Q.Name the processes from which chlorine is obtained as a by-product. What will happen if an aqueous solution of NaCl is subjected to electrolysis?

Down's process: electrolysis of fused NaCl gives Na at the cathode and $Cl_2$ at the anode. Chloralkali process: electrolysis of aqueous NaCl gives NaOH (in solution) + $H_2$ (cathode) + $Cl_2$ (anode). If aqueous NaCl is electrolysed: $H_2$ is liberated at the cathode (water reduced in preference to $Na^+$), $Cl_2$ at the anode (Cl$^-$ oxidised preferentially), leaving NaOH behind: $2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2$. …

Step 1 – Processes that yield chlorine as a by-product

Chlorine gas is obtained as a valuable by-product in at least two important electrometallurgical/industrial electrolysis processes:

1. Down's process — the electrolytic extraction of sodium metal from fused (molten) sodium chloride (with some $CaCl_2$ added to lower the melting point). At the cathode, $Na^+$ is reduced to Na metal; at the anode, $Cl^-$ is oxidised to $Cl_2$ gas, which is collected as a by-product.
2. Chloralkali process — the electrolysis of concentrated aqueous NaCl (brine) to manufacture sodium hydroxide. Chlorine is liberated at the anode as an industrially important by-product (along with hydrogen at the cathode).

Step 2 – What happens on electrolysing aqueous NaCl specifically

When an aqueous solution of NaCl is electrolysed (using inert electrodes):

• At the cathode: although $Na^+$ ions are present, water is preferentially reduced (since $Na^+$ is very difficult to reduce in aqueous solution, being a highly reactive metal ion), liberating hydrogen gas and leaving hydroxide ions behind:

• At the anode: chloride ions are preferentially oxidised over water (due to overvoltage effects favouring $Cl^-$ discharge under the conditions used), liberating chlorine gas:

• In solution: the $Na^+$ ions remain in solution together with the $OH^-$ ions generated, effectively giving sodium hydroxide.

Step 3 – Overall reaction

$2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electrolysis}} 2NaOH(aq) + Cl_2(g) + H_2(g)$ …

Name the two classic processes (Down's process for Na, chloralkali process for NaOH) that yield $Cl_2$ as a by-product, then work through what is discharged at each electrode when aqueous NaCl spe …

• Saying $Na^+$ is reduced at the cathode when aqueous NaCl is electrolysed — in aqueous solution, water is reduced instead, giving H$_2$ (this only changes to metallic Na deposition in the FUSED-salt Down's process). …