Thermodynamics Flashcards
Boyle’s equation
p∝1/V
Charles’ and Gay-Lussac’s equations
V∝T
p∝T
Ideal Gas Law
pV = nRT
1 bar in pascals
1 bar = 1 x 105 Pa
Pa in SI base units
kg s-2 m-1
0oC in kelvin
0oC = 273.15K
Dalton’s Law of Partial Pressures
For gases A and B:
p<sub>A</sub> = n<sub>A</sub> RT / V p<sub>B</sub> = n<sub>B</sub> RT / V
p = pA + pB
Assumptions made by the Ideal Gas Law
The molecules take up no volume
There is no interaction between molcules
Van der Waals equation of State
p = (nRT / V-nb) - (n2a / V2)
For reaction A + 2B <–> C + D, what is the equilibrium constant, K?
|C||D| / |A||B|2
Symbol for internal energy change
ΔU
System definition
What we are interested in
eg reactants / products or biological cell
Surroundings definition
Everything else in the universe apart from the system
Open system definition
Can exchange matter and energy with surroundings
eg a beaker with ether, that is left to evaporate
Closed system definition
Can only exchange energy with its surroundings (not matter)
eg a reaction in a sealed tube
Isolated system definition
Completely insulated from surroundings (cannot exchange energy with surroundings)
Relationship between internal energy, U and enthalpy, H
H = U + pV
Definition of enthalpy
A measure of the total energy of a thermodynamic system. It includes the internal energy (sum of kinetic and potential energies), plus an additional term, pV (energy required to ‘make room for it’ by displacing its surroundings
1st Law of Thermodynamics
ΔU = q + w = ΔH - pΔV
q = heat transfer to/from surroundings w = work done on/by surroundings
Work done =
-force x distance = -pressure x ΔV = -Δn x RT
= nRT x ln (V1 / V2)
Which properties depend on the path you take?
Heat and work
What properties do not depend on the path taken?
- Called state functions
ΔU, T, V, p, H, S, G
For an ideal gas at a constant temperature…
ΔU = 0
q = -w
(𝛿U / 𝛿V)T = 0
What is the specific heat capacity, C of a substance?
The amount of heat needed to raise the temperature of a substance by 1 degree
C = (𝛿q / 𝛿T) J K-1 mol-1
For constant pressure, ΔH =
Cp x ΔT
An exothermic process…
Releases heat , ΔH < 0
An endothermic process…
Absorbs heat, ΔH > 0
What does ΔrHo(298.15 K) mean?
Standard enthalpy change of reaction at 298.15 K
Define entropy
The measure of the molecular disorder of a system
2nd Law of Thermodynamics
The entropy of a system and surroundings increases in the course of a spontaneous change
Trouton’s rule
Δvap S is approximately 85 J K-1 mol-1 for most liquids (NOT H2O)
What process is accociated with ΔHfus?
Solid to liquid
Entropy change for A(s) –> B(aq) + C(aq)
Negative
Third Law of Thermodynamics
A perfect crystal at 0 K has zero entropy
Gibbs free energy, G =
H - TS (at const. p)
Helmholtz free energy, A =
U - TS (at const. V)
dG =
Vdp - SdT
For a reversible change, dU =
q + w
ΔGo =
-RTln(K) = ΔH - TΔS
K is always…
unitless
The Van’t Hoff equation
(𝛿 lnK / 𝛿T) = (ΔH / RT2)
F, degrees of freedom that we can change independently eg temp, pressure, composition =
C - P + 2
C = chemical independent constituent
P = phase
For calculating ΔHf at a different temperature, use the equation…
T2
ΔH = ʃ ΔCp dT
T1
For calculating ΔS for 2 temperatures, use…
ΔS = Cp x ln(T2 / T1)
For calculating ΔS using ΔH, use…
ΔS = ΔH / T
dU =
dq + dw
For an instantaneous change…
dq = 0
At constant volume (isochoric)…
dV = 0
so dw = -pdV = 0
so ΔU = q
T2
q = ʃ dq = CV x ΔT
T1
At constant pressure (isobaric)…
dw = -pdV
so Δw = -ʃ p dV
so ΔU = Δq - pΔV
For no heat change…
H = U + pV
Δq = 0
ΔU = Δw
Name A, B, X, Y and Z
Give brief explanations of A and B
A = Critical point - cannot get transition between gas and liquid after this point
B = Triple point - gas, solid and liquid are all stable at this T, p and V
X = Solid
Y = Gas
Z = Liquid
