3. Properties of Ionic Solutions & Biochemical Energetics Flashcards
Define chemical potential
- The free energy associated to specific species and its amount in a mixture
Or in a more understandable way:
- The free energy (G) per mole under a given set of conditions
How would the chemical potential of 1mM glucose solution compare to the chemical potential of 2mM glucose solution?
The 2mM glucose solution would have double the chemical potential of the 1mM one
What is the equation for the chemical potential?
mu = G/n
mu = chemical potential G = free energy of the system n = number of moles in system
What type of measurement is chemical potential?
Intrinsic property (independent of how much of the material is present). Mu depends on the environment of the species instead.
What is the equation for delta G in a multicomponent system?
Hint: involving mu and n
deltaG = (Sigma)(mu)i.(delta)ni
delta G = change in free energy of system
Sigma = sum of ….
(mu)i = chemical potential of a species
(delta)ni = change in number of moles of a species
Does the chemical potential change during a reaction?
Yes, according to chemical change and extent of chemical reaction
What is the Gibbs-Duhem equation?
deltaG = (sigma)(mu)i.(delta)ni + (sigma)ni.(delta)(mu)i
(mu)i = chemical potential of a species
(delta)ni = change in number of moles of a species i
(delta)(mu)i = change in chemical potential of a species
ni = number of moles of a species i
What is the Gibbs-Duhem equation for the reaction A–>B?
deltaG = (mu)A(delta)nA + (mu)B(delta)nB + nA(delta)(mu)A + nB(delta)(mu)B
When does the chemical potential change? According to what?
Chemical potential changes when the concentration of the reactants change
According to the Gibbs-Duhem equation
How does the chemical potential change over the course of a reaction? Why?
In a two component mixture (A–>B) if component A chemical potential increases then muB decreases. This is because the sum of the free energies of each species must equal 0 (remain constant)
What is the equation for the relationship between chemical potential and chemical potential of standard state?
mu = mu0 + RTln([i]/[i0])
[i] = concentration of i [i0] = concentration of i in standard state
How do the equations for the relationship between chemical potential and chemical potential of standard state vary depending on whether a gas, liquid/solvent, solute or solid is being used?
Gas: instead of [i] (concentration of liquid) the partial pressures of the gas are used - symbol - Pi
Liquid/solvent: [i0] is concentration of pure liquid
Solute: [i], [i0]
Solid: (mu)i = (mu)i0 - all pure solids are in their standard states by definition
What is the partial pressure of a gas at standard state?
1atm
What is the concentration of the solute at standard state?
1M (1 mol/dm3)
What mole fraction is required for a solution to have ideal behaviour?
If there is a solvent with a mole fraction (x) close to 1 then ideal behaviour applies. This is because there are mostly solvent molecules so solvent molecules mostly only see other solvent molecules
What is the equation generated by Raoult to relate the mole fraction and chemical potential? How is this equation different if you took it from the solutes perspective?
(mu)A = (mu)A* + R.T.ln(xA)
(mu)A = chemical potential of solvent A
(mu)A* = chemical potential of PURE solvent A
RT = u no dis
xA = mole fraction of solvent A
If you took it from the solutes perspective then the mole fraction of solute B would be almost 0 so B would only see solvent A molecules. This is so NOT-ideal
Use Henry’s law
(mu)B = (mu)B* + RT.ln(xB.KB)
KB = constant with dimensions of pressure xB = mole fraction of B
ORRRRR
(mu)B = (mu)B* + RT.ln(aB)
aB = activity of B
How do electrolyte solutions (e.g. NaCl solute) affect the colligative properties?
If electrolytes dissolve in solution then 2x as many dissolved species! WOAH FUCK! Therefore the colligative properties will be more pronounced than those predicted using M of solute.
Furthermore, the electrostatic forces between the ions are stronger than the VDW forces (between solvent molecules) so the forces between molecules are not all even and this just fucks shit up. SO not ideal!
Give an example of some important ionic interactions in biochemistry (3)
- The interactions of proteins with other proteins e.g. cytochrome c oxidase with cytochrome c
- Enzyme-substrate reactions - coulombically influenced, shown by pH dependence of activity
- Equilibrium electrochemistry e.g. nerve action potential, chemiosmotic theory
How does pH affect the activity of enzymes?
Create a bell-shaped activity curve when comparing with pH. This issue to the pH affecting the protonation of the amino groups present in the active site.
Give an example of 2 important ionic interactions in the ANIMAL KINGDOM
Ampullae of lorenzini in sharks - can’t be bothered to google this but assuming the sensory shit in their nose
Electric eels
Out of molarity (mol/dm3) and molality (mol/kg), which is temperature dependent? Why?
