Thermodynamics and chemical equilibrium: States of matter; Henry's Law and Raoult's Law

Question 1

What is ΔH?

Answer 1

the heat flow into the system at constant pressure

Question 2

What is entropy?

Answer 2

a thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work; qualitatively, entropy is a measure of how evenly energy is distributed in a system

Question 3

What is the second law of thermodynamics?

Answer 3

a system with constant volume and internal energy will adopt a configuration that maximises its entropy

Question 4

What is Gibbs free energy?

Answer 4

the maximum energy available in a system to do non-expansion work under conditions of constant temperature and pressure

Question 5

What is temperature?

Answer 5

a measure of how much the internal energy changes when the entropy is varied while the volume is held constant; reflects the amount of kinetic energy of the molecules in the system

Question 6

How is temperature defined?

Answer 6

T = (∂u/∂S)v

Question 7

What units does temperature have?

Answer 7

Kelvin

Question 8

What is heat capacity?

Answer 8

a measure of how much the internal energy or enthalpy changes as the temperature is varied

Question 9

How is heat capacity at constant volume defined?

Answer 9

C(v) = (∂u/∂T)v ≈ (ΔU/ΔT)v

Question 10

How is heat capacity at constant pressure defined?

Answer 10

C(p) = (∂H/∂T)p ≈ ΔH/ΔT

Question 11

What is another definition of heat capacity?

Answer 11

C ≈ q/ΔT

where q is the heat supplied to the system

Question 12

What is an extensive property of the system?

Answer 12

the value depends on the amount of substance in the sample

Question 13

What is specific heat?

Answer 13

the amount of heat per unit mass required to raise the temperature by one degree

Question 14

Is specific heat an extensive or intensive property?

Answer 14

an intensive property

Question 15

Define specific heat

Answer 15

c(p) = C(p)/m

where m is the mass of the sample

Question 16

What is molar heat capacity?

Answer 16

the amount of heat per mole required to raise the temperature by one degree; C(p,m)

Question 17

Define G

Answer 17

G = H - TS

Question 18

Define ΔG

Answer 18

ΔG = ΔH - TΔS at constant T

Question 19

Define the standard equilibrium constant

Answer 19

ΔG0 = - RTlnK0

Question 20

What is the standard state?

Answer 20

state of a system chosen as standard for reference by convention; the state of the pure system at standard pressure (1 bar)

Question 21

What is the standard state of a solvent?

Answer 21

the state of the pure solvent at standard pressure

Question 22

What is the standard state of a solute?

Answer 22

the hypothetical state of the solute at standard molality (most commonly 1 mol.kg^-1), standard pressure and exhibiting ideal-dilute behaviour

Question 23

What is the problem with the chemists’ definition of the standard state of the solute?

Answer 23

it leads to a standard state of H+ ions in solution corresponding to a pH of zero, which does not occur in biological systems

Question 24

What is an alternative definition of the standard state of the solute?

Answer 24

molality of 1 x 10^-7 mol.kg^-1 (pH = 7 for H+)

Question 25

What configuration will a system under conditions of constant temperature and pressure adopt?

Answer 25

that of lowest possible Gibbs free energy

Question 26

What is Gibbs free energy a function of?

Answer 26

T, p, n(A), n(B)

where n(A) and n(B) are number of moles of A and B, respectively

Question 27

Considering a two-component system such as that represented by the simple equilibrium A⇌B, how does the Gibbs free energy depend on the amount of A and B present?

Answer 27

dG = (∂G/∂n(A)) dn(A) + (∂G/∂n(B)) dn(B)

Question 28

Define the chemical potential

Answer 28

μ(A) = ∂G/∂n(A)
μ(B) = ∂G/∂n(B)

Question 29

Define dG

Answer 29

dG = μ(B) dn(B) + μ(A) dn(A)
= [ (μ(B) - μ(A) ] dn(B)

therefore ∂G/∂n(B) = μ(B) - μ(A)

Question 30

What is the meaning of G taking a minimum value?

Answer 30

there is no thermodynamic advantage to changing the composition; the system is at equilibrium

Question 31

What is the relationship between μ(A) and μ(B) at equilibrium?

Answer 31

at equilibrium, ∂G = 0, therefore μ(B) - μ(A) = 0 and μ(A) = μ(B)

Question 32

What happens to the chemical potential at equilibrium?

Answer 32

at equilibrium, the chemical potentials of all components of a mixture are equal across all phases

Question 33

Define the chemical potential relative to a standard

Answer 33

μ(x) = μ(x)0 + RT ln a(x)

where a(x) is the relative activity of x