3.1 and 3.2 Redox and standard electrode potential and Redox reactions

Define oxidation

Loss of electrons

Define reduction

Gain of electrons

Oxidation number of group 1 metals

Always +1

Oxidation number of group 2 metals

Always +2

Oxidation number of oxygen

Usually -2, except perioxides where it is -1

Oxidation number of hydrogen

Usually +1, except metal hydrides where it is -1

Oxidation number of Florine

-1

Oxidation number of chloride

-1, unless bonded to F or O

Oxidation number of ions

Charge of ion.

If a substance is reduced, does the oxidation number increase or decrease?

decreases

What is a disproportionation reaction

A single substance is simultaneously oxidised and reduced

5 steps to construct a redox equation?

1. Balance atoms

2. Add water to balance oxygens

3. Add H⁺ to balance hydrogens

4. Add electrons to balance hydrogens

5. Combine to make a redox equation

What is a half cell?

Comprises of an element in 2 different oxidation states.

Structure of a simple half cell?

Strip of metal placed in an aqueous solution of its ions.

How to tell If a half cell is likely to loose or gain electrons?

Use a standard electrode as a reference.

Label the 5 parts of the standard hydrogen electrode.

1. Copper wire

2. Hydrogen gas at 1atm

3. Platinum electrode

4. 298K

5. H⁺ (aq) at 1moldm^-3 (e.g. HCl)

Why is a platinum electrode used?

It is inert.

Label the 9 parts of the copper half cell connected to a SHE (electrolysis)

1. Copper wire

2. Voltmeter

3. Standard hydrogen electrode

4. H₂ gas (1atm)

5. platinum electrode

6. H⁺ at 1moldm^-3

7. Salt bridge

8. Copper

9. Cu²⁺ (aq) (1moldm^-3)

What happens in the system when two half cells are connected? Equation for this.

Copper electrode will get bigger and solution will become paler, as electrons flow from the hydrogen half cell to the copper half cell.

Cu²⁺ + 2e^- → Cu

Why is a salt bridge needed? Give an example.

KNO₃, needed to connect and complete the circuit. KNO₃ suitable as it is soluble, so no precipitate formed.

Rule for direction of electron flow in half cells?

The most positive E^∘ goes forward (right) and the other backwards (left)

How to calculate E^∘

Most positive - most negative

What indicates a feasible reaction?

Positive E^∘

What indicates a spontaneous reaction?

A voltage reading above 0.4V

How to allow 2 half cells reaction to run to completion

Remove voltmeter, but no reading will be available.

2 limitations of half cells

1. Rate could be slow due to high activation energy

1. Conditions may be different in actual situations

How to read mixture of different oxidation states?

Use a platinum electrode (e.g. iron with a mix of Fe²⁺ and Fe³⁺)

Find the forward reaction, backwards and overall reaction, feasibility and whether this reaction is spontaneous?

Cu²⁺ (aq) + 2e^- ⇌ Cu (s); E⁰ = +0.34V

Sn⁺ (aq) + 2e^- ⇌ Sn (s); E⁰ = -0.14V

Cu²⁺ (aq) + 2e^- → Cu (s): more positive, so forwards)

Sn → Sn²⁺ 2e^-

Overall reaction: Cu²⁺ + Sn → Cu + Sn²⁺

E⁰ = 0.34 - (-0.14) =0.48V

Reaction feasible and spontaneous

How to create a spontaneous reaction if voltage isn’t above 0.4V

Change conditions.