Thermodynamics

Question 1

Boyle’s equation

Answer 1

p∝1/V

Question 2

Charles’ and Gay-Lussac’s equations

Answer 2

V∝T

p∝T

Question 3

Ideal Gas Law

Answer 3

pV = nRT

Question 4

1 bar in pascals

Answer 4

1 bar = 1 x 10⁵ Pa

Question 5

Pa in SI base units

Answer 5

kg s^-2 m^-1

Question 6

0^oC in kelvin

Answer 6

0^oC = 273.15K

Question 7

Dalton’s Law of Partial Pressures

Answer 7

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 = p_A + p_B

Question 8

Assumptions made by the Ideal Gas Law

Answer 8

The molecules take up no volume

There is no interaction between molcules

Question 9

Van der Waals equation of State

Answer 9

p = (nRT / V-nb) - (n²a / V²)

Question 10

For reaction A + 2B <–> C + D, what is the equilibrium constant, K?

Answer 10

|C||D| / |A||B|²

Question 11

Symbol for internal energy change

Answer 11

ΔU

Question 12

System definition

Answer 12

What we are interested in

eg reactants / products or biological cell

Question 13

Surroundings definition

Answer 13

Everything else in the universe apart from the system

Question 14

Open system definition

Answer 14

Can exchange matter and energy with surroundings

eg a beaker with ether, that is left to evaporate

Question 15

Closed system definition

Answer 15

Can only exchange energy with its surroundings (not matter)

eg a reaction in a sealed tube

Question 16

Isolated system definition

Answer 16

Completely insulated from surroundings (cannot exchange energy with surroundings)

Question 17

Relationship between internal energy, U and enthalpy, H

Answer 17

H = U + pV

Question 18

Definition of enthalpy

Answer 18

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

Question 19

1st Law of Thermodynamics

Answer 19

ΔU = q + w = ΔH - pΔV

q = heat transfer to/from surroundings
w = work done on/by surroundings

Question 20

Work done =

Answer 20

-force x distance = -pressure x ΔV = -Δn x RT

= nRT x ln (V₁ / V₂)

Question 21

Which properties depend on the path you take?

Answer 21

Heat and work

Question 22

What properties do not depend on the path taken?

• Called state functions

Answer 22

ΔU, T, V, p, H, S, G

Question 23

For an ideal gas at a constant temperature…

Answer 23

ΔU = 0

q = -w

(𝛿U / 𝛿V)_T = 0

Question 24

What is the specific heat capacity, C of a substance?

Answer 24

The amount of heat needed to raise the temperature of a substance by 1 degree

C = (𝛿q / 𝛿T) J K^-1 mol^-1

Question 25

For constant pressure, ΔH =

Answer 25

C_p x ΔT

Question 26

An exothermic process…

Answer 26

Releases heat , ΔH < 0

Question 27

An endothermic process…

Answer 27

Absorbs heat, ΔH > 0

Question 28

What does Δ_rH^o(298.15 K) mean?

Answer 28

Standard enthalpy change of reaction at 298.15 K

Question 29

Define entropy

Answer 29

The measure of the molecular disorder of a system

Question 30

2^nd Law of Thermodynamics

Answer 30

The entropy of a system and surroundings increases in the course of a spontaneous change

Question 31

Trouton’s rule

Answer 31

Δ_vap S is approximately 85 J K^-1 mol^-1 for most liquids (NOT H₂O)

Question 32

What process is accociated with ΔH_fus?

Answer 32

Solid to liquid

Question 33

Entropy change for A_(s) –> B_(aq) + C_(aq)

Answer 33

Negative

Question 34

Third Law of Thermodynamics

Answer 34

A perfect crystal at 0 K has zero entropy

Question 35

Gibbs free energy, G =

Answer 35

H - TS (at const. p)

Question 36

Helmholtz free energy, A =

Answer 36

U - TS (at const. V)

Question 37

dG =

Answer 37

Vdp - SdT

Question 38

For a reversible change, dU =

Answer 38

q + w

Question 39

ΔG^o =

Answer 39

-RTln(K) = ΔH - TΔS

Question 40

K is always…

Answer 40

unitless

Question 41

The Van’t Hoff equation

Answer 41

(𝛿 lnK / 𝛿T) = (ΔH / RT²)

Question 42

F, degrees of freedom that we can change independently eg temp, pressure, composition =

Answer 42

C - P + 2

C = chemical independent constituent

P = phase

Question 43

For calculating ΔH_f at a different temperature, use the equation…

Answer 43

T₂

ΔH = ʃ ΔC_p dT

T₁

Question 44

For calculating ΔS for 2 temperatures, use…

Answer 44

ΔS = C_p x ln(T₂ / T₁)

Question 45

For calculating ΔS using ΔH, use…

Answer 45

ΔS = ΔH / T

Question 46

dU =

Answer 46

dq + dw

Question 47

For an instantaneous change…

Answer 47

dq = 0

Question 48

At constant volume (isochoric)…

Answer 48

dV = 0

so dw = -pdV = 0

so ΔU = q

T₂
q = ʃ dq = C_V x ΔT
T₁

Question 49

At constant pressure (isobaric)…

Answer 49

dw = -pdV

so Δw = -ʃ p dV

so ΔU = Δq - pΔV

Question 50

For no heat change…

Answer 50

H = U + pV

Δq = 0

ΔU = Δw

Question 51

Name A, B, X, Y and Z
Give brief explanations of A and B

Answer 51

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