D - Ions Flashcards
What is the equilibrium expression for reaction
aA + bB → cC + dD?
What are the units of K?
K = ([C]c[D]d)/([A]a[B]b)
Units of K are variable, but for the expression K has no units
What is the expression for reaction quotient Q for reaction
aA + bB → cC + dD
and how is it different to K?
Q = ([A]a[B]b)/([C]c[D]d)
Q gives the status of the reaction by mass balance.
Q can have many values whereas K only has one.
What happens when Q < K, Q > K and Q = K?
Q < K → the reaction will favour the products
ΔG = negative
A + B → C + D
Q > K → the reaction will favour the reactants
ΔG = positive
A + B ← C + D
Q = K → the system is at equilibrium
ΔG = 0
A + B ⇌ C + D
What is the Arrhenius definition of acids and bases?
Acid → increases the concentration of H3O+
Base → increases the concentration of OH-
How are pKa and pKb related to the strength of the acid or base?
Low pKa = equilibrium lies to the right = strong acid
Low pKb = equilibrium lies to the right = strong base
pKa and pKb are the pH at the equilibrium point; the lower the value of pKa/b, the sooner the species will dissociate
What are the equations for Ka and Kb?
HA(aq) + H2O(l) → A-(aq) + H3O+(aq)
Ka = ([A-][H3O+])/[HA]
BOH(aq) → B+(aq) + OH-(aq)
Kb = ([B+][OH-])/[BOH]
What is a buffer?
Buffers resist change in pH by providing a source and sink for protons.
The solution contains roughly equal amounts of a weak acid and its conjugate base.
The pH will be roughly maintained when small amounts of acid or base are added.
What are the steps involved in designing a specific buffer?
1 → Choose an acid + conjugate base that has its pKa ≈ pH needed
2 → Calculate the ratio of HA : A- using the Henderson-Hasselbalch equation.
3 → Check with pH meter and adjust if necessary.
What happens when a sparingly soluble salt dissolves in water, and what is the equilibrium expression?
An equilibrium is set up between the undissolved salt and the dissociated ions:
MX(s) → M+(aq) + X-(aq)
Ksp = [M+][X-]
(The reactant is not included because it is presumed to have a concentration of 1M)
MX2(s) → M+(aq) + 2X-(aq)
Ksp = [M+][X-]2
Ksp → solubility product
What happens when a sparingly soluble electrolyte is dissolved in one of its ions?
The solubility decreases a lot.
Eg. AgCl dissolving in 0.1M NaCl
As the concentration of Cl- is a lot higher than it would be on dissolving in water, the concentration of Ag+ must be lower in order to keep the value of Ksp constant.
What is the difference between voltage and current?
Voltage → the driving force of a reaction; gives the direction of a reaction
Current → gives the quantity of electricity
What is the generic form of an electrochemical cell (no Pt)?
(Red)(s) | (Ox)(l) ‖ (Ox)(l) | (Red)(s)
Left(Red) + Right(Ox) → Left(Ox) + Right(Red)
‖ = salt bridge
What is the generic form of an electrochemical cell with Pt, and why is Pt used?
Pt | (Red)(l) , (Ox)(l) ‖ (Ox)(l) , (Red)(l) | Pt
Left(Red) + Right(Ox) → Left(Ox) + Right(Red)
The Pt provides a solid surface for the reaction to occur on
‖ = salt bridge
How are electrode potentials measured?
They are all measured against a standard Hydrogen electrode, which is used as a reference because it has a voltage of 0V.
Electrode reaction: H+(aq) + e- → ½H2(g)
The H2 gas must be at standard pressure (1 atm) and the acid at standard concentration (1M).
Why might testing electrodes with biological applications involve using 2 standard electrodes?
The standard Hydrogen electrode will have a very low pH due to the concentration of H+ required.
There may be another electrode with the same reaction as the standard H2 electrode but at pH 7.0.
What is the equation for calculating the cell electromotive force (EMF)?
EºCELL = EºRIGHT - EºLEFT
or
EºCELL = EºRED - EºOX
What is the equation for calculating Gibb’s free energy from Eº?
ΔGº = - n F Eº
n = number of electrons
F = Faraday constant (96485.332 C mol-1)
Eº = standard redox potention (V)
How can cell measurements at non-standard conditions be corrected to standard conditions?
Nernst equation:
E = Eº - [RT/nF] ln Q
where
E = measured EMF
Q = reaction quotient
[RT/F] = 0.02569V at 25ºC
What is an ion-selective electrode?
It has an electrode potential dependent on one specific ion in an external solution, which develops a junction potential with the internal solution also containing that ion.
What is a glass electrode?
Aka a pH probe.
It is sensitive to H+ ions.
Usually consists of an Ag/AgCl electrode dipping in HCl solution, which is contained in a tube with a thin-walled glass bulb at the end.
This bulb is made of special conducting glass that allows diffusion of H+ ions and, therefore, allows a junction potential to develop across its walls.
What is the difference between a galvanic cell and an electrolytic cell?
Galvanic cell → produces electricity. The reaction will proceed until equilibrium.
Electrolytic cell → A non-spontaneous reaction is driven by an external source of current.
What is the Nernst equation?
E = Eº - (RT/nF) ln Q
How do you convert between Amperes and moles of electrons?
Amperes x time = Coulombs
96495 Coulombs = 1 Faraday
1 Faraday = 1 mole of electrons
