Electrolysis

Question 1

What are electrolytic reactions?

Answer 1

Electrolytic reactions are non-spontaneous and convert electrical energy to chemical energy. These reactions proceed by passing an electric current through the electrolyte from an external power source.

Question 2

Differences between galvanic cells and electrolytic cells.

Answer 2

1) Gal cells undergo spontaneous chemical reactions, generating electrical energy while electrical energy is supplied to Electrolytic cells to drive a non-spontaneous chemical reaction.
2) In galvanic cells, reactions are carried out in two separate containers or half cells while in Electrolytic cells, reactions are carried out in one container. Electrolytic cells have two electrodes and one electrolyte that is shared between them. It does not matter if the reactants are in contact in an Electrolytic cell because it undergoes non-spontaneous reactions and so the reaction won’t occur unless it is driven by an external power source.
3) In gal cells, oxidants and reductants are not in contact while in electrolytic cells, they are in contact.
4) In galvanic cells, the salt bridge allows the movement of ions from one half cell to another to maintain electrical neutrality. Electrolytic cells don’t have a slat bridge.
5) In galvanic cells, the anode is the negative electrode and the cathode is the positive electrode. In electrolytic cells, the anode is the positive electrode and the cathode is the negative electrode.
6) In galvanic cells, the polarity of electrodes is determined by reaction occurring at each electrode. In electrolytic cells, the polarity of electrodes is imposed by power supply.

Question 3

How can electrolysis be set up?

Answer 3

Electrolysis can occur in one container or in a U-Tube.

Question 4

Principles of electrolysis

Answer 4

The most powerful oxidant present will react with the powerful reductant. In electrolysis, the most powerful oxidant is found lower on the electrochemical series than the most powerful reductant.
In electrolysis, water is considered when the reactants are aqueous.
The anodes of electrolytic cells are positive due to the power source withdrawing electrons. The cathodes are considered to be the negative electrodes as the power source forces an excess of electrons to it, causing reduction to occur. (Oxidation at anode, reduction at cathode).

Question 5

The cell emf

Answer 5

E(cell) = E(oxidant) - E(reductant)
For electrolytic cells, this gives an estimate of the minimum potential difference that must be applied to bring about the required electrolysis reaction. Usually, a potential greater than that calculated is applied (called decomposition potential).

Question 6

Predicting reactions

Answer 6

Whenever an aqueous solution of a compound is electrolysed, it is possible the water will be involved in the reaction at either, or both, electrodes. Therefore, when predicting the products of a reaction in an electrolytic cell, we must consider the oxidation and reduction reactions of water.
Steps:
1) Identify all species present.
2) Locate species on electrochemical series.
3) Underline the species present.
4) Circle strongest oxidant and reductant.
5) Write the half and full equations.

Question 7

Predicting reactions notes

Answer 7

• The concentration of solution must be considered.
• If one species is consumed the next strongest oxidant or oxidant will react
• NO3- and SO42- are not reduced because they are negative and so are not attracted to the cathode (negative) note: these species are at their highest oxidation state - they cannot be oxidised further
• If two competitors have ver similar E. values, they will probably be discharged simultaneously.
• In aqueous solutions, water may be involved in either the oxidation or the reduction reaction, or both.

Question 8

Competition at the Anode

Answer 8

For:
Cl2(g) + 2e- 2CL- (aq) +1.36
O2(g) + 4H+(aq) + 4e- 2H2O(l) +1.23

Due to the similarity in the potential difference for these two half cells, we might expect that either or both reactions could occur. This is dependent on concentration.

If the solution is:

• Very dilute (0.1-0.5M): Only H2O is oxidised.
• Dilute (0.5-2M): H2O and CL- are both oxidised.
• Greater than 2M: Only CL- is oxidised (due to its high concentration)

Question 9

What are the uses of electrolytic cells in the industry?

Answer 9

• Recharging secondary cells
• Industrial production of sodium hydroxide, chlorine and hydrogen
• Extraction of reactive metals like sodium or aluminium from their ores
• Electroplating
• Electrorefining

note: electrolysis is not generally used for the production of chemicals due to the high cost of electricity involved. It is, however, not possible to easily obtain reactive elements like Na, Cl and Al by other means.

Question 10

What does commercial viability depend on?

Answer 10

• Location close to electrical power
• Cheap and available electrolytes
• Cheap electrodes
• Accessible transport
• Continuous operation
• Using low temperatures

Question 11

Molten electrolytes and what are they used in?

Answer 11

There is no water present in these electrolytes. These electrolytes are molten salts and used in the production of metals such as Na, K, Ca and Al. The aqueous solution of these salts are not used as water is a stronger oxidant than the metal ions Na+, K+, Ca+ and Al+ and so water would be preferentially reduced and no metal would form. The use of a molten electrolyte involves higher energy expenditure and so it is not necessary to heat the electrolyte continuously (the electric current keeps it molten). However, this means there is greater wear on the cell because of the high temperature.

Question 12

Obtaining Sodium through electrolysis

Answer 12

Sodium is extracted from molten sodium chloride by electrolysis.