5.2.3: Redox and electrode potentials

Question 1

Standard electrode potential

Answer 1

the e.m.f. (electron motive force) of a half cell connected to a standard hydrogen half cell under the standard conditions of 298K, 100kPa and 1 moldm⁻³.

Question 2

What is the electrode made out of for a hydrogen half cell?

Answer 2

Platinum

Question 3

How to set up a half cell with two ions of the same metal?

Answer 3

Two ions in solution at 1 molmoldm⁻³ EACH

Platinum electrode

Question 4

Reducing agent

Answer 4

Is itself oxidised

Question 5

Oxidising agent

Answer 5

Is itself reduced

Question 6

Why might a reaction that is feasible by its Eᶿ values not actually happens?

Answer 6

• Conditions are non-standard (state how…)

* Activation energy is too high

Question 7

A positive Eᶿ value means

Answer 7

the metal is readily reduced (REDuction Is More Positive)

Question 8

A negative Eᶿ value means

Answer 8

the metal is readily oxidised

Question 9

What happens at the electrode with the more reactive metal?

Answer 9

The more reactive metal loses electrons and is oxidised –> chucking out electrons, XS of e- –> is the negative electrode

Question 10

What happens at the electrode with the less reactive metal?

Answer 10

The less reactive metal gains electrons and is reduced –> deficit of e- –> is the positive electrode

Question 11

Charge carriers in the salt bridge

Answer 11

Ions

Question 12

Charge carriers in the wire

Answer 12

electrons

Question 13

Predicting feasibility from Eᶿ value

Answer 13

Positive Eᶿ value means the redox reaction is feasible.

Question 14

Features of a primary cell

Answer 14

• Non-rechargeable

* Electrical energy produced by reduction and oxidation at the electrodes –> however reactions cannot be reversed

Question 15

Features of a secondary cell

Answer 15

• Rechargeable

* Cell reactions can be reversed during charging –> regenerates chemicals so that cell can be used again

Question 16

Types of cell

Answer 16

• Primary
• Secondary
• Fuel cells

Question 17

What are redox titrations? Why do them?

Answer 17

Redox titration: a titration that uses transition metal elements

• Transition metals have variable oxidation states so useful as oxidising/reducing agents because they readily give accept/give out electrons.
• Often change colour with oxidation state; easy to see when reaction has finished

Question 18

What are redox titrations used to calculate?

Answer 18

The concentration of the oxidising/reducing agent

Question 19

How to calculate the concentration of reducing agent (redox titration)?

Answer 19

Use manganate ions as oxidising agent

1) Pipette fixed volume unknown reducing agent into conical
2) Add excess H₂SO₄ to the flask to provide any H⁺ needed
3) Gradually add MnO₄⁻ (oxidising agent) from burette, swirling conical
4) Stop when purple MnO₄⁻ colour becomes permanent; = endpoint because no more reducing agent to react with MnO₄⁻
5) Repeat to calculate mean titre

Question 20

How to calculate the concentration of an oxidising agent (redox titration)?

Answer 20

Where IO₃⁻ is the oxidising agent in Stage 1
Stage 1
1) Measure out 25cm³ of potassium iodate solution
2) Add excess KI. This oxidises the iodide to iodine

Stage 2

1) Add sodium thiosulfate dropwise to flask from stage 1. End point is pale yellow
2) Add 2cm³ starch. Dark blue if iodine present.
3) When dark blue colour disappears, all the iodine has been reduced to iodide
4) Use titre of sodium thiosulfate to calculate moles of iodine. Then ➗3 for the moles (hence conc.) of iodate