Electrolysis and Electrochemical Cell



1. General
During electrolysis, electricity is passed through an electrolyte (aqueous or molten ionic compound), resulting in the decomposition of the electrolyte.

Solid ionic compounds cannot undergo electrolysis, as the ions are held strongly together in the lattice, and are unable to move about to conduct electricity.

In an electrolysis circuit,

  • Cathode or negative electrode is connected to the negative terminal of the battery. Cathode will attract positive ions or cations. Reduction takes place at the cathode, as the cation gains electrons.
If there are more than 1 cations in the solution, the one that is lowest in the reactivity series will be preferentially discharge (or reduced).


electrolytic cell, discharge, reactivity series, reaction at cathode
Reactivity Series and order of discharge for electrolytic cell

  • Anode or positive electrode is connected to the positive terminal of the battery. Anode will attract negative ions or anions. Oxidation takes place at the anode, as the anion loses electrons.

If there are more than one anion in the solution, use the following to decide which anion will be discharged:
- OH- will be preferentially discharged in dilute/ aqueous solutions, unless solution is concentrated halide solutions. 
- When solution is a concentrated halide solution (e.g concentrated sodium chloride solution), then the anion that will be preferentially discharged will be the halid (e.g. chloride).
- NO3- SO42- and CO32- will not be discharged (for your syllabus).
  
  • An inert electrode (e.g graphite, and platinum) is one that does not react. A reactive electrode is one that will react.
  •  A typical experimental setup for electrolysis is represented by the diagram below:

Electrolytic cell diagram, cathode and anode diagram, electrolysis




2. Electrolysis of molten ionic compounds (using inert electrodes)
Watch this video here for an overview of electrolysis of molten compounds.

Example. Determine the products at the cathode and anode, when molten zinc chloride undergoes electrolysis, using graphite electrode.
Step 1. Write out the ions present in zinc chloride.
Ions present are Zn2+ and Cl-.
Step 2. Cations go to the cathode.
Zn2+ at cathode.
Step 3. Cations gain electrons to form atom.
Zn2+ + 2 e- ---> Zn
Step 3. Anions go to the anode.
Cl- is at anode.
Step 4. Anions lose electrons to form molecules.
2Cl- ---> Cl2 + 2e -
Step 5. Products are zinc and chlorine.
Step 6. To write balanced equation, note that zinc chloride forms chlorine and zinc.
Hence, ZnCl2 ---> Cl2 + Zn

3. Electrolysis of aqueous ionic compounds (using inert electrodes)

Note that in aqueous solutions, H+ and OHare present due to the partial dissociation of water. Hence, as there are more than 1 cations and anions, you need to determine which ion will be preferentially discharged.

Example. Determine the products at the cathode and anode, when aqueous sodium chloride solution undergoes electrolysis, using graphite electrode.
Step 1. Write out the ions present in aqueous sodium chloride.
Ions present are Na+, Cl-, H+ and OH.
Step 2. Cations go to the cathode.
Naand Hat cathode.
Step 3. Determine which cation will be discharged.
Since sodium is above hydrogen in the reactivity series, hydrogen is discharged.
Step 4. Cations gain electrons to form atom.
2H+ + 2 e- ---> H2
Step 5. Anions go to the anode.
OHand Cl- are at anode.
Step 6. Determine which anion will be discharged.
Since solution is dilute, OHwill be discharged.
Step 7. Anions lose electrons to form molecules.
4OH- ---> 2H2O + O2 + 4e-
You will notice that this reaction essentially involve removal of H+ and OHions (which are from water). Hence, the concentration of sodium chloride increases as the reaction proceeds.

Example. Determine the products at the cathode and anode, when concentrated sodium chloride solution undergoes electrolysis, using graphite electrode.
Step 1. Write out the ions present in aqueous sodium chloride.
Ions present are Na+, Cl-, H+ and OH.
Step 2. Cations go to the cathode.
Naand Hat cathode.
Step 3. Determine which cation will be discharged.
Since sodium is above hydrogen in the reactivity series, hydrogen is discharged.
Step 4. Cations gain electrons to form atom.
2H+ + 2 e- ---> H2
Step 5. Anions go to the anode.
OHand Cl- are at anode.
Step 6. Determine which anion will be discharged.
Since solution is concentrated, Clwill be discharged.
Step 7. Anions lose electrons to form molecules.
2Cl- ---> Cl2 +  2e-
Reaction involves removal of and . Towards the end of the reaction, Na+ and OH- are the main ions left. Solution becomes more alkaline as the reaction proceeds.

Example. Determine the products at the cathode and anode, when aqueous copper (II) sulfate solution undergoes electrolysis, using graphite electrode.
Step 1. Write out the ions present in aqueous copper (II) sulfate.
Ions present are Cu2+, SO42-, H+ and OH.
Step 2. Cations go to the cathode.
Cu2+ and Hat cathode.
Step 3. Determine which cation will be discharged.
Since hydrogen is above copper in the reactivity series, copper is discharged.
Step 4. Cations gain electrons to form atom.
Cu2+ + 2e- ---> Cu
Step 5. Anions go to the anode.
OHand SO42- are at anode.
Step 6. Determine which anion will be discharged.
OH- will alway be discharged in aqueous solution, unless solution is concentrated halide. Thus, in this case, OH- is discharged.
Step 7. Anions lose electrons to form molecules.
4OH- ---> 2H2O + O2 + 2e-
Reaction removes OH- and Cu2+.  Colour intensity decreases as reaction proceeds, since copper (II) ions are removed. The solution is left with mainly H+ and SO42-at the end of the reaction. Solution becomes more acidic as reaction proceeds. 

4. Electrolysis of aqueous ionic compounds (using reactive electrodes)
Example. Determine the products at the cathode and anode, when aqueous copper (II) sulfate solution undergoes electrolysis, using copper electrode.

Step 1. Write out the ions present in aqueous copper (II) sulfate.
Ions present are Cu2+, SO42-, H+ and OH.
Step 2. Cations go to the cathode.
Cu2+ and Hat cathode.
Step 3. Determine which cation will be discharged.
Since hydrogen is above copper in the reactivity series, copper is discharged.
Step 4. Cations gain electrons to form atom.
Cu2+ + 2e- ---> Cu
Copper is precipitated at the cathode.
Step 5. Reactive electrode is used. Anode will dissolve (or anode metal will form cation). The reaction at the anode will be:
Cu ---> Cu2+ + 2e-  
Reaction effectively involves transferring copper from the anode to the cathode. The anode decreases in size, while the cathode increases in size.
There is no change in concentration of Cu2+. Colour intensity remains unchanged. 

5. Purification and electroplating
In purification, the metal to be purified will be placed at the anode. The solution will also contain the ion of the metal to be purified. The anode will then dissolve to form ions. At the cathode, the pure metal is being precipitated.

In electroplating, the item to be plated is placed at the cathode, and the metal used for plating is placed at the anode. The solution will also contain the ion of the metal to be plated.The anode dissolves, and the metal used for the plating process is precipitated on the item.

6. Electrochemical cell or Simple cell
In the simple cell, a different metal is used for both the cathode and the anode. The difference in potential between the two metals result in a potential difference, and hence electricity is being produced. The greater the difference in reactivity of the two metals, the higher the potential difference of the cell. If two similar metals are used, the potential difference is 0.

The anode will be the more reactive metal, and the cathode will be the less reactive metal.

Note that in electrolysis, a battery or a source of electricity is needed. However, in a simple cell, electrical energy is being produced.



A setup of the simple cell is shown in the diagram below:
Fuel Cell, electrodes, cathode, anode, voltmeter

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