5.2.3 Redox and electrode potentials

Question 1

what are the two redox titrations you have to know

Answer 1

Potassium manganate under acidic conditions Fe²⁺/MnO₄⁻

Sodium thiosulfate for determination of iodine I₂/S₂O₃²⁻

Question 2

How to carry out a manganate titration

Answer 2

• add purple KMnO₄ to burette
• add Fe²⁺ and excess H₂SO₄ (to reduce MnO₄⁻ ions) to conical flask
• manganate decolourises as it is being added. The endpoint is judged when there is a permanent pink colour, meaning MnO₄⁻ ions are in excess

Question 3

What’s the overall equation for a reaction between manganate and iron ions

Answer 3

MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 4H₂O(l) + 5Fe³⁺(aq)

Question 4

What’s the overall equation for a reaction between iodine and thiosulfate ions

Answer 4

2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻

Question 5

How to carry out an iodine/thiosulfate titration

Answer 5

• add Na₂S₂O₃ to burette
• add oxidising agent and excess of KI to conical flask
• ox agent reacts with iodide ions to produce iodine, which turns soln yellow brown
• titrate this soln with Na₂S₂O₃. the iodine is reduced back to iodide ions and the brown colour gradually fades to yellow
• end point is unclear, so add starch as an indicator. At end point blue black colour will disappear as all the iodine will have reacted with the thiosulfate

Question 6

standard electrode potential

Answer 6

the e.m.f of a half cell connected to a standard hydrogen electrode. Shows the relative tendency to be reduced and gain electrons

Question 7

describe the hydrogen half cell

Answer 7

• acid solution containing 1.0 moldm⁻³ H⁺
• H₂ (g) at 298K and 100kPa
• inert platinum electrode

Question 8

voltaic cell

Answer 8

converts chemical energy to electrical energy

Question 9

what are the two types of half cells you can have

Answer 9

• metal/metal ion

- ion/ion (Pt electrode)

Question 10

salt bridge (what is it, purpose)

Answer 10

strip of filter paper soaked in concentrated electrolyte solution, allows ions to flow. Doesn’t react with the contents of either beaker
eg. KNO₃

Question 11

the equilibrium is always written so that the forward reaction is…

Answer 11

reduction

Question 12

The more negative the E⦵ value,

Answer 12

the greater the tendency to lose electrons and undergo oxidation

Question 13

The more positive the E⦵ value,

Answer 13

the greater the tendency to gain electrons and reduction

Question 14

How would you represent the hydrogen electrode using vertical lines and square brackets?

Answer 14

Pt | H₂(g) | 2H⁺(aq) ||

Question 15

what does || indicate

Answer 15

salt bridge

Question 16

what does | indicate

Answer 16

the boundary between two phases

Question 17

limitations of predictions of reaction feasibility made using E⦵ values

Answer 17

• Ea may be too high so ROR is v slow
• conc of solutions may not be 1.0 moldm⁻³
• other non standard conditions

Question 18

how does a fuel cell generate a voltage?

Answer 18

uses the energy from the reaction of a fuel with oxygen.

can operate continuously without being recharged

Question 19

what are primary cells?

Answer 19

non-rechargeable batteries

reactions at electrodes cannot be reversed

Question 20

what are secondary cells?

Answer 20

rechargeable batteries

reactions at electrodes can be reversed during recharging