Electrolysis

[jowrose]

Ok, I understand the general reaction of electrolysis;

 

The anode takes water and creates oxygen molecules, hydrogen ions, and electrons.

 

The cathode takes water and electrons and creates hydrogen molecules and hydroxide ions.

 

Furthermore, when a salt is dissolved in the water, electrolysis separates the two components of the salt, the anion to the anode and the cation to the cathode.

 

What I am confused about is the role the cathode and anode have in this process. Say I'm using copper electrodes and NaCl as the electrolyte. Sodium is drawn to the cathode, where it combines with hydroxide ions? The chlorine is drawn to the anode, and it combines with hydrogen ions? And what about the copper? Does it combine with hydroxide ions at the cathode, and oxygen at the anode?

 

I am confuzzled.

[woelen]

Electrolysis is nothing more than a redox reaction, with the cathode acting as a strong reductor and the anode acting as a strong oxidizer. In fact, these artificial reductors and oxidizers are incredibly powerful. If a voltage of just a few volts is applied, then the reducing power and oxidizing power exceeds the power of the strongest chemical reductors and oxidizers.

 

Now, back to the concrete situation of NaCl, dissolved in water, with copper anode and copper cathode.

 

Near the anode, the following things are present:

 

H2O

Na(+) ions

Cl(-) ions

Cu metal

 

The anode acts as oxidizer. What happens now is that the most easily oxidized species is oxidized. In the above case, this is the copper metal. It is oxidized to Cu(+) ions, or Cu(2+) ions.

 

If a graphite anode is used, then the available compounds for oxidation are:

 

H2O

Na(+) ions

Cl(-) ions

C (carbon of anode)

 

Now, the Cl(-) ions are the easiest oxidized species, but they are close to H2O. So, what happens now is that Cl(-) is oxidized to Cl2: 2Cl(-) --> Cl2 + 2e. As a side reaction you also have oxidation of water. This is because the oxidation of water is just a little bit harder than oxidation of chloride.

 

At the cathode a similar reasoning can be made. Over there we have

 

H2O

Na(+) ions

Cl(-) ions

Cu metal

 

Now, we have to look at what is easiest reduced. In this case, that is the water.

 

Now suppose that we add some NiCl2 to the solution of NaCl. In that case we have the following things around the cathode:

 

H2O

Na(+) ions

Cl(-) ions

Cu metal

Ni(2+) ions

 

Now, the easiest reduced species is not water, but nickel ion. The nickel ions are reduced to metallic nickel. When the applied voltage is sufficiently high, then still the reduction of water also occurs as a side reaction.

 

So, you see, electrolysis is a redox-reaction, with different reactions competing at the electrodes. Which species is actually oxidized/reduced depends mainly on the ease of oxidation/reduction, but also at the applied voltage and the concentration of the reactants.

[jowrose]

Ah, thanks. Tis getting clearer.

 

So you can use an activity series of metals to determine which of those will be oxidized, but what about nonmetals? Is there a given rule to determine which elements will be reduced or oxidized, or is there just some chart somewhere?