Chemistry Y6: Thermodynamics Flashcards
What is the 1st law of thermodynamics?
Energy can be converted from one form to another but the total amount of e energy in the universe is constant.
What is the two types of energy stores for substances?
Kinetic and potential energy
What can change the internal energy (U) of a system?
1) Transfer of heat (to and from surroundings)
2) Work being done (on or by the system)
What is another way of saying U = U(system) FINAL -U(system) INITIAL?
U = KE + KP
- Kinetic and potential energy
What is the equation for C (SHC) at constant volume (V)?
Cv = ( d̂U/d̂T)V = dQ/dT
At constant pressure what does dH equal?
dH=dQ
= to the amount of heat transferred between the systems and the surrounding during a process
what is the 2nd law of thermodynamics?
1) heat never flows spontaneously from cold body to a hot body
2) Any spontaneous change must be accompanied by an increase in entropy of the universe
What is the mathematical definition of entropy?
ds = dQ/T
Jk-1mol-1
Which is bigger Latent heat of fusion or the Latent heat of vaporisation?
Vaporisation
What are the stationary parts on a graph of entropy vs Temperature?
Melting and Boiling
* stationary as energy is being used to change state
What is the calculation of change in Entropy (ΔS) from T2 to T1?
ΔS = C ln (T2/T1)
T = temperature (k)
What is the definition of statistical entropy / Equation for S across changing states?
S = Kb ln(W)
Kb = Boltzmann constant
W = number of states
What is the 3rd law of thermodynamics?
S= Kb ln(W)
For a perfect crystal at 0K, since there is only 1 possible configuration (W=1), S =0
As S= Kb ln(1) = 0
What is the calculation of change in Entropy (ΔS) from V2 to V1?
ΔS= nRln (V2/V1)
Does a system favour having a high or low entropy
High entropy
The feasibility of a reaction based on the difference between the Reactants and products is known as
Gibbs free energy , G
What is the equation for Gibbs free energy at constant temp and pressure?
ΔG = ΔH-TΔS
what are the limitations of Gibbs?
- If pressure isn’t constant
- if we think of internal energy in Helmholts energy, F
What is the equation for Helmholtz energy?
-When will a reaction be spontaneous?
F= U-TS
when 𝝙F < 0
-Same as 𝝙G
What if pressure isn’t constant when calculating Gibbs
You have to calculate in terms of U, internal energy, rather than using enthalpy.
This used Helmholtz energy, F
F = U - TS
What is the gibs equation when we vary temperature and pressure
dG= VdP - SdT
How do we take partial derivatives of G where Pressure is constant?
(d̂G/d̂T)P = -S
How do we take partial derivatives of G where temperature is constant?
(d̂G/d̂P)T = V
If temperature increases, what happens to S and G
S always increases
G always decreases
dG=-SdT
Does G vary more with a smaller or larger volume?
A larger volume
dG=VdP
As pressure increases what happens to G
Since molecular volume is getting smaller, the effect on G is getting Smaller.
dG= VdP
What is sublimation?
(s) -> (g)
At constant temp what does dG equal?
dG = VdP
V always +ve so, G always increases with pressure
What is the ideal gas equation?
PV=nRT
What is the ideal gas equation for 1 mol of gas
V=RT/P
What is the equation linking ideal gas to ΔG
ΔG= RT ln(P2/P1)
Units Kj.mol-1
What is 𝝙G⁰, and what is its equation?
it is the different in Gibbs energies of R’s and P’s when they’re in their standard states
𝝙G⁰= sum G⁰ Products - sum G⁰ Reactants
What are the standard states?
Temp = 298K
Pressure = 1atm
Concentration = 1 mol.L-1
What is the most stable phase of a material
Which ever has the lowest Gibbs energy
What is the equation we use when reactants and products are not in standard states to get change in Gibbs (ΔG)
ΔG = ΔG⁰ + RTlnQ
When Q= K what does ΔG equal?
Turn this into an equation
ΔG = 0
0=ΔG⁰ + RTlnK
If, 0=ΔG⁰ + RTlnK
What does ΔG⁰ equal?
ΔG⁰ = -RTlnK
What is the quadratic formula and when do you. need to use it?
