Electrode potentials and cells

How do you make electrochemical cells?

Two different metals are dipped in salt solutions of their own ions and connected by a wire

Voltmeter on wire to measure voltage

What are the two reactions happening within an electrochemical cell?

Oxidation and reduction

What can a salt bridge be made out of?

Filter paper soaked in potassium nitrate

Why is a salt bridge used?

Completes the circuit

Allows ions to flow between the half cells and balance out the charges

Ions can’t flow through the wire

Which direction do electrons flow in electrochemical cells?

From the most reactive metal to the least

(Negative electrode to positive electrode)

What metal is used for an electrode when half cells have a solution with two aqueous ions of the same element?

Platinum

What is the EMF?

The voltage between two half cells

How do we draw conventional representations of electrochemical cells?

Most negative electrode potential goes on the left. Least negative on the right

Write left hand as oxidation reaction. Write right hand as reduction reaction

Single line between element and ion as different phases

Comma between chemicals in same phase

Ions nearest to salt bridge

2 lines Salt bridge separates reaction

How do we draw a conventional representation if a reaction stays in the same phase?

Write Pt on the outside and seperate using a single line

What is oxidation and reduction in terms of electrode potentials?

Oxidation is the most negative electrode potential

Reduction is the most positive electrode potential

How do we calculate the cell potential?

Ecell = E right side - E left side

What are standard conditions used for standard electrode potentials?

100kpa

298K

1 mol dm-3

How do you measure the standard electrode potentials?

Measure against the standard hydrogen electrode

Hydrogen gas is bubbled through a solution of H+ ions and a platinum electrode is used

How do we write the standard hydrogen electrode?

Always write on the left hand side

What is an electrochemical series?

List of electrode potentials for different electrochemical half cells

They are used to predict the direction of reaction (oxidation or reduction)

How do we work out the cell potentials ?

E cell = E reduced - E oxidised

How can we work out if a reaction is feasible or not?

If EMF is positive, reaction is feasible

If EMF is negative reaction isn’t feasible

What do we use batteries for?

Power/charge things

Zinc/carbon non-rechargeable cell

Overall : 2NH₄⁺ (aq) + Zn (s)  → 2NH₃ (g) + H₂ (g)  + Zn²⁺ (aq)

• Zn (s) →   Zn²⁺ (aq)  +  2e^-   

• 2MnO2 (s) + 2NH4 + 2e- —> 2NH3 (aq) + H2O (l)

• EMF = 1.50 V

Lithium cells

Rechargeable

Electrode: lithium cobalt oxide and graphite

Electrolyte : lithium salt in an organic solvent

Overall: Li (s)  + CoO₂ (s)  →   Li ⁺ (CoO₂) ^– (s)

Negative electrode: Li (s) →   Li⁺ (s)  +  e^– 

Positive electrode: Li⁺ (s)  + CoO₂ (s)  +  e^– →   Li ⁺ (CoO₂) ^– (s)

EMF = 3.6 V

Lead acid battery

Rechargeable

Used in cars

Anode- lead dioxide

Cathode - lead

Electrolyte - sulfuric acid

Pb (s) +  SO₄^2- (aq)  →   PbSO₄ (s)  +  2e^-

PbO₂ (s) +  4H⁺ (aq) +  SO₄^2- (aq) +  2e^- →  PbSO₄ (s)  + 2H₂O (l) 

Overall: PbO₂ (s) +  4H⁺ (aq) +  2SO₄^2- (aq) +  Pb (s) →  2PbSO₄ (s)  + 2H₂O (l)

EMF = 2.06 V

Nickel/cadmium cells

Rechargeable

Anode - Cadmium

Cathode - NiO(OH)

Electrolyte - potassium hydroxide

Cd (s) +  2OH^-  (aq) → Cd(OH)₂ (s)  +  2e^-

NiO(OH) (s) + H₂O (l) + e^- → Ni(OH)₂ (s) +   OH^-  (aq)

Overall: 2NiO(OH) (s) + 2H₂O (l) + Cd (s) → 2Ni(OH)₂ (s) + Cd(OH)₂ (s)

What is a fuel cell?

The chemicals are stored seperately outside the cell and fed in when electricity is needed. They are used in electric vehicles

Alkaline hydrogen oxygen fuel cell

Electrolyte - KOH

Overall : 2H₂ (g) + O₂ (g)  →   2H₂O (l)

Hydrogen fed into negative electrode :

2H₂ (g) + 4OH^– (aq)  →  4H₂O (l) +  4e^–

^{Oxygen fed into positive electrode:}

O₂ (g) +  2H₂O  +  4e^– →  4OH^– (aq)

Advantage of fuel cells

• more energy converted into kinetic energy compared to an internal combustion engine

• Waste product is water

• Don’t need to be recharged

Disadvantages of fuel cells

• energy needed to make H2

• Hydrogen is flammable

• Limiting refuelling stations