Molarity, temperature affects volume
Can get away with using molarity as most biochemistry stuff is undertaken at 25deg.
What is the equation for the ionic strength of a solution?
I = 0.5.(sigma).Mi.(zi^2)
I = ionic strength sigma = sum of Mi = concentration of one ion zi = charge of one ion
EXAMPLE: 1mol/kg of Na2SO4, 0.02mol/kg of NaCl
I = 0.5[(2x1^2)+(1x(-2)^2)+(0.02x1^2)+(0.02x(-1)^2)
Gimme 4 facts that always apply to ionic solutions, and gimme them now
- Electrolyte solution is globally electroneutral
- All ions in solution are in perpetual random motion (amount of motion depends on thermal energy)
- Like charges are repelled, opposite charged attracted
- Cloud of cations around an anion and vice versa
What is the equation for activity?
a = (gamma).(c/c0)
a = activity gamma = activity coefficient c = concentration of solute c0 = concentration of solute under standard state (usually 1mol/kg)
c/c0 cancels as both units of molality
What is the activity?
Basically the mole fraction, but corrected for the effects of non-ideality by the coefficient
How does the solution behave if it has an activity coefficient tending towards 1?
The solution behaves ideally - or is at infinite dilution
If we are considering the dissolving of an electrolyte, how do we calculate the activity?
We calculate the activity for each ion (+ve and -ve), but it is impossible to separate the ion activities (don’t know dissociation etc) so we use the mean ionic activities:
a+/- = sqrt(a+.a-)
in NaCl a+ = a-
Which two ways may we define the mean activity coefficient?
- a+/- = sqrt(a+.a-)
2. Also can define in terms of the mean activity coefficient
Which equations would we use to work out the mean activity, activity coefficient and solute concentration of the salt MpXq?
p and q are subscript
MpXq –> PM^2+ + qX^2-
a+/- = ((a+^p).(a-^q))^(1/(p+q))
gamma+/- = ((gamma+^p).(gamma-^q))^(1/(p+q))
c+/- = ((c+^p).(c-^q))^(1/(p+q))
Find c+/- for the following solute:
La2(SO4)3 = 2La^3+ + 3SO4^2-
[La2(SO4)3] = M mol/kg
c+ = 2M c- = 3M
c+/- = [(2M)^2.(3M)^3]^1/5
c+/- = 2.55M mol/kg
What is likely causing the non-ideality of the ionic solutions?
The electrostatic interactions because they are long range and strong (compared to other IMG)
What is the ion atmosphere? Who coined this term?
The ion atmosphere (a statistical phenomenon) is the presence of a surplus of oppositely charged ions around every ion in solution
Debye and Huckel
What is the Debye length?
The distance between the central and the edge of the ionic imbalance
What is the equation for the Debye length?
rD = (((epsilon0)(epsilonr).RT)/(2.(F^2).p.I)))^0.5
rD = debye distance epsilon0 = permittivity of a vacuum epsilonr = relative permittivity RT = Gas constant and temp F = Faraday constant p = density of solvent I = Ionic strength
NGL this is confusing, look at page 11 for less confusins.
What does the Debye-huckel model allow us to calculate?
gamma+/- (the mean activity coefficient)
What is the Debye-huckel model (5)?
- Long range electrostatic interactions cause electrolyte solutions to be strongly non-idea
- Interaction between 2 neutral molecules decreases by 1/(r^6)
- BUT coulomb interactions decrease by 1/r
- So coulombic interactions are probs responsible for departures from ideality
- Ion atmosphere - at any point in time an ion is surrounded by a spherical haze of oppositely charged ions
What are three characteristics of the mean activity coefficient of electrolyte solutions?
- mean activity coefficient has large distortions at low concentration
- Strong electrolytes have a mean activity coefficient which passes through a minimum
- More highly charged ions (e.g. +2/-2) have stronger effects
Which two effects cause the characteristics shown by a log(mean activity coefficient) against sqrt(I) graph?
- Interionic attraction - lowers the free energy of ions in solution, so lowers gamma+/-
- Ions attract solvent dipoles - reducing free energy of water - using Gibbs-Duhem if G (of water) decreases then G+/- increases and so does gamma+/-
What are the 7 main assumptions of the Debye-Huckle theory?
- Electrolyte assumed to be completely dissociated at all concentrations
- Ion pair formation is disregarded - IPF occurs at high conc. or in solvents with low dielectric constants
- Ions are regarded as hard spheres of radii a
- Ions are in constant random thermal motion
- Only coulomb interactions between ions are considered (the long range ones)
- Solute/solvent interactions are ignored
- Solvent is considered a continuous medium with relative permittivity epsilonr