In K ( rate) calculations
____________
X= -b+/- _/ (b²-4ac/2a)
what is the equation for Ui (chemical potential) ?
Ui = (d̂G/d̂ni)PTn
what is total Gibbs free energy of a binary mixture?
G= nAUB + nBUB
n= no. moles
U= chemical potential
what does U depend on?
Temperature / pressure
-so G can vary with n/u
combine G= nAUB + nBUB
and dG = VdP- SDT
dG = VdP -SdP + UAdnA + UBdnB
write an equation of dG and its relationship with T/P
dG = VdP- SDT
What is ΔG reaction?
It is the difference in chemical potentials at the composition of the mixture
-G of a reaction system tries to minimise itself
-Reaches 𝝙G at equilibrium (lowest point on graph)
Variation of 𝝙G with composition- show how chemical potential of component i varies
-What does this tell is
Ui = Ui ⁰ + RT lnai
ai = activity of i
Direction a reaction will move
What is Le Chatalier’s principle
When a system at equilibrium is changed, the system will adjust to decrease the effect of that change
Say 𝝙H is negative, what happens when you increase/decrease temperature?
℃ Increase = R’s lose heat, exo favoured= forward reaction, K increases
℃ Decrease = R’s gain heat, endo favoured = backwards reaction, K decreases
if 𝝙S is +ve, how does ΔG⁰ change with temp?
if 𝝙S is -ve, how does ΔG⁰ change with temp?
- Increased temp = Increased G
- Increased temp = decreased G
What states are relevant in entropy
only (g)
What is the chemical potential of a substance in a mixture?
It is the contribution of that substance to the total Gibbs energy of the mixture
What is the Vant Hoff equation and what does it tell us
LnK2-Lnk1= ΔH⁰/R [1/T1 - 1/T2]
Tell us how K changes as temperature changes
what is vapour pressure?
the pressure exerted by a vapour
-Related to the (s)/(l) evaporation rate
-More volatile a substance = greater VP
-increased temp = increase KE= greater VP
What is the Clausius- Clapeyron equation and what does it tell us?
LnP2 - LnP1 = ΔH vap/R [1/T1 - 1/T2]
It tells us how P changes as temperature changes
What is the effect of altitude on bp?
atm pressure decreases at higher altitudes
-So water boils at lowere temperatures
When does SVP = ATM pressure of surroundings
boiling point
Since VP increases with temp, for temperatures > ATM pressure the bp H2O > 100℃
when is bp of H2O below 100 ℃?
At lower pressures
What is Gibbs phase rule
F = C -P +2
F = number of degrees of freedom
C = number of components in the system
P= number of phases at equilibrium
what do phase diagrams show?
they plot temperature vs pressure
*illustrates at what T/P the different (s), (l), (g) phases of matter
What is triple point on a phase diagram?
The temperature and pressure at which all 3 phases co-exist (s), (l), (g)
What is the critical point?
On the (l) (g) boundary, its where the boundary disapears and the substance becomes a supercritical fluid
What are supercritical fluids?
They are substances that can move through objects like gases. But they dissolve things like a liquid
Describe a CO2 phase diagram
*The (s) - (l) equilibrium line has a +ve slope
*(S) CO2 is denser than (l) CO2
*At 1atm, if temp increases the phase change is (s) - (g) -Sublimation
*Can’t be a liquid at P > 5.11
Describe a H2O phase diagram
- The (s) -(l) equilibrium line has a -ve slope
- Ice is less dense than (l) water
- As P increases, ice melts to form water because the same mass occupies less volume
*Triple point is P=00.06ATM, T=0.01℃
How many distinct solid phases of ice are there?
16
What are polymorphs
Solid material can exists in different conditions in the form of different crystalline structures
What is the phase rule?
F = C - P + 2
F = dof (independent variables you can change without changing No. of phases at equilibrium)
C = NO. of components in the system (Chemically independents constituent of the system)
P = No. of phases at eq.
If,
I > II and,
UAI > UAII, is the reaction feasible?
dG = -UAdnA (I) + UAdnA (II)
Yes, ΔG will occur, as overall ΔG will be <0
I -> II (forward reaction)
What is the mathematical definition of a components contribution to G in terms of U?
