of the system.
U is a state function

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11
Q

What does U being a state function mean for a process?

A

The change in energy going from A to B is independent of the path taken

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12
Q

What is the equation for change in energy (first law equation)?

A

dU = dQ + dW

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13
Q

What is the differential of work done by expansion?

A

-PdV

P is external pressure and V is volume

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14
Q

What is the differential of work done by extension?

A

Fdl

(F is tension and dl is change in length

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15
Q

What are Ideal Gas assumptions?

A
  1. The particles do not take up any volume
  2. The particles do not interact (except through perfectly elastic collisions)

The interaction energy of two particles in an ideal gas is zero regardless of their separation

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16
Q

When is the interaction energy of two particles not zero?

A

At high pressures because all substances experience Van der Waals attractive interactions

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17
Q

What happens in an adiabatic expansion?

A

No heat enters the system but work is done, dQ = 0

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18
Q

What happens in an isothermal expansion?

A

The temperature remains constant, dU = 0

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19
Q

What work is done if P remains constant during a volume change?

A

dW = -PdV

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20
Q

How many degrees of freedom do vibrations contribute?

A

Two because there is potential and kinetic energy

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21
Q

How many degrees of freedom does a monatomic molecule have?

A

3 (all translational)

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22
Q

How many degrees of freedom does a diatomic molecule have?

A

6 (3 translational, 2 rotational and 1 vibrational)

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23
Q

What is the classical equipartition theorem?

A

States that each degree of freedom contributes 1/2 k to the heat capacity Cv

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24
Q

For a real gas what is the internal energy U a function of?

A

Volume and temperature

dU = dU/dT dt + dU/dV dV

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25
What is the relationship between Cv and Cp for an ideal gas?
Cp - Cv = nR (1 mol of ideal gas, the difference in the heat capacities is simply R
26
What does a heat engine dp?
Absorbs heat Qh from a hot reservoir at temperature Th and coverts part of it to work W and discards heat Qc to a cold reservoir (heat sink) at Tc
27
What is the Second Law? | arrow of time
Heat cannot by itself pass from a colder to a hotter body OR The entropy of the universe approaches a maximum (identifies which processes are accessible by way of a spontaneous change)
28
What is Entropy, S?
A state function which increases during a spontaneous change.
29
When does the entropy of an isolated system increase?
During a spontaneous change (dS > 0) | There are no physical processes which lead to a decrease in entropy
30
What kind of processes generate entropy (disorder)?
Irreversible processes | therefore they occur spontaneously
31
What happens when you generate entropy?
the ability to do useful work is lost (more useful work is retained the smaller the difference in temperature between the two reservoirs
32
What is the equation for entropy?
dS = dQ/T
33
When does the minimum possible entropy change occur?
For reversible processes
34
What is the Clausius Inequality?
dS => dQ/T
35
How do you convert from J K-1mol-1 to JK-1?
Multiply by the number of moles n
36
What are the units of heat capacity?
JK-1 JK-1mol-1 JK-1kg-1
37
What is a conservative field?
One for which the work done is independent of the path
38
What are examples of conservative fields?
Thermodynamic potentials, gravity and electrostatic fields
39
What is an extremum Principle in Nature shown in thermodynamics?
The equilibrium state of the system under different conditions is determined by the extremum of the appropriate thermodynamic potential
40
When does the equilibrium state of an isolated system occur?
When the entropy reaches maximum value
41
How does the second law show the extremum principle?
A process is spontaneous so entropy increases and system therefor moves to a state of higher entropy if the path is available
42
When is a state of equilibrium reached?
dS = 0 (this describes a reversible process which occurs equally well in both directions
43
What are the 6 thermodynamic potentials?
``` Entropy (S) Internal Energy (U) Enthalpy (H) Helmholtz free energy (F) Gibbs free energy (G) Chemical potential (mu) ``` (all state functions)
44
What happens to a system as it is maintained at constant volume and energy?
It will evolve to maximise the entropy (S)
45
What happens to a system at constant entropy and volume?
It will evolve to minimise the energy (U) | There is an accompany increase in entropy of the surroundings as energy flows out as heat
46
What is the equation for enthalpy?
H = U + PV
47
What happens to the entropy as the enthalpy remains constant?
It increases as then there can be no increase in entropy of the surroundings
48
What happens to the enthalpy if the entropy stays constant?
The enthalpy must decrease as then it is essential to have increase in entropy of the surroundings
49
What is the Helmholtz Free Energy equation?
F = U - TS
50
What is Gibbs Free Energy equation?
G = H - TS
51
In the rationale for the van der waals equation, what are the parameters of a and b?
b accounts for the finite volume of gas particles and a takes into account the attraction between the particles
52
What is the expansion coefficient a?
The rate of change of volume(V) with temperature (T) per unit volume at constant pressure (P)
53
What is the isothermal compressibility, Kt ?
A measure of the change of volume under the influence of pressure at constant T
54
What does free energy seeks?
Minimum (most -ve) at equilibrium
55
What are U and H are determined by?
The strength of the intermolecular interactions
56
What happens at low temperatures?
U wins and you get solids
57
What happens at high temperatures?
S wins and you get gases
58
What is a phase diagram?
Summarises the conditions of temperature and pressure under which a substance exists in different phases, e.g. solid/liquid/gas
59
What do boundaries between regions on a phase diagram show?
The values of P and T at which the two phases coexist in equilibrium
60
What does the hatched region in a phase diagram indicate?
phases are metastable
61
What are first order phase transitions (e.g melting and boiling) associated with?
Latent heat (L)
62
What happens during a first order transition?
The heat supplied to the system doesn't increase T but is absorbed in converting the substance from one phase to another This implies that there is a discontinuous change in the entropy and therefore a singularity (an infinite) heat capacity at the transition
63
What is the Clausius-Clapeyron equation?
The differential equation that describes the phase equilibrium curve It shows how the boiling/freezing point of a substance changes with pressure (calculate dP/dT along the phase equilibrium curve)
64
Which variables must refer to the same quantity of substance (eg molar entropies and volumes) ?
delta S and delta V
65
What does the Clapyeron equation tell us?
How the transition temperature T changes with pressure Describes the condition for the two phases to be in equilibrium (i.e the phase boundary)
66
What is the orientation of the the enthalpy of melting?
It is positive
67
How is the Clapyeron equation derived?
by setting the chemical potential to be the same for both phases
68
What does the Helmholtz equation relate?
The variation of the energy with the volume for any gas, real or ideal
69
What is the Nernst Heat Theorem?
That at lower and lower temperatures the change in G and the change in H become closer together He found this by measuring the change in G and change in H for chemical reactions that started and the finished at the same temperature
70
What were Nernst's observations?
change in H - change in G tends towards zero as T tends towards zero which also implies that TdeltaS tends to zero
71
What is the third law of thermodynamics?
The entropy change in a process tends to zero as the temperature T tends to absolute zero OR As the absolute zero of temperature is approached the entropy of all bodies tends to zero
72
What does the third law tell us about heat capacities?
As T tends to zero, C tends to zero
73
What is the third law of thermodynamics in direct disagreement with?
Classical Equipartition (which breaks down at low temps due to quantum effects
74
The work required to perform refrigeration and remove heat shows what?
That as the temperature tends to zero the work required to cool the system tends to infinity
75
What is one way of reaching low temperatures?
Adiabatic demagnetisation which makes use of the paramagnetic properties of a substance
76
What does the third law imply about absolute zero?
The entropy curves of a substance coincide (at T = 0) as absolute zero is approached If the third law were not true then absolute zero could be reached

Thermodynamics (1 decks)

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