Electrolysis of Aqueous Solution

Electrolysis of Aqueous Solution

  1. An aqueous solution is solution of water of a substance. For example, if you heat sodium chloride until it melts, it is called molten sodium chloride, but if you dissolve sodium chloride in water, it is called aqueous sodium chloride.
  2. Electrolysis of aqueous solution is different from electrolysis of molten electrolyte because an aqueous solution contain more types of ions.

Example:
Ions presence in molten sodium chloride are Na+ and Cl–


NaCl –> Na+ + Cl–

Ions presence in sodium chloride solution are Na+, H+, O2-and Cl–

NaCl –> Na+ + Cl–
H2O –> 2H+ + O2-

Conclusion:
In an aqueous solution, it can be more than 1 positive and negative ions.

Selective Discharge

  1. When the ions move to the anode and cathode, only 1 negative ion and 1 positive ion will be selected to be discharged, and this is called selective discharge.
  2. There are three main factors that affect the selective discharge in an electrolysis:
    1. position in the electrochemical series
    2. the concentration and
    3. the type of electrode

Factors Affecting the Selective Discharge – Electrochemical series

  1. The tendency of ions to be selected to be discharged at an electrode depends on their position in a series called the electrochemical series.
  2. The ions at the top of the list is more difficult to be discharged, but as we go down the table, they become easier to be discharged. 
  3. During electrolysis, the ion in the lower position will be selected to be discharge.
  4. Figure below shows the electrochemical series for the positive and negative ions.

Example: Electrolysis of Aqueous Sulphuric Acid

  1. Figure above shows the set up of apparatus used to investigate the electrolysis of aqueous sulphuric acid.
  2. Carbon electrodes are used as the anode and cathode.
  3. The molecules of sulphuric acid ionise to form hydrogen ions and sulphuric ions.
H2SO4 → 2H+ + SO42-
  1. In an aqueous solution, water molecules will also ionise to form hydrogen ions and hydroxide ions.
H2O → H+ + OH–
  1. Therefore, the ions that present in the solution are H+ , SO42-, H+ and OH–
At anode (Positive Electrode)
  1. The negative ions (SO42- and OH–) will be attracted to the anode.
  2. OH– ions is lower than SO42- ions in electrochemical series, hence the OH– ions will be selected to be discharged at anode.
  3. The OH– ions discharged to form water and oxygen gas.
4OH– → 2H2O + O2 + 4e

Observation:
Colourless gas bubbles are released around anode. When a glowing wooden splinter is inserted into the test tube that contain the gas, the wooden splinter light up.

At cathode (Negative Electrode)

  1. The positive ions (H+) are attracted to cathode.
  2. The H+ ions are discharged to form hydrogen molecule.
2H+  + 2e →  H2

Observation:
Colourless gas bubbles are released around cathode. When a lighted wooden splinter is brought close to the mouth of the test tube that contain the gas, a “pop” sound is produced.

Note:
Selective discharge occur at anode when there are more than one type of ions present. The ions located lower in the electrochemical series is selected to be discharge.

Factors Affecting the Selective Discharge – Concentration

If the concentration of a particular ion is high, it may be selected to be discharged even though it is higher in the electrochemical series compares with another ion present in the solution. Example
 
Electrolysis of Diluted Hydrochloric Acid Electrolysis of Concentrated Hydrochloric Acid
Ions presence at Anode:
Cl–, OH–
Cathode:
 H+
 Ions presence at Anode:
Cl–, OH–
Cathode:
H+
Observation at Anode: Colour gas is produced. When a glowing wooden splinter is inserted into the test tube that contain the gas, the splinter is rekindled. Cathode Colour gas is produced. When a lighted wooden splinter is brought close to the test tube that contain the gas, a “pop” sound produced.. Observation at Anode: A greenish yellow gas produced. When a blue litmus paper is inserted into the test tube that contain the gas, the blue litmus paper turn red and then become colourless. Cathode Colour gas is produced. When a lighted wooden splinter is brought close to the test tube that contain the gas, a “pop” sound produced.
Half Equation of the Reaction at Anode:
 4OH– → 2H2O + O2 + 4e
Cathode:
 H+ + 2e → H2
 Half Equation of the Reaction at Anode:
2Cl– → Cl + 2e
Cathode:
 H+ + 2e → H2
Note: At anode, the position of hydroxide ion (OH–) is lower compare to chloride ion (Cl–). However, chloride is selected to be discharged because its concentration is much higher than he concentration of hydroxide ion.
 

Additional Note:

  1. Nevertheless, concentration is not a determining factor. It only affect the selective discharge of the ions which is very close to each other in electrochemical series.
  2. The chart below summurises the ions that likely to be selected if they are presence with hydrogen ions at cathode or hydroxide ions at anode with high concentration.
 

Factors Affecting the Selective Discharge – Types of Electrode

Type of Electrode

  1. There are 2 types of electrode:
    1. Inert electrode
      Inert electrodes do not react with electrolyte and does not take part in the chemical reaction. (Example: Carbon, platinum)
    2. Active electrode
      Active electrode will react with the electrolyte and take part in the reaction. (Example: Copper, Silver, Mercury)

Example: Electrolysis of Copper(II) Sulphate with Carbon/Copper Electrodes


Using Carbon Electrode
Using Copper Electrode
Ions presence at
Anode:
OH–, SO42-
Cathode:
H+, Cu2+
 Ions presence at
Anode:
OH–, SO42-
Cathode:
H+, Cu2+
Observation at
Anode:
Colourless gas is produced. When a glowing wooden splinter is inserted into the test tube that contain the gas, the wooden splinter is rekindled.

Cathode
The electrode become thicker.

Colour of the Solution:
Become paler		 Observation at
Anode:
The electrode become thinner.

Cathode
The electrode become thicker.

Colour of the Solution:
Remain unchanged
Half Equation of the Reaction at
Anode:
4OH– → 2H2O + O2 + 4e
Cathode:
Cu2+ + 2e → Cu
 Half Equation of the Reaction at
Anode:
Cu → Cu2+ + 2e
Cathode:
Cu2+ + 2e → Cu

Note:
For the electrolysis of copper(II) sulphate solution by using copper electrodes, both the hydroxide and sulphate ions are not selected to be discharged, Instead, the copper electrode ionise to form copper(II) ions. The electrode has taken part in the reaction!