Thermodynamics Flashcards
Define an ideal gas
A gas which has no molecule interaction and the molecules take up negligible space
Define the 3 types of thermodynamic systems
Open - matter and energy can transfer to surrounding
Closed - only energy can transfer to surrounding
Isolated - nothing can transfer to surrounding
What is meant by a micro state system?
Describe every atom/molecule
Only works for extremely small systems (less than a mol)
What is meant by a macrostate system? And detail intrinsic and extrinsic properties
Describes overall properties
Intrinsic - temp and pressure
Extrinsic - volume and moles
What is the relationship between pressure and volume, volume and temp, volume and moles?
And how are extrinsic properties made into intrinsic ones?
Pressure = K/volume
Volume = K.Temp
Volume = K.moles
Remove size from extrinsic ie volume/moles = Vm = intrinsic, mass/volume = density = intrinsic
Detail the ideal gas law
R = PV/NT
Universal gas constant = pressure.volume/temp.moles
What is a state variable?
Explains the current state of a material, not the route used to get there
Path taken does not affect the state variable
Which transition path will a material take?
The one with the lowest energy
Name 3 state variables
Pressure and volume are state variables, Energy = PV meaning energy is also a state variable
Describe joules experiment
Isolated water tank, turbine connected to pulley (weight released = turbine spin), as turbine spun water temp increased
What is the first law of thermodynamics?
Δu = q - w
Change in internal energy = heat flux - work done (energy out -energy in)
What is the energy loss caused by atom bonding called?
Enthalpy of bond formation
Describe enthalpy
H = U + PV
Enthalpy = internal energy + pressure.volume
Energy of a system
Is enthalpy intrinsic or extrinsic?
Extrinsic as it contains volume
Make it intrinsic by dividing it by moles = molar enthalpy
What is Cv and why is it preferred over Cp?
Cv = specific heat capacity at a constant volume (Cp at constant pressure), Cv is intrinsic so is preferred (Cp varies with temp)
Define Cv
Cv = 3R/Tm R = universal gas constant, Tm = melting temp
What is meant by latent heat of melting?
Energy released by a material when solidifying = energy taken to break material bonds + melt material
Difference in H between solidus and liquidus lines
Define entropy and how to make it intrinsic
Degree of mixing/disorder within a system
δs = δq/T
q = heat transfer, T =temp
Entropy/moles = molar entropy = intrinsic
δs = δH/T
How can entropy be used to tell of a process is reversible or not?
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
What does the conservation of energy mean for enthalpy?
Means enthalpy must be conserved between system and surroundings δHsys = - δHsur
Define stable, meta stable and unstable
Stable - won’t move without large energy input
Meta - move with some energy input
Unstable - will spontaneously move
If a liquid is undercooled what state is it in?
Unstable state - can spontaneously transform into a liquid
How do you calculate the entropy change between solidus and liquidus lines for an undercooled liquid?
Entropy is a state variable = route doesn’t matter
- δs from A to Tm
- Hm
- δs from Tm to B (where A and B on different lines at T1 but δs different as C different)
- Work out entropy change of surroundings (= δH/T1) finding δh over the three points
State the 3 laws of thermodynamics
1 - ΔU = q - w (internal = heat flow - work done)
2- δs = δq/T (entropy = heat flow/Temp)
3 - at 0k entropy = 0 for everything
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
Define Gibbs free energy
G = H - TS
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
When is thermodynamic equilibrium achieved?
When G = 0 as energy can’t be reduced further
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
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
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
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
What is an ideal solution?
Bond A-B has the same energy as bond A-A = B-B
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
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
What changes the chemical potential energy of an element?
Phase it’s in, composition, pressure and temperature
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
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
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
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
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
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
What are the 3 types of grain boundaries?
Twist boundary
Low angle symmetric tilt boundary
Unsymmetrical tilt boundary
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 (
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)
When is grain boundary energy minimised?
When they coincide with the specific planes and when most of the grains share the same boundary
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)
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
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
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)
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)
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)
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
Can crystal have more than one interface present?
Yes, most particles have coherent + incoherent interfaces at different sites within the lattice
