Thermodynamics and chemical equilibrium: States of matter; Henry's Law and Raoult's Law Flashcards
What is ΔH?
the heat flow into the system at constant pressure
What is entropy?
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
What is the second law of thermodynamics?
a system with constant volume and internal energy will adopt a configuration that maximises its entropy
What is Gibbs free energy?
the maximum energy available in a system to do non-expansion work under conditions of constant temperature and pressure
What is temperature?
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
How is temperature defined?
T = (∂u/∂S)v
What units does temperature have?
Kelvin
What is heat capacity?
a measure of how much the internal energy or enthalpy changes as the temperature is varied
How is heat capacity at constant volume defined?
C(v) = (∂u/∂T)v ≈ (ΔU/ΔT)v
How is heat capacity at constant pressure defined?
C(p) = (∂H/∂T)p ≈ ΔH/ΔT
What is another definition of heat capacity?
C ≈ q/ΔT
where q is the heat supplied to the system
What is an extensive property of the system?
the value depends on the amount of substance in the sample
What is specific heat?
the amount of heat per unit mass required to raise the temperature by one degree
Is specific heat an extensive or intensive property?
an intensive property
Define specific heat
c(p) = C(p)/m
where m is the mass of the sample
What is molar heat capacity?
the amount of heat per mole required to raise the temperature by one degree; C(p,m)
Define G
G = H - TS
Define ΔG
ΔG = ΔH - TΔS at constant T
Define the standard equilibrium constant
ΔG0 = - RTlnK0
What is the standard state?
state of a system chosen as standard for reference by convention; the state of the pure system at standard pressure (1 bar)
What is the standard state of a solvent?
the state of the pure solvent at standard pressure
What is the standard state of a solute?
the hypothetical state of the solute at standard molality (most commonly 1 mol.kg^-1), standard pressure and exhibiting ideal-dilute behaviour
What is the problem with the chemists’ definition of the standard state of the solute?
it leads to a standard state of H+ ions in solution corresponding to a pH of zero, which does not occur in biological systems
What is an alternative definition of the standard state of the solute?
molality of 1 x 10^-7 mol.kg^-1 (pH = 7 for H+)
What configuration will a system under conditions of constant temperature and pressure adopt?
that of lowest possible Gibbs free energy
What is Gibbs free energy a function of?
T, p, n(A), n(B)
where n(A) and n(B) are number of moles of A and B, respectively
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?
dG = (∂G/∂n(A)) dn(A) + (∂G/∂n(B)) dn(B)
Define the chemical potential
μ(A) = ∂G/∂n(A) μ(B) = ∂G/∂n(B)
Define dG
dG = μ(B) dn(B) + μ(A) dn(A)
= [ (μ(B) - μ(A) ] dn(B)
therefore ∂G/∂n(B) = μ(B) - μ(A)
What is the meaning of G taking a minimum value?
there is no thermodynamic advantage to changing the composition; the system is at equilibrium
What is the relationship between μ(A) and μ(B) at equilibrium?
at equilibrium, ∂G = 0, therefore μ(B) - μ(A) = 0 and μ(A) = μ(B)
What happens to the chemical potential at equilibrium?
at equilibrium, the chemical potentials of all components of a mixture are equal across all phases
Define the chemical potential relative to a standard
μ(x) = μ(x)0 + RT ln a(x)
where a(x) is the relative activity of x
