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Question
Explain the following:
When a dilute aqueous solution of sodium chloride is electrolysed between platinum electrodes, hydrogen gas is evolved at the cathode but metallic sodium is not deposited. Why?
Solution
\[\ce{NaCl -> Na+ + Cl-}\]
\[\ce{H2O <=> H+ + OH-}\] or
\[\ce{H2O + H2O <=> H3O+ + OH-}\]
H+ (H3O+) and Na+ move to the cathode. Hydrogen is evolved at the cathode because, despite the high concentration of Na+, H+ acquires electrons more readily than Na+ and is also lower than Na+ in the electrochemical series.
\[\ce{H+ + e- -> H}\]
\[\ce{H + H -> H2}\]
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