Chemistry - 3.1.11: Electrode Potentials and Electrochemical Cells

What happens if you place two different metals in a salt solution and connect them together?

An electron current flows where the electrons pass from the more reactive metal to the less reactive

In what direction do electrons flow?

From the most reactive metal to the least reactive - from the more negative to the more positive

What happens when a rod of metal is dipped into a solution of its own ions? Write an equation for this.

An equilibrium is set up, e.g.:

Zn (s) ⇌ Zn²⁺ (aq) + 2e^-

Electrode

An arrangement where a rod of metal is dipped into a solution of its own ions

Why is a salt bridge used in electrodes instead of a piece of wire?

Wires can only transfer electrons, while salt bridges can transfer ions

Draw an electrode

Draw two electrodes connected together to measure the potential difference.

Why is a standard electrode needed?

To compare the tendency of different metals to release electrons - other half cells can be connected to it for comparison

Why is Pt used as the electrode for half cells with no solids? (2)

1. It is a conductor of electricity

2. It is unreactive

Describe the standard hydrogen electrode and its conditions (4)

• Hydrogen gas is bubbled in

• 1.00 mol dm^-3 solution of H⁺ ions

• Platinum metal and wire used

• 298 K, 100 kPa

What is the potential of the standard hydrogen electrode?

0

The electromotive force

The potential of a cell connected to the standard hydrogen electrode

How do you represent the electromotive force?

E

EƟ

Standard electrode potential - the potential of the second cell at standard conditions

What does it mean if EƟ is negative?

The electrodes are better at releasing electrons (i.e. better reducing agents) than hydrogen

How can you change the electrical potential of an electrode?

By changing the conditions e.g. the concentration of ions, the temperature or the pressure

Do the number of electrons involved in a reaction affect EƟ?

No

How do you find the potential difference obtained by connecting two standard electrodes together?

By finding the difference between the two EƟ values

In a conventional cell diagram, how do you represent the phase boundary?

A vertical solid line

In a conventional cell diagram, how do you represent the salt bridge?

A double vertical solid line

In a conventional cell diagram, where is the species with the highest oxidation state written?

Next to the salt bridge

6 Features of a conventional cell diagram

1. ROOR

2. Reduced species on right, oxidised species on left

3. | to separate species in different states

4. , to separate species in the same state

5. || to represent a salt bridge

6. Pt (s) electrode if species is not in the solid state

How can you tell the direction of a redox reaction from standard electrode potentials?

From the signs of the electrodes - electrons flow from the more negative (reducing agent / oxidised one) to the more positive (oxidising agent / reduced one)

Battery

A number of cells connected together

3 types of cells

Non-rechargeable, rechargeable or fuel cells

What is a risk associated with the use of non-rechargeable cells?

Electrolytes used may be acidic so corrosive - batteries must be disposed of properly

How are rechargeable batteries recharged?

By applying an external voltage greater than the voltage of the cell to drive the electrons in the opposite direction, reversing the cell reactions

What reaction takes place at the positive electrode of a lithium cell?

Li⁺ + CoO₂ + e^- → LiCoO₂

What type of cell is a lithium cell?

Rechargeable, portable

What are the benefits associated with the use of lithium in lithium ion cells? (3)

1. Light as lithium is the least dense metal

2. Solid polymer electrolyte so cannot leak

3. Charge can be topped up at any time (doesn’t have to be fully discharged to recharge efficiently)

What are the electrodes made out of in lithium cells?

Positive: Lithium cobalt oxide (LiCoO₂)

Negative: Carbon

What reaction takes place at the negative electrode of a lithium cell?

Li → Li⁺ + e^-

What overall reaction takes place in an alkaline hydrogen-oxygen fuel cell?

2H₂ (g) + O₂ (g) → 2H₂O (l)

What reaction takes place at the negative electrode of an alkaline hydrogen-oxygen fuel cell?

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

What reaction takes place at the positive electrode of an alkaline hydrogen-oxygen fuel cell?

O₂ (g) +2H₂O (l) + 4e^- → 4OH^- (aq)

What electrolyte is used in an alkaline hydrogen-oxygen fuel cell?

NaOH solution

What metal is used for the electrodes in an alkaline hydrogen-oxygen fuel cell?

A porous platinum-based material

In an alkaline hydrogen-oxygen fuel cell, how are the electrodes separated?

By a semi-permeable membrane

What are the benefits associated with the use of alkaline hydrogen-oxygen fuel cells? (3)

1. The only by-product is water, so can be used as drinking water by astronauts

2. It doesn’t produce carbon dioxide, so doesn’t contribute to global warming

3. Hydrogen and oxygen are supplied continuously - can be operated without stopping to recharge

What are the costs associated with the use of alkaline hydrogen-oxygen fuel cells? (4)

1. Hydrogen may be obtained from crude oil - a non-renewable resource

2. Hydrogen filling stations would have to be built so hydrogen-powered vehicles can be used

3. Hydrogen gas is highly flammable so would be difficult to store and transport

4. Hydrogen gas takes up a large volume, so would be difficult to be stored

How do you calculate the Emf of a cell?

E(reduced) - E(oxidised) = E(right) - E(left)

Why is the electrode potential for the standard hydrogen electrode equal to 0.00V?

By definition

Why do hydrogen-oxygen fuel cells operating in acidic and alkaline conditions have the same emf?

The overall reaction is the same

Why might the hydrogen-oxygen fuel cell not be considered carbon neutral?

Hydrogen may be being made by an energy source that is not ‘carbon neutral’ (e.g. fossil fuels)

When drawing a conventional cell diagram, where must the standard hydrogen electrode always be?

On the left

Why might the actual EMF of a cell be different from the EMF calculated using standard electrode potentials?

The cell might not be under standard conditions

Electrochemical series

A list of electrode potentials in order OR a list of half cells in order of electrode potential

How do you set up electrodes to measure the electrode potential of a cell? (3)

1. Immerse strip of metal in the solution

2. Connect the solutions with a salt bridge

3. Use a voltmeter to measure the emf of the cell

Why is a salt bridge needed in cells?

To complete the circuit by allowing ions to flow

How might you change a circuit used to measure the emf of a cell so that the cell reaction will go to completion?

Replace the voltmeter with a lamp / wire / ammeter

Why would the EMF of a cell decrease to 0?

The concentration of ions in the two solutions become equal

What is an advantage of using a fuel cell instead of an internal combustion engine?

Internal combustion energy wastes more energy / fuel cell converts more of the available energy from combustion of hydrogen into kinetic energy of the car

Conventional representation for an alkaline hydrogen-oxygen fuel cell

Pt (s) | H2 (g) | OH- (aq), H2O (l) || O2 (g) | H2O (l), OH- (aq) | Pt (s)

What type of reaction takes place at the positive electrode?

Reduction

What type of reaction takes place at the negative electrode?

Oxidation

Why might a salt solution not be suitable for the salt bridge?

One of the ions of the salt would react with the ions of the electrodes

Why are Pt / C rods used in cells without solid reactants?

To allow electrons to flow / to provide a reaction surface

Why might a cell not be rechargeable?

The reaction is not reversible