pH, pKa and Electrochemistry Flashcards
What is the equation for pH?
pH = -log10[H+]
What is the equation for Kw, or ion product?
Kw = [H+][OH-] = 1 x 10-14
At 25°
What is a weak acid, and what is the equation for Ka?
A weak acid is one that doesn’t fully dissociate into its ions. The acid dissociation constant, Ka, measures the position of equilibrium.
Ka = [H+][A-]/[HA]
What is the Henderson-Hasselbalch equation, and what are its uses?
pH = pKa + log10[A-/HA]
It can be used to determine the charge on a protonatable group at any pH.
It can be used to calculate amounts of reagents needed to make a buffer solution at a specific concentration and pH.
What is pKa and what is the equation for it?
pKa is the pH at which [A-] = [HA]
pKa = -log10[Ka]
What is a buffer and how does it work?
A buffer is a solution that minimises the change in pH caused by any additions of acid or alkali.
Weak acids can act as buffers around their pKa (+- 1 unit) because at this point the solution is a mixture of HA and A-.
When acid is added, A- will react with it forming HA. When alkali is added, HA will react with it forming A- and H2O.
The change in pH will be lowest when pH = pKa.
What is the equation for K, the equilibrium constant, and what happens when K < 1 and K > 1 ?
For reaction A + B → C + D
K = [C][D]/[A][B]
K > 1 → the reaction will move spontaneously in the direction that favours the products
K < 1 → the reaction will move spontaneously in the direction that favours the reactants
What is the equation for standard Gibbs free energy, and what happens when K = 1, K > 1 and K < 1 ?
ΔG° = - RT lnK
K = 1 → the reaction is already at equilibrium and ΔG° = 0
K > 1 → the larger the value of K, the more negative the ΔG°, and the more the reaction will favour the products
K < 1 → the smaller the value of K, the more positive the value of ΔG°, and the more the reaction will favour the reactants
What is the definition of Gibbs free energy?
It is the amount of energy that can be used to do useful work by a reaction that starts under standard conditions. This equates to:
ΔG° = ΔH° - TΔS°
That is, ΔG° is the energy released that is available for driving other processes, having adjusted for entropy changes of the surroundings.
What is a redox reaction, and how can the redox potential be measured?
A redox reaction is one where the oxidation of one species is coupled to the reduction of another species.
Redox reactions generate electrode potentials, whose magnitude is dependent on the redox potential of the species involved.
Redox potential is a measure of the tendency of a chemical species to acquire electrons and be reduced.
In order to quantitate this, it must be measured against a standard reference cell that has a fixed, defined potential, ie. the hydrogen electrode.
What is the structure of an electrochemical cell referenced against a hydrogen electrode?
Two reaction vessels, one of which is the metal reaction and the other H+ + e- ⇔ 1/2 H2
The hydrogen electrode is catalysed by a platinum electrode, and the metal reaction may also be catalysed by a platinum electrode is both of the reacting species are aqueous.
A volt metre is set up linking the two electrodes together.
A salt bridge links the two reaction vessels to allows transfer and neutrality of electrons.
What is the equation for ΔGº using ΔE?
ΔGº = - n F ΔE (Jmol-1)
Where
n = number of electrons involved
F = Faraday constant = 96,487 JV-1mol-1
ΔE = difference betwee the Em values of the redox couple
What is the difference between Em and Eh values?
Em is the potential generated when the reaction has reached its midpoint.
Eh is the potential generated using the relative concentration of the redox couple.
Em can also be called Eº, and Eh called E.
What is the Nernst equation, and how can it be used for an electrochemical cell?
Eh = Em + RT/nF x ln[Oxidised/Reduced]
Eh = Em + 0.059/n x ln[Oxidised/Reduced]
Where
R = 8.3145 Jmol-1K-1
T = 298K
F = Faraday constant = 96487 JV-1mol-1
n = number of electrons involved
If the Em is known and the Eh can be measured, then the ratio of oxidised:reduced species can be directly calculated.
How does a pH probe work?
Two reference electrode are connected together, so the potential difference between them is fixed.
There is a barrier between them that is permeable to H+ ions, and these will cause a potential difference across the barrier that is related to the ion gradient.
This will create an offset in the potential measured between the two electrodes, which can be calibrated to measure the concentration of H+ ions.
The pH probe consists of a glass bulb connected to a glass body, inside which a silver wire is connected to a silver chloride tip. This is filled with a neutral KCl buffer solution. This is connected to another electrode immersed in KCl solution.
When the glass bulb is dipped in the test solution, any H+ ions present will penetrate the outer boundary or gel layer of the glass bulb. Any other larger charged ions in the test solution will not be able to penetrate it. This also occurs on the inside of the bulb with the KCl solution.
If the H+ concentration on the inside differs from that on the outside, then a potential difference forms and this can be measured.
