Mammoth Memory

Examples of electrolysis with different substances

Example 1: NaCl and water

 

Chlorine and hydrogen are given off from the electrodes and are caught

 

So what is happening at the electrodes?

Starting with the basic elements we have:-

Hydrogen and oxygen

In the solution we have hydrogen and oxygen

 

Sodium and chlorine

 

We also have chlorine and sodium

As molecules

Molecular structure of salt and water

But dissolved in solution with each other, chemists say we get:-

 

Chemists electrolysis understanding of salt and water

At the cathode

 

Rule one says hydrogen can only form on the cathode, sodium cant so all of its electrons are stripped and given to the hydrogen

Rule 1 says hydrogen forms at the cathode not sodium (as sodium is more reactive than hydrogen). So at the cathode electrons are given out (to the hydrogen ions).

 

With the electron hydrogen ions become H

But as hydrogen always goes around in pairs (as in a diatomic gas), two hydrogen atoms now combine to form a hydrogen gas.

 

Hydrogen atoms have to be paired to be come a gas

 

The half equation is:

                                                   H+ (aq)      +          ?          →        H2 (g)

To balance this                        2H+ (aq)     +          ?          →        H2 (g)

And the electron balance      2H+ (aq)     +          2e-          →        H2 (g)

Or                                                2H+ (aq)                             →        H2 (g) – 2e-

 

At the anode

 

Rule 2 says only halogens can form at the anode not hydroxide

Rule 2 says a halogen (Cl) forms at the anode and not a hydroxide (OH-).

So at the anode electrons are taken away from the chlorine.

 

A chlorine electron is stripped from the atom to be come Cl

 

But as chlorine always goes around in pairs (as in a diatomic gas), two chlorine atoms now combine to form chlorine gas …

 

Chlorine then bonds with another to form chlorine gas

 

The half equation is:

                                              Cl- (aq)             +          ?          →        Cl2 (g)

To balance this                    2Cl- (aq)           +          ?          →        Cl2 (g)

And the electron balance   2Cl- (aq)           +          2e-         →        Cl2 (g)

Or                                            2Cl-                                           →        Cl2 + 2e-

 

The sodium and hydroxide ions just combine:

Sodium and hydroxide ions just combine

 

They combine to give NaOH or (Na+OH-)

They give NaOH

 

NOTE: Within NaOH there is an ionic bond between sodium and oxygen and a covalent bond between oxygen and hydrogen.

This is a good example of how there can be different types of bond within one compound.

 

More Info