Question

How will the pH of brine (aqueous $NaCl$ solution) be affected when it is electrolysed?

Answer 1

You should know that; brine solution refers to the high concentration of salt ($NaCl$) in water (${{H}_{2}}O$). Electrolysis is the process in which ionic substances are broken into simpler substances when an electric current is passed through it.

Complete answer:
Given that,
An aqueous solution of brine undergoes electrolysis. So, we have to find out the change in pH.
We know that brine solution has a composition of sodium chloride ($NaCl$) and water (${{H}_{2}}O$). So, on electrolysis it will tend to form sodium hydroxide ($NaOH$) and chlorine. The chlorine and sodium hydroxide formed in the process have to be separated in the reaction, when they come in contact with each other.
As we know, electrolysis is done on two electrodes i.e. cathode (negative) and anode (positive). So, during the electrolysis of brine, the cathode attracts the sodium ions and hydrogen ions from sodium chloride and water, respectively. Hydrogen ions get reduced by gaining electrons and hydrogen molecules at cathode. The reaction is shown below:
$2H+2{{e}^{-}}\to {{H}_{2}}(g)$
While, the anode attracts the negative hydroxide ions from water and chloride ions from sodium chloride. So, the chloride ions are oxidized by the electron loss to offer chlorine molecules at anode.
$C{{l}_{2}}\to 2Cl+2{{e}^{-}}$
While, the hydroxide ions with the sodium ion tend to form sodium hydroxide.
$N{{a}^{+}}+OH\to NaOH$
We can see that there is formation of sodium hydroxide which is a strong base. And bases have generally greater value of pH.
Hence, the pH will be increased during the electrolysis of brine solution.

Note:
It is important to note that, higher will be the hydrogen ion concentration less will be the value of pH whereas, more is the hydroxyl ion concentration, more will be the value of pH. Electrolysis is where ionic compounds are separated to form simple compounds and it works only if the compound contains ions. Covalent compounds cannot undergo electrolysis, as they have neutral atoms.