The Periodic Table

The Alkali Metals - Electrolysis of Sodium Chloride in Water.

This page shows the electrolysis of sodium chloride
which has been dissolved in water. You get
different products if the s
odium chloride is completely dry.

What is the Electrolysis of Brine?

Sodium chloride dissolved in water is called brine.
Electrolysis of brine gives hydrogen gas at the cathode and
chlorine gas at the anode. You must know how to test
for hydrogen and chlorine gas. Sodium hydroxide remains
dissolved in the solution. The picture below shows
what happens to the ions during the electrolysis of brine.
This is an important industrial process making
hydrogen gas, chlorine gas and sodium hydroxide.

Electrolysis of Sodium Chloride in Water

What are Half Equations?

The reactions at each electrode are called half equations.
The half equations are written so that
the same number of electrons occur in each equation.

2H+  +  2e-    arrow   H2 (hydrogen gas at the (-)cathode).
2
Cl-  -  2e-   arrow    Cl2  (chlorine gas at the (+)anode).

Hydrogen ions gain electrons (reduction) to form hydrogen atoms.
The hydrogen atoms combine to form molecules of hydrogen gas.

Chloride ions lose electrons (oxidation) to form chlorine atoms.
The chlorine atoms combine to form molecules of chlorine gas.

The overall reaction is

2NaCl(aq)  +   2H2O(l)   arrow   2Na+(aq)  +   2OH-(aq)  +   Cl2(g)+   H2(g)


Electrolysis
of hydrochloric acid also gives
hydrogen at the cathode and chlorine at the anode but
hydrochloric acid is more expensive than brine
and you do not get sodium hydroxide left in the solution.

See more detail of the electrolysis on the next page.

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