137A - thermodynamics Flashcards
what is isobaric
reaction at constant pressure
what is isochloric
reaction at constant volume
what is a state function + examples
just values (doesn’t matter how you got there) e.g. enthalpy, entropy, pressure
what is a path function
value when it matters how you got there e.g. health, work done
what is an extensive property
properties dependant on amount of “stuff” you have e.g. mass, internal E, heat capacity
what is an intensive property
properties not dependant on amount of “stuff”/ molar properties e.g. specific heat cap, molar enthalpy
how many degrees of freedom does every atom have
3
how many degrees of freedom does a molecule have
3N (N= number of atoms in molecule)
why do linear molecules only have 2 rotational degrees of freedom not 3
because rotating around z doesn’t make any difference
equation for working out vibrational degrees of freedom for linear molecule
x= 3N-5
equation for working out vibrational degrees of freedom for non-linear molecule
X=3N-6
What is an isolated system
no transfer of matter or energy
what is an open system
matter or energy can be removed or added
what is a closed system
cannot take matter but can transfer energy In heat or work
what is an adiabatic system
no transfer of energy by heat, but work can be done on system by surroundings+ visa versa
What are energy levels are rarely populated unless at very high temperatures
vibrational. At low temps only translational are occupied. As temp increases distribution increases over more levels
Describe the spacing in energy levels. Why are transitional levels well populated?
translational are very closely spaced, then rotational, then vibrational which are higher in energy. Relative population of levels depends on the energy gap between them
How much does each degree of freedom contribute to the molar internal E
RT/2
What is the internal E of a diatomic gas with 6 degrees of freedom
E= 3RT
Why do vibrational degrees of freedom not contribute to the internal E
only active degrees of freedom contribute; vibrational levels are largely spaced and rarely occupied
define the terms of the first law of thermodynamics
△U= internal energy
q= energy in heat
w= energy in work
What is the first law of thermodynamics
the internal energy of an isolated system is constant
How many degrees of freedom does N2 have and its molar internal E at 25°
3x2= 6
3 trans, 2 rot, 1 vibrational
3+2=5 x (RT/2) = 6190Jmol-1
How many degrees of freedom does O3 have and its molar internal E at 25°
3x3= 9 degrees freedom
3 trans, 3 rot, 3 vib
3+3=6 x (RT/2) = 7.4kJmol-1
units of internal E
kJ/ J
units of molar internal E
kJmol-1 or Jmol-1
Why can’t you use equipartition theory to determine the internal energy of ethanol at room temperature
It’s a liquid at room temperature, equipartition theory tells us about ideal gases
if the system is doing work on the surrounding the work done is positive/negative
negative/exothermic (energy transferred from system to surroundings)
if the surroundings is doing work on the system the work done is positive/negative
positive/ endothermic (energy transferred from surroundings to system)
if work done is negative, the internal E of the system rises/falls
falls
if the reaction is exothermic does the internal E of the system rise/fall
fall
if the reaction is endothermic does the internal E of the system rise/fall
rise
what is isothermal expansion
temp remains constant when a gas expands
label the terms (enthalpy change at constant pressure)
△H= enthalpy change
△U = change in internal E
p△v = work done
How else can this equation be written as when containing the ideal gas law
△H = △U + △nRT
what is the work done when something expands
work done by system on surroundings, which means work done is negative
**what does this equation mean w = -w
w = -w
w= work done on system
-w= work done by system
define the heat capacity of a material
amount of energy required to raise the temp of a material by 1K
why are heat capacities at constant pressure bigger than heat capacities at constant volume (Cp = Cv + R)
because if we are at constant pressure the gas has to do work in expanding
does heat capacity vary with temperature
yes
how much does each transitional DOF contribute to the heat capacity
1/2 R
how much does each vibrational DOF contribute to the heat capacity
R
Define the terms (relation between enthalpies at 2 different temps) △H (T) = △H (T2) + Cp x(T-T2)
△H= enthalpy at first temp
△H (T2) = enthalpy at second temp
Cp = difference in heat capacity between products and reactants are constant pressure
what temp are standard enthalpies of formation measured at
25°
what phase change is △H vapourisation
liquid to gas
what phase change is △H fusion
solid to liquid
why does enthalpy of vapourisation> fusion
it takes more E to go from liquid to gas than solid to liquid (complete bond breaking)
Heat capacity units
JK-1mol-1
what is a bomb calorimeter
container where volume of the system cannot change but pressure can when the reaction happens
when a bomb calorimeter is involved in a question with a heat capacity, is the heat capacity Cv or Cp
Cv (capacity at constant volume)
what is the second law of thermodynamics (entropy)
the entropy of a system tends to increase
what does this equation say
the entropy increase is greater at lower temperatures for a given amount of heat
what does it mean if a reaction is spontaneous
will happen without needing work being done
equation to work out total permutations in a system
example of macrostate arrangement
what is a microstate arrangement
equation for the probability of any given microstate occurring
no. micro states (we care about)/ total no. permutations
**label units in the equation: q= Cp x n x △T
q= energy in form of heat
Cp= heat capacity
n= moles
△T= change in temp
Consider a system with two bulbs and 20 particles. How many micro states are there where all 20 particles are in one
2 microstates (all in bulb A or all in B)
