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Thermodynamics Flashcards

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1
Q

Define an ideal gas

A

A gas which has no molecule interaction and the molecules take up negligible space

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2
Q

Define the 3 types of thermodynamic systems

A

Open - matter and energy can transfer to surrounding
Closed - only energy can transfer to surrounding
Isolated - nothing can transfer to surrounding

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3
Q

What is meant by a micro state system?

A

Describe every atom/molecule

Only works for extremely small systems (less than a mol)

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4
Q

What is meant by a macrostate system? And detail intrinsic and extrinsic properties

A

Describes overall properties
Intrinsic - temp and pressure
Extrinsic - volume and moles

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5
Q

What is the relationship between pressure and volume, volume and temp, volume and moles?
And how are extrinsic properties made into intrinsic ones?

A

Pressure = K/volume
Volume = K.Temp
Volume = K.moles
Remove size from extrinsic ie volume/moles = Vm = intrinsic, mass/volume = density = intrinsic

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6
Q

Detail the ideal gas law

A

R = PV/NT

Universal gas constant = pressure.volume/temp.moles

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7
Q

What is a state variable?

A

Explains the current state of a material, not the route used to get there
Path taken does not affect the state variable

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8
Q

Which transition path will a material take?

A

The one with the lowest energy

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9
Q

Name 3 state variables

A

Pressure and volume are state variables, Energy = PV meaning energy is also a state variable

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10
Q

Describe joules experiment

A

Isolated water tank, turbine connected to pulley (weight released = turbine spin), as turbine spun water temp increased

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11
Q

What is the first law of thermodynamics?

A

Δu = q - w

Change in internal energy = heat flux - work done (energy out -energy in)

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12
Q

What is the energy loss caused by atom bonding called?

A

Enthalpy of bond formation

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13
Q

Describe enthalpy

A

H = U + PV
Enthalpy = internal energy + pressure.volume
Energy of a system

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14
Q

Is enthalpy intrinsic or extrinsic?

A

Extrinsic as it contains volume

Make it intrinsic by dividing it by moles = molar enthalpy

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15
Q

What is Cv and why is it preferred over Cp?

A

Cv = specific heat capacity at a constant volume (Cp at constant pressure), Cv is intrinsic so is preferred (Cp varies with temp)

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16
Q

Define Cv

A 
Cv = 3R/Tm
R = universal gas constant, Tm = melting temp
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17
Q

What is meant by latent heat of melting?

A

Energy released by a material when solidifying = energy taken to break material bonds + melt material
Difference in H between solidus and liquidus lines

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18
Q

Define entropy and how to make it intrinsic

A

Degree of mixing/disorder within a system
δs = δq/T
q = heat transfer, T =temp
Entropy/moles = molar entropy = intrinsic
δs = δH/T

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19
Q

How can entropy be used to tell of a process is reversible or not?

A

change in entropy of system + change in entropy of surrounding = entropy total
If Stotal = 0 = reversible
Stotal > 0 process is irreversible
Stotal < 0 process can’t happen

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20
Q

What does the conservation of energy mean for enthalpy?

A

Means enthalpy must be conserved between system and surroundings δHsys = - δHsur

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21
Q

Define stable, meta stable and unstable

A

Stable - won’t move without large energy input
Meta - move with some energy input
Unstable - will spontaneously move

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22
Q

If a liquid is undercooled what state is it in?

Study These Flashcards
A

Unstable state - can spontaneously transform into a liquid

23
Q

How do you calculate the entropy change between solidus and liquidus lines for an undercooled liquid?

Study These Flashcards
A

Entropy is a state variable = route doesn’t matter

  1. δs from A to Tm
  2. Hm
  3. δs from Tm to B (where A and B on different lines at T1 but δs different as C different)
  4. Work out entropy change of surroundings (= δH/T1) finding δh over the three points
24
Q

State the 3 laws of thermodynamics

Study These Flashcards
A

1 - ΔU = q - w (internal = heat flow - work done)
2- δs = δq/T (entropy = heat flow/Temp)
3 - at 0k entropy = 0 for everything

