lecture 6 Flashcards
what type of function is entropy
it’s a state function
- current value doesn’t depend on route taken to get there
if a system starts and ends at the same place,, what is its entropy
0
in a reversible reaction at constant temp,, the heat added is:::
minus the work done
U = q + w = 0
in expansion,, what results in an increase in entropy
V > 0
0 > p
Cp of monatomic gas
5nR /2
entropy of fusion is equal to what
enthalpy of fusion / T
- for all phase transitions under equilibrium conditions
S= q/t
but q = H at standard pressure
entropy change of phase change
change S = change H / T
for vaporisation
sublimation
under equilibrium conditions
entropy changes on raising temperature,, what do we do when we heat things + phase transitions occur
total change in S =
H(fus)/T. + Cp ln(T1/To) + H(vap)/T
Cp ln(t1/to) = S when raising temp at a constant pressure.
H/T = S when phase changes are occurring.
we need to add them bc as we go from liquid to solid,, we heat it ( S when increasing temp) then it melts (S when phase change occurs) so we must add them if we want to find the total S from liquid -> gas.
when is Cp constant with Temperature
in liquid water
that’s why we do the H/T + Cp ln(T/T) when finding S for increasing temp and phase changes in liquids
bc Cp is constant with temp
when is Cp not constant with T
low temperatures
solid phase
Cpm equals what at low temperatures near 0k
aT^3
for non metallic solids
entropy, for changes in temp in non metallic solids, near 0k (very low temperatures) is equal to what
change in S = 1/3 Cpm
what is the entropy of anything at 0k ( 0 kelvin,, very low temperatures)
entropy is 0
apart from imperfect crystals( their imperfections add disorder)
how is entropy 0 at 0k
equation: S = k ln (W)
at 0k,, W = 1
so S = 0
when can absolute entropy be found for lower temperatures
if we know the heat capacity at all lower temperatures
standard enthalpy isss
enthalpy at standard conditions
what is Sm
ABSOLUTE (third law) molar entropy at standard pressure and a given temperature
what does Sm equal for elements in their standard forms
NOT 0
it has a value,, it is not the same as S ( standard enthalpy)
impurities that don’t allow the 3rd law of thermodynamics to be true
elements and compounds that aren’t ordered the same way
isotopes present in the compound
overall entropy is equal to what
the sum of entropies going from Ok -> XK
when do spontaneous changes happen
when the total entropy of the universe increases
in a perfect crystal,, if the temp is 0k,, can the atoms change arrangement
nope
it only has one way of ordering atoms
it cannot decrease its disorder any further than what it is at 0K
bc the atoms aren’t moving, they’re perfect so no disorder due to isotopes etccc.
perfect crystals at 0k,, cannot decrease entropy by any more
what does the third law of thermodynamics help us do
it helps us to define the entropy of any substance at any temperature
what is the overall entropy of a substance at a temperature
sum of entropy changes to get to that temperature
how is Sm different to S
at 298k
if the substance is in its standard state,, S is 0
Sm would be entropy it takes to get from 0 -> 298
spontaneous process equation
S universe = S system + S universe > 0
equilibrium (reversibility) if ,…,,.
🔺S universe = 0
if entropy of the universe is 0,, the reaction is in equilibrium
the reaction is spontaneous iffff
entropy of universe ( surroundings + system) is larger than 0. THE TOTAL ENTOPY OF THR UNIVERSE INCREASES
second law of thermodynamics
when do spontaneous changes happen
when total entropy of the universe increases
how to find entropy of surroundings
-( Hf pro - Hf rea)/T
what does it mean if the entropy of surroundings is negative but entropy of shatter is positive
the reaction is endothermic
takes in energy
laws of thermodynamics
- energy cannot be created or destroyed,, only converted between forms : U=q+w
- entropy of an isolated system increases: reactions occur spontaneously when Suni = Ssurr + Ssys > 0
- entropy of a perfect crystal at 0K is 0
