Question

During the electrolysis of molten $NaCl$,some water was added. What will happen? This question has multiple correct options
  (A) Electrolysis will stop
  (B) Hydrogen will be evolved
  (C) Some amount of caustic soda will be formed
  (D) A fire is likely

Answer 1

Hint: In electrolysis the passage of a direct electric current through an ionic substance that is either dissolved in a suitable solvent or molten takes place. This passage results in chemical reactions at the electrodes. On adding water to the electrolysis of molten sodium chloride different products are formed.


Complete step by step solution:
- Let's start with the concept of electrolysis. It is the process of separation of compounds and bonded elements by passing an electric current through them. Electrolysis uses a direct electric current (DC).
- The major process of electrolysis is the exchange of ions and atoms by the addition or removal of electrons to the external circuit. The process consists of two electrodes and an electrolyte in most common cases.
- The electrodes mentioned above are called cathode and anode and oppositely charged ions are attracted by each electrode. At anode oxidation of neutral molecules or ions occurs, and at the cathode reduction of ions or neutral molecules takes place.
- In the given question electrolysis of molten sodium chloride is taking place. In this process, sodium chloride is melted at a temperature above $801{}^\circ C$and into the molten sodium chloride two electrodes are inserted followed by the passing of an electric current. This will imitate chemical reaction at electrodes.
- When electrons enter the melt, the sodium ions are migrated to the cathode and are reduced to sodium metal and the reaction can be written as follows,
$N{{a}^{+}}+{{e}^{-}}\to Na$
- The chloride ions migrate towards the anode and the anode undergoes reduction and the reaction can be written as follows
$C{{l}^{-}}\to \frac{1}{2}C{{l}_{2}}+{{e}^{-}}$
The overall reaction can be written as
$2NaCl\to 2N{{a}_{(s)}}+C{{l}_{2(g)}}$
When water is added to this process of electrolysis there would be some changes. As we know water can be both reduced and oxidized and hence it competes with the dissolved sodium ion and chloride ions and as a result hydrogen is being produced. The new reaction can be written as follows
$Na+{{H}_{2}}O\to NaOH+\frac{1}{2}{{H}_{2}}+Heat$
The sodium hydroxide is also called Caustic soda and since the reaction is highly exothermic due to the vigorous reaction between sodium and water there is a probability of fire.


Therefore, the answers are options (B), (C) and (D).


Note: We should be careful while using the terms anode and cathode. As we mentioned cathode is the electrode at which reduction takes place. It can be positive in the case of a galvanic cell where electrical energy is produced from chemical reaction and can be negative when electrical energy is supplied to the cell for chemical decomposition. Also, in anode oxidation takes place and in an electrolytic cell it can act as positive and generally it acts as negative due to its tendency to give off electrons.