Water

Electrolysis of Water

Let's begin the study of electrolysis by