RTlnPa/P
PA = P*A Xa
PA = partial pressure of A
P*A = Vapour pressure of pure component A
XA= mole fraction of A in the mixture
What is the calculation for UA mixture?
UA mixture = UA pure + RT ln XA
What is the driving force of mixing
*XA < 1. so that,
*Ln XA <0. so that,
*UA is less in the mixture than the
pure gas (at constant P) =DRIVING FORCE
so , ΔG<0
= SPONTANEOUS MIXING
What is the equation for mixing 4+ ideal gases?
Gfinal -Ginitial = RT/n (Sum of i) XiLnXi
what does ni/n equal?
Xi
How will the mixing of 4 gases be spontaneous?
if Xi < 1 , all terms in lnXi must be <0
So that 𝝙G of mixing is <0
= Spontaneous mixing
-at constant T/P
What is the equation for ΔS of mixing
ΔS mixing = -(dG mixing/ dT)
= - (R/n [Sum of I] XiLnXi
ΔS = ALWAYS +ve
=Driving force , as ΔG = ΔH-TΔS
when mixing 2 components A and B what does it mean if:
1.Mole fraction of A =1
2.Mole fraction of A =0
3.Mole fraction of a = 0.5
1.AAAAAA
2.BBBBBB
3.ABABAB
For 𝝙S of mixing, what equation do we need to remember to find 𝝙G mixing
𝝙G mix = RT/ n x (SUM of i Xi lnXi)
Sub in to
S= -(d̂G/d̂T)P
What are the driving forces for mixtures of non-ideal gases?
ALWAYS:
1. Xi<1
2. so lnXi <0
3. so 𝝙G mixing <0
What is Raolt’s law?
Partial VP of each component and it’s given by:
Pi= P*I Xi
Xi = mole fraction
P*I= vp when pure
In a mixture how do find VP
P= PA + PB
Add the partial pressures
What is Euler’s number?
Natural log E = 2.718
When is mixing not spontaneous?
Between polar and non-polar substances e.g. oil and water
* Energy lost by breaking bonds isnt compensated by maing bonds
*SO, 𝝙H mixing is +ve > +ve 𝝙S
*SO, 𝝙G = +ve
NOT SPONTANEOUS
(immiscible)
What does the equation:
𝝙U = 𝝙Q =P𝝙 mean?
Expansion of gas when heated
-Work done depends on pressure in the system and change in volume
-Work done on the system depends on gas changes and pressure in the system
What are differentials and what do they represent?
“differentials” refer to infinitesimal changes in thermodynamic variables, which are used to describe how a system’s properties change in response to changes in its state.
-Usually in terms of one variable in relation to another.
If y=3X²Z⁴
And we only want to know the relationship between X and Y, what do we do?
We keep X constant so,
(dy/dx)z = 6XZ⁴
What is the formula for working out differentials, if Y=aX^n
dy/dx = anX^n-1
How would you get the overall change in internal energy?
you would add the differentials where volume is kept constant and whee temperature is kept constant
du= (d̂U/d̂T)V. dT + dU=(d̂U/d̂V)T. dV
What is a state function?
It is a property whose value doesn’t depend on path taken to reach a specific value
-Conditions and history independent of value
e.g. Internal function, Entropy, Gibbs
What is a path function?
path functions have values that depend on the path taken from initial state -> final state
e.g. Work, heat transfer
What is the definition of heat capacity?
The ability of a system to absorb heat as a function of temp
-Energy required to heat 1g by 1K
Enthalpy is = internal energy + (——)?
P x V
so H= U + PV
In regards to entropy and specific heat what does CV equal?
CV = dQ/dT = T(d̂S/d̂T)v
At melting what does 𝝙S equal?
𝝙S = 1/L
L = Latent hat of fusion
At boiling what does 𝝙S equal?
𝝙S = L/T
L= Latent heat of vaporisation
What is bigger latent heat of fusion or vaporisation?
Lv > Lf
what is the relation ship between entropy and temperature
If T increases, S increases
If dS = dQ/T and C= d̂Q/d̂T
merge these equations if relation to dS
dS = Cd̂T/T
What can often hinder maximum entropy?
What equation relates to max Entropy?
Bonds and IM forces
S =Kb ln(W)
j.k-1mol-1