25
How are entropy and enthalpy related to specific heat capacity?
δH = integral Cp respect to T δs = integral δH/T respect to T When pressure is constant
26
Define Gibbs free energy
G = H - TS
27
How can Gibbs free energy be used to determine a systems reversibility?
G = 0 then in equilibrium If G > 0 then can’t proceed If G < 0 then spontaneous irreversible reaction occurs
28
When is thermodynamic equilibrium achieved?
When G = 0 as energy can’t be reduced further
29
Draw the Gibbs free energy vs T graph for a pure element and detail it
Straight liquidus and solidus line, liquidus starts higher and they intersect Intersection = melting temp System will be in state of lowest graph as energetically favourable, driving force to solidify below Tm as solidus is lower than liquidus
30
What is the Gibbs free energy of a mixture (ignoring configuration)?
G = XaGa + XbGb | Where Xa = Na/N (%A atoms in structure), Ga = Gibbs energy of an a atom
31
What is entropy of mixing?
Measure of how many possible configurations there are Sm = KN (Xa.lnXa + Xb.lnXb) K = Boltzmann constant, N = amount of atoms, Xa = Na/N, Sm = entropy of mixing
32
What is the Gibbs free energy of a mixed solution?
``` G = -TR(Xa.lnXa + Xb.lnXb) R = universal gas constant, T = temp, Xa = Na/N ```
33
What is an ideal solution?
Bond A-B has the same energy as bond A-A = B-B
34
What entropy is beneficial for spontaneous reactions?
High entropy as mixing as means there are lots of possible microstates = more likely to move into one
35
What is the Gibbs free energy of an ideal solution?
G = μa.Xa + μb.Xb Where μa = Ga + RT.lnXa, μb = Gb + RT.lnXb Ga = Gibbs energy of A atoms, R = universal gas constant, T = temp, Xa = Na/N
36
What changes the chemical potential energy of an element?
Phase it’s in, composition, pressure and temperature
37
When are two phases of varying composition in chemical equilibrium?
When c helical potential of atoms are equal (G energy can’t be lowered by swapping atoms) - this is the lowest point at any stage of an G vs comp graph
38
Draw and label a free energy vs composition graph and label
``` Two curves (solidus trough on left, liquidus in right) with liquidus trough higher than solidus Draw tangent between two troughs When tangent = L+S has lowest G = mixed phase stable, where there isn’t a tangent = lowest line stable Graph Only valid for constant temp ```
39
How are phase diagrams created from G vs comp graphs?
Have various graphs at different temps, draw the tangents and plot the points on a temp vs comp graph, connect the dots to created the phase diagram
40
What are interfaces, what do they do and give some examples
Faces that separate things and they stop dislocation movement Examples: free surface of a crystal, grain boundaries and interphase interfaces
41
What’s the free energy of an interface?
AY is the interfacial energy, A = area of interface, Y = free energy of the interface G = G° + AY G° = Gibbs energy of pure material
42
What are grain boundaries and what parameters dictate them?
2D defects that desperate identicle crystals that have different orientation (if not identical then are interphase interfaces) Amount of disorientation, grain boundary angle, translation vector and type of boundary
43
What are the 3 types of grain boundaries?
Twist boundary Low angle symmetric tilt boundary Unsymmetrical tilt boundary
44
Describe a twist boundary and unsymmetric tilt boundary
Crystal are tilted along a rotational axis | Grains tilted away from each-other but at different angles away from the centre axis (
45
Describe a low angle symmetric tilt boundary
Formed by stacking dislocations, angle is correlated to distance between the grains, each boundary stores energy, both grains are same angle away from axis (symmetric) and tilted away from eachother (tilt boundary)
46
When is grain boundary energy minimised?
When they coincide with the specific planes and when most of the grains share the same boundary
47
Are high or low angle grain boundaries energetically favourable?
Low angle grain boundaries have less misorientarion = lower energy However, over 15° the energy penalty levels off (increasing non-linearly until this point)
48
What is the broken bond model for a solid/gas interface?
It says that for a given interface a number of broken bonds/metre are formed Ysv = 0.15.Ls/Nbb Where Ysv = free energy of solid/vapour interface, Ls= latent heat of sublimation (melting and vaporisation), Nbb = number of broken bonds on the surface
49
What is the broken bond model for a solid/liquid interface?
There are a number of broken bonds formed/metre at an interface, as liquids are more dense = more broken bonds Ysl = 0.45.Lm/Nbb Ysl= free energy of solid/liquid interface, Lm= latent heat of melting, Nbb = number of broken bonds on surface
50
Draw two examples of a strain-free coherent interface
1- comp changes but structure is identical (energy penalty from AB bonds) 2- comp changes and what B is bonded is different but no lattice strains (penalty from AB bonds)
51
Draw a coherent interface with lattice mismatch, a semi-coherent interface and a in-coherent interface
1- comp changes and lattice not aligned (0.5a out) = strained 2- comp changes and lattice lined up, but dislocation at interface 3- no lattice planes continue across interface (most usual case)
52
What is meant by coherent interface?
Same number of atoms in each side of the interface and when there are lattice planes that are continuous (even if they’re strained)
53
How can the energy be lowered at a solid/solid interface?
Coherent strain free interfaces have the lowest energy as have the most lattice sites on both crystal and no strain fields are present
54
Can crystal have more than one interface present?
Yes, most particles have coherent + incoherent interfaces at different sites within the lattice

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