5 redox equilibria

what direction are redox reactions written in

the direction of reduction

how are potentials defined

defined against the standard hydrogen electrode (SHE)

equation for E^⦵_cell

E^⦵_cell = E^⦵_reduction - E^⦵_oxidation

which half-reaction is reduction

the higher reducing potential (more positive)

what is the overall equation?

calculate E^⦵_cell

when is a reaction spontaneous in terms of E^⦵_cell

if E^⦵_cell is positive

what is the Nernst equation

what are the terms in the Nernst equation

how can lnK be found

effect of E° on K

as E° increases, K increases

when is a reaction spontaneous in terms of E°, K and ΔG

equation for K_redox

how can this be rearranged to give the activity of electrons in a redox couple

how can this be expressed

can be expressed E_h - redox potential (different to reduction potential)

equation for redox potential

what is pE

what is it analagous to

reducing power of a solution

analagous to pH = acidity

equation for reducing power of a solution

how else can this be expressed

what assumption is made for this equation

α_electrons = [ ]_electrons

what does the pE scale indicate

gives the ability/likelihood of a species to be oxidised or reduced in a certain environment

what are the three regions of the pE scale

oxic, suboxic and anoxic

oxic

• pE

• electrons?

• what happens

high pE, low electron availability, species oxidised

what happens in the suboxic region

species likely to be half oxidised, half reduced

anoxic

• pE?

• electrons?

• what happens

low pE, high electron availability, species reduced

equation for redox potential

what do the terms mean

what is the conversion factor from electron activity to redox potential (in Volts)

298K

pE is dimensionless

what happens in an electrolytic cell

an external voltage is applied to drive a nonspontaneous reaction

how can K_sp/K_a & K_b/K_f be found using redox

why is this effective

practical example?

couple to redox reactions and measure using half-cell potentials

coupled equilibria change the value of E°

e.g. potentiometric titrations

what can be added to balance redox reactions

H⁺, OH^-, H₂O

steps to balance a chemical reaction in acidic solution?

1. write out half reactions

2. balance all atoms except H and O

3. balance O with H₂O

4. balance H with H⁺

5. balance charge with e^-

6. multiply by lowest common multiple so same number of electrons on both sides

balance this equation in acidic solution

what happens if pH is increased?

reaction depends on [H⁺] so if the pH is increased the reaction proceeds to the right

what can be used to indicate water quality

dissolved oxygen concentration

what is a Winkler titration

[O_2(aq)] is measured using iodine-thiosulfate titration

what happens in the first step of a Winkler titration

• what is mixed

• pH of solution?

• what reacts

• what is formed

the water sample is mixed with manganese (II) in a basic solution

manganese (II) ions react with hydroxide ions to form a white precipitate

equilibrium equation for the first step of Winkler titration (mixing of manganese with hydroxide)

K_sp for Mn(OH)₂?

is this low or high? what does this tell you?

K_sp = 5 × 10^-2

low, not very soluble (forms white ppt)

what effect means solubility depends on pH? what is the effect on this reaction for higher pH

due to the common ion effect

at higher pH, the concentration of [OH^-] increases, pushing equilibrium to precipitation

what happens if oxygen is present

show chemical eqn and charges of Mn

a redox reaction takes place

what are the oxidising equivalents?

how can Gibbs free energy of the redox reaction be found

using standard redox potentials

write the half equations for O and Mn

which is reduced and which is oxidised?

calculate E°_cell?

E°_cell = 0.4 - 0.2 = 0.2V

calculate lnK

where does the equilibrium lie and what does this mean for oxygen

what is step 2 of the Winkler titration

(after Mn mixed with hydroxide then Mn(OH)₂ has reacted with O to form MnO(OH)₂)

the solid MnO(OH)₂ is added to a solution containing sulfuric acid and potassium iodide

chemical equation for solid MnO(OH)₂ being added to sulfuric acid and potassium iodide?

what does acidification cause?

MnO(OH)₂ dissolves as OH^- reacts with H⁺ which forms H₂O and pushes the equilibrium to the right

is manganese oxidised or reduced? why? what are the oxidation states?

what other species is involved and oxidised/reduced (which?)?

manganese (IV) is reduced to manganese (II) due to the coupled reaction with iodide which is oxidised to iodine

write the reduction half reactions for the acidic step 2 of the Winkler titration

what are the oxidising equivalents for the acidic step 2 of the Winkler titration

include step 1’s oxidising equivalent

calculate E°_cell

calculate the E°_cell values at pH 0 and pH 10

E° for Mn(IV)/Mn(II) at pH 0 is +0.2V

calculate the values of K at pH 0 and pH 10

chemical equation for the titration of iodine with thiosulfate

name the sulfur-containing ion formed

what are the reduction half-reactions

oxidising equivalents for step 3 of the Winkler reaction

combine the oxidising equivalents for steps 1-3 of the Winkler titration

how is a potentiometric Winkler titration performed

• what is added and how

• what is measured (physically how)

• automated addition of S₂O₃^-

• potential difference measured between platinum and calomel electrodes

equation for E_cell in a potentiometric titration

E_cell = E_Pt - E_cal

for the Winkler titration, give the equation for E_Pt

what does SCE stand for

Saturated Calomel Electrode

draw a diagram of an SCE and indicate what is happening at the other electrode

what is the calomel half-reaction

what is E_SCE relative to SHE

what is the half reaction at the platinum electrode for a potentiometric Winkler titration

calculate E

what is it relative to?

shape of a graph of potential (y) vs volume (x) for potentiometric Winkler

what is the equivalence point for potentiometric Winkler

what is the end point for potentiometric Winkler

experimental observation of the equivalence point → point of inflection on the titration curve

how is the end point found for potentiometric titration

calculate and classify the dissolved oxygen level

how does complexation affect redox (pairs)

complexation stabilises two oxidation states in a redox pair differently - this affects the redox potential of the metal

how to denote a reduction pair

Fe(III)/Fe(II)

how is this redox pair denoted

first step to determine which species is most strongly complexed by CN?

what can be deduced about the complexation of CN to Fe from these equations and potentials

Iron (III) is more difficult to reduce when complexed to cyanide, E° is smaller

CN is stabilising the Fe(III) oxidation state

next step to determine which complex is most stable

find formation constants and ratio between them

how to find ratio between formation constants

write the equilibrium for the coupled redox processes

then calculate K and E°_cell

rearrange using the Nernst equation to find E

how can the ratio be found to determine relative stability of the complexes

what does this say about the relative stabilities of the iron cyanide complexes

draw a diagram to show how the solubility of AgCl could be determined electrochemically

for this potentiometric titration

• what is added into what

• what are the concentrations at the equivalence point

• silver nitrate added to NaCl solution

• at the equivalence point [Ag⁺] = [Cl^-]

example of a reference electrode used in potentiometric titrations? draw diagram

write a balanced equation for potentiometric titration of this with Cu/CuSO₄ reference electrode

what are the half-equations?

equation for E_meas (potentiometric titration)

calculate E_meas

against?

if at the equivalence point [Ag⁺] = [Cl^-], what is K_sp?

equation for E° involving E°_cell and parts of Nernst

calculate K_sp

• E = 0.19 V

• E_Cu/Cu²⁺ = 0.462V

• [Cu²⁺] in electrode = 0.1M

what is an advantage of using potentiometric titration to determine solubility

very low K_sp values can be accessed through equilibrium potentials rather than direct detection of solutes