# 19.1 Electrochemical cells

### Voltaic Cells

• Electromotive Force (EMF): The energy supplied by a source divided by the electric charge transported through the source
• In voltaic cell, a cell potential is generated, resulting in movement of electrons from anode to cathode via external circuit.
• Cell potential: the potential difference between the cathode and the anode when the cell is operating
• Under standard conditions, cell potential is called Standard Cell potential
• Eᶱcell = Eᶱcat – Eᶱan
• In order to calculate Eᶱcell for a spontaneous cell, the cathode is taken as the more positive value from the two electrodes
• The more positive one is also the strongest oxidizing agent

### Standard Hydrogen Electrode (SHE)

• Consists of an inert platinum electrode in contact with 1 mol dm-3 hydrogen ions and hydrogen gas at 100 kPa and 298 K. This is an example of a gas electrode
• Standard electrode potential of a single half-cell cannot be measured on its own. Has to be relative to another cell
• Standard electrode potentials are measured relative to SHE
• SHE has Eᶱcell of 0V
• The reduction half equation corresponding to the SHE cell is
•

### Cell potential and Gibbs free energy

• Spontaneous:
• Eᶱcell is positive, ΔG is negative
• Non-Spontaneous
• Eᶱcell is negative, ΔG is positive
• When ΔG is 0, Eᶱcell is 0
• Both are related by following equation:
• Where:
• n= amount, in mol, of electrons
• F = Faraday’s constant = 96500 C mol-1

### Electrolytic Cells

• Convert electrical to chemical energy
• In SL, we looked at electrolysis of molten salt, now we will look at types of electrolysis
• The higher the reduction potential, higher the tendency to react
1. Electrolysis aqueous NaCl
1. Concentrated
• You have to take into account water as well
• At cathode, water is reduced to create hydrogen gas
• At anode, Cl is oxidized to create Cl2 gas
2. Diluted
• At cathode, hydrogen ions are reduced to create hydrogen gas
• At anode, water is oxidized to produce oxygen gas
• This is equivalent to electrolysis of water
2. Electrolysis of CuSO4
1. Inert graphite (carbon) electrodes
• Electrodes don’t take part in reactions
• At cathode, copper ions are reduced to create copper deposits
• At anode, water is oxidized to produce oxygen gas
2. Active copper electrodes
• Electrodes take part in reaction
• At cathode, copper ions are reduced to create copper deposits
• At anode, sludge of impurities is found
• Process known as electrorefining in which the impurities in copper are separated from copper itself
• Also the basis of electroplating in which a thin layer of metal is deposited onto cathode of another
3. Electrolysis of water
• Water is poor conductor of electricity
• Electrolysis of water is done in dilute solutions of sulfuric acid or sodium hydroxide using inert Pt electrodes
• At cathode, hydrogen ions are reduced to create hydrogen gas
• At anode, water is oxidized to produce oxygen gas

### Factors affecting amount of product formed

1. Current
• Higher the current, greater yield
• Q=It
2. Duration of electrolysis
• Longer the time, greater yield
3. Charge on the ion
• Na+ required 1 mol of electrons however Pb2+ requires 2 mols of electrons