1st Law
What is the first law? (Asta, 2018)
internal energy is conserved
sum of work and heat
U is a state function, w and q are path functions
What is the second law in words only? (Asta, 2018)
dS greater than or equal to dq/T
What are the conditions for Boltzmann's approximation?
particles are identical and indistinguishable
high temperature low density
# states >>>> # particles (classical regime)
The entropy change along a reversible path between two states is the same as along an irreversible path between those states (Exam 1, 2018)
True. Entropy is a state function and entropy change is independent of path, regardless of whether or not the path is reversible.
What is band theory? Where is it derived from? (Electronic properties, Al Balushi, 2020)
N atoms in a material would have N molecular bonding and antibonding orbitals, where the spacing between the energy levels is so small that we can think of them as continuous bands
How would you explain thermodynamics, entropy, enthalpy to the president? (Doyle, 2015)
thermodynamics: macroscopic framework of energy flows and how they affect properties of system
entropy: measure of disorder; always increasing in the universe
enthalpy: energy associated with flow of matter
What do the two cases in the inequality in the second law correspond to? (Asta, 2018)
= refers to reversible process
> refers to irreversible process
What is configurational entropy? (Digby, 2013)
entropy associated with configuration
liquid has higher configurational entropy than solid
A device uses a chemical reaction to create electrical energy. The conversion efficiency of this device, defined as the ratio of the electrical work divided by the enthalpy of the reaction, is limited by the Carnot efficiency.
False. Carnot's theorem applies to thermodynamic cycles, or a heat engine where thermal energy is converted to mechanical work. Devices that convert a fuel's chemical energy directly into electrical work, such as fuel cells, can exceed the Carnot efficiency.
Define and qualitatively derive Bragg's law (Characterization, Scott, 2016)
draw the planes, show path distance has to be integer multiple of wavelength for constructive interference to occur
What's the relationship between enthalpy and heat at constant P? How does enthalpy relate to heat capacity? (Dibgy, 2013)
H = TdS + VdP so at constant P, H = TdS = n*cp*dT
From the 2nd law is entropy always increasing for a system? If not, give an example of a system with decreasing entropy. (Ceder 2019)
not always increasing for the system, but process can happen spontaneously as long as it increases entropy of universe
eg: cooling process
Explain the differences between Einstein and Debye models of specific heat. (Digby 2013)
Einstein assumes fixed vibrational frequency and that particles oscillate independently of each other
Debye says there's a range of vibrational frequencies and that particles don't oscillate independently
Both asymptote at 3NkbT. Einstein model not appropriate at low T
All state functions are conserved quantities (Exam 1, 2019)
False - entropy is not conserved.
What's the difference between Fermi Level and Fermi Energy? (Adair, 2020)
Fermi level = highest energy at which 50% of carrier occupancy is attained. Increases with T
Fermi energy = energy of highest occupied electronic states of a system of fermions at 0 K (doesn't change with T because definition doesn't apply to finite T)
How do you get Gibbs free energy from first and second laws? What's the Gibbs free energy equilibrium condition? (Digby, 2013)
First law gives dU = dq + dw
second law gives dq = TdS (reversible)
dG = dU - TS + PV = -SdT + VdPminimized at equilibrium
Why does minimizing G maximize S?
dG = -SdT + VdP
sign difference means that lowering G raises S
Which model (Einstein or Debye) do you use to measure heat capacity at low temperatures and why? (Digby, 2013)
Debye because there are energies available
Entropy change of a system undergoing a reversible process is always zero.
False - entropy can and usually does change for many reversible processes
What ways can a screw dislocation move? (Madelyn, 2020)
climb
can't cross-slip because no extra half-plane
Draw a Carnot cycle. What is the efficiency of a Carnot engine? What is the efficiency of a refrigerator?
2. adiabatic expansion
3. isothermal compression
4. adiabatic compression
total S = 0 for cycle, which you can use to find Qc and Qhengine efficiency = (Th - Tc)/(Th)
refrigerator efficiency = (Tc)/(Th - Tc)
If I have my (hotter) coffee cup at constant P and closed system with the room, how will it change (Ceder 2019)
heat will flow from hotter coffee to cooler surroundings (2nd law)
at constant P, enthalpy of coffee will decrease since heat flow is negative
What are the heat and work terms of the first law in terms of energy and probability?
heat: sum of energies * change in probabilities
work: sum of probabilities * change in energy levels
A many particle Hamiltonian is always the sum of single particle Hamiltonians. (Exam 2, 2019)
False - only if we assume independent particles or independent degrees of freedom
What's the difference between electron diffraction and x-ray diffraction? (Characterization, Wu, 2016)
X-rays will interact with electrons in the sample while EM beams interact with both electrons and nuclei, so electron diffraction gives more chemically-specific information
X-ray techniques like XPS can also give chemically-specific info but typically only for top 10nm of material (penetration depth depends on material)
Electrons have smaller wavelengths so higher resolution