Quiz 1 Flashcards
What is the first law of thermodynamics?
Energy is conserved , in any process energy can neither be created or destroyed
What is an open system?
heat and matter exchange with the surroundings
What is a closed system?
only heat exchanges with the surroundings
What is an isolated system?
no heat or matter exchange with the surroundings
What are heat and work (general)
- different means energy is transferred in and out of a system
How does heat transfer energy?
due to temperature changes between system and surroundings
How does work transfer energy?
due to unbalanced forces between system and surroundings
How is the change in internal energy defined?
sum of heat transferred and work done is internal energy
What is a state function?
- depends only on the difference between initial and final STATES, not the path
- internal energy, enthalpy
What is a path function?
- processes that transform one state into another
- work and heat, a system does not contain work or heat but rather these are processes that transform one state into another
What is an intensive quantity?
- doesnt depend on the amount of substance present
- temperature
- pressure
What is an extensive quantity?
- depends on the amount of the substance present
- mass, internal energy
What molecular level properties determine C?
- at constant volume, C depends on internal energy
- U in turn depends on the degrees of freedom a given molecule/substance has (translation, rotation, vibration)
Heat capacity units
(J/K) extensive quantity
Molar heat capacity
(J/K mol) intensive quantity
Specific heat capacity
(J/ K g) heat capacity per mass
What is the equipartition theorem?
- every quadratic term to the energy is equal to 1/2RT
Apply Equipartition theorem to the oscillator.
Etotal = E kin + E pot
Etotal = RT
Heat capacity of Monatomic Ideal gases
Cvm = 3/2 R
Heat capacity of diatomic ideal gases
- Equipartition theorem predicts
C = 7/2 R - However the vibrational mode is not significantly populated at room temperature so C = 5/2 R
What does large heat capacity indicate?
A lot of heat is required to change the temperature
What is the heat capacity equation?
dq = C dt
q = C(T2- T1)
Heat capacity of simple solids
C = 3R
Why does the equipartition theorem only work at high temperature?
- the underlying assumption is that the total energy is equally spread over all modes
- that is not correct at low temperatures because the solids follow the laws of quantum mechanics
- only in the limit of high temperature is the classical limit approached
How does heat capacity change with temperature?
- Heat capacity decreases at decreasing temperature
- At low temperature, high frequency mode of vibration does not contribute to the total energy of the solid, thereby explaining the decay of C with low temperature
What factors affect the heat capacity of complex solids?
- increase with molar mass (more vibrational and rotational modes are available), also heavy atoms give rise to low-energy vibrations
- liquids have greater heat capacities at the melting temperature than corresponding solids
- more flexible molecules have high heat capacities than more rigid molecules
- general trend of C increasing w temp
Why is Cp larger than Cv?
because not only do we increase the temp when we add heat, we also do pressure volume work against the atmospheric pressure
What is the relationship between Cp and Cv ideal gas?
Cp = Cv + nR
What is the enthalpy equation?
H = U + PV
When is the change in enthalpy equal to the heat transferred at constant pressure? delta H = Qp
- no additional work other than pV work (no electrical work)
- the process must be carried out reversibly
What is the irreversible, isothermal expansion of an ideal gas?
- sudden removal of weight
w = - pex (vf- vi)
What is the reversible isothermal expansion of an ideal gas?
- we remove infinitiesimally small amounts of sand in a stepwise manner so that pex = pgas
wr = -nRT ln(V2/V1)
Derive enthalpy equation
H = U + pV
dH = dU + Vdp + pdV
at constant pressure dp = 0
dH = dU + pdV
Positive work
surroundings are doing work on the system
Negative work
system doing work on the surroundings
What does internal energy change equal during isothermal expansion?
delta u = q + w = 0
q = -w
Ideal gas law
PV = nRT
What is R and units?
8.314 j / mol k
.08206 L atm / K mol
For expansion is irreversible or reversible work larger magnitude?
rev > irrev
What is internal energy?
Internal energy is the sum of all of the energies including chemical, kinetic, and potential
Why does an ideal diatomic gas have a higher heat capacity than an ideal monatomic gas?
A diatomic gas has more degrees of freedom because it also occupies rotational and sometimes vibrational states at room temperature. Monatomic gases only occupy 3 translational modes +2 rotational modes at room temperature
Describe the internal energy of an ideal gas
only depends on temperature, not pressure or volume
