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Thermodynamics 2 > Lectures 1 & 2 > Flashcards

Lectures 1 & 2 Flashcards

1
Q

what is an extensive property?

A

an extensive property depends on the amount of substance
e.g mass volume, enthalpy, entropy, internal energy, helmholtz energy, gibbs free energy

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2
Q

what is an intensive property?

A

an intensive property takes the same value irrespective of the amount of substance
e.g. temperature, pressure, density, viscosity, specificheat capacity, mole fraction, molar volume

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3
Q

what is a state function/ variable?

A

the value only depends on the current state of the substance. only takes into account the initial and final values
e.g. temperature, pressure, all energies (U,S,A,G,H)

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4
Q

what is a path function?

A

a path function depend on the history of the system by which the system has reached current state
e.g. work, heat

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5
Q

what is energy?

A

energy is the ability of a system to do work

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6
Q

what is heat?

A

heat is a form of energy associated with the movement of atoms and molecules in any material. the higher the temperqature of a material, the faster the atoms are moving, and hence the greater the amount of energy present as heat

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7
Q

what is the molecular interpretation of heat?

A

the transfer of energy that makes use of chaotic molecular motion

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8
Q

what is the notation for heat

A

δQ=T dS
where δQ is a path function and T dS

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9
Q

what is work?

A

work is the energy required to move an object against a force

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10
Q

what is the molecular intrepretation of work?

A

work is the transfer of energy that makes use of organised molecular motion

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11
Q

derive an expression for expansion/ compression work

A

dW= F dx
P=F/A
A dx = V
dW= -P dV

where δW is path function

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12
Q

how are changes in internal energy U achieved?

A

achieved by heating or cooling, or by doing work on the system or extracting work from the system

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13
Q

write an expression for the change in internal energy

A

ΔU= Q+W

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14
Q

write a statement of the first law?

A

the internal energy of an isolated system is constant
ΔU isolated = 0

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15
Q

what is the fundamental equation of internal energy

A

dU = T dS -P dV

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16
Q

derive the fundamental equation for internal energy

A

dU=δQ+ δW from the first law
δW= -P dV define expansion/compression work
δQ = T dS a relation between heat and entropy

therefore: dU = T dS -P dV

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17
Q

does the fundamental equation for internal energy apply for reversible or irreversible processes?

A

the fundamental equation for internal energy is a state function (only depends on the initial and final states) therefore it doesnt matter if the process is reversibl or irreversible. this is a general statement

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18
Q

how do you calculate internal energy for constant volume processes?

A

for a constant volume process
-P dV= 0
therefore we need to measure
ΔU=Qv

which we can gather from molecular information like the equipartion theorem and maxwell molecular velocity distribution

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19
Q

what is the equation for enthalpy?

A

H=U+PV

20
Q

is enthalpy a state or path function

A

ΔH=Hfinal -Hinitial
therefore, enthalpy is a state function

21
Q

derive an equation for the change in enthalpy

A

dH= dU+d(PV)
= T dS -P dV + P dV + V dP
dH= T dS + V dP

22
Q

how would you measure and calculate ΔH?

A

ΔH is related to the heat supplied at constant pressure
ΔH=Qp
for an endothermic reaction ΔH>0
for an exothermic reaction ΔH<0

23
Q

what is the second law in terms of entropy?

A

if an isolated system undergoes change, it will change in such a way that its entropy will increse or, at best, remain constant

S tot ≥ 0

24
Q

what does spontaneous change mean?

A

spontaneous means it occurs with no need for external influence or driver
( no implication on the rate of the process, the process can be fast or slow)

25
Q

how does spontaneous change affect entropy

A

the entropy of an isolated sytem increases in the course of spontaneous change

26
Q

what is the entropy at equilibirum

A

ΔS tot = 0

the system is at a maxium in entropy, since direction of spontaneous change is to increase entropy

no entropy change

27
Q

what does entropy relate

A

entropy is a quantity that relates heat absorbed from or delivered to a reservoir at a given temperature

dS = δQrev/ T

28
Q

what is Qrev

A

Qrev is the energy (heat) transferred reversibly to the system at absolute temperature T

29
Q

what is the total entropy?

A

the total entropy involves the system and its surroundings

dStot = dSsys + dSsurr ≥ 0

30
Q

why does δQsurr= - δQsys

A

because the surroundings act as a heat source for the system

31
Q

why are the surroundings unaltered by the heat loss

A

because the surroundings are arbitrarily large ‘infinite’ and therefore the heat exchanged is infinitesimal

32
Q

if the surroundings are unaltered by the heat loss what does that mean for the entropy of the surroundings

A

dSsurr= δQsurr/Tsurr

33
Q

derive dS≥ δQ / T

A

dStot = dSsys + dSsurr ≥ 0
∂Qsurr= -∂Qsys (surroundings act as heat source to the system)
dSsurr= ∂Qsurr/Tsurr (entropy of surroundings)
therefore,
dSsurr=- ∂Qsys/Tsurr (from above relation)
dStot = dSsys - ∂Qsys/Tsurr ≥ 0 (from above relation)
dSsys ≥ ∂Qsys/Tsurr

then, if the system is a thermal equilibrium with its surroundings Tsurr=Tsys=T

therefore, dSsys≥ δQsys / T

then we normall drop the ‘sys’ subscript to form the clausius inequality

dS ≥ δQ/T

34
Q

what is the clausius inequality?

A

dS ≥ δQ/T

or
TdS -δQ ≥ 0

35
Q

what is clausius inequality for macrocscopic quantities

A

ΔS≥Q/T

TΔS -Q ≥ 0

36
Q

derive an equation for helmholtz free energy A

A

TΔS -Q ≥ 0 second law
@ specified T,V

Q= ΔU
TΔS -ΔU ≥ 0 by definition -ΔA

therefore
ΔA=ΔU -TΔS

ΔA(V,T)≤ 0

ΔA=A(T2,V2)-A(T1,V1)

37
Q

what is the helmholtz free energy?

A

the helmholtz free energy A provides the direction of spontaneous change in processes where volume V and temperature T are independent (specified) variables

A=U-TS

38
Q

what is the helmholtz free energy in terms of differentials

A

dA=dU-d(TS)
dU=TdS-PdV
dA=TdS-PdV-TdS-SdT
dA=-PdV-SdT

39
Q

what is the gibbs free energy

A

the gibbs free energy G provides the direction of spontaneous change in processes where the pressure P and temperature T are independent (specified) variables

40
Q

derive the gibbs free energy equation

A

TΔS -Q ≥ 0 second law
@ specified T,P

Q= ΔH
TΔS -ΔH ≥ 0 by definition -ΔA

therefore
ΔG=ΔH -TΔS

ΔG(P,T)≤ 0

ΔG=G(T2,P2)-G(T1,P1)

ΔG=0 if at equilibrium
ΔG< 0 if spontaneous

41
Q

what is the gibbs free energy in terms of differentials

A

G=H-TS
dG=dH-TdS-SdT

dH=TdS+VdP

dG=TdS+VdP-TdS-SdT
dG=VdP-SdT

42
Q

what is the entropy for real systems

A

absolute entropies for real systems re strictly positive

S>0 for real systems

43
Q

what is ΔG at equilibrium?

A

0

44
Q

what is ΔG during spontaneous change

A

less than 0

45
Q

what does ΔG≤ 0 determine?

A

determines the direction of spontaneous change at given P and T

Thermodynamics 2 (3 decks)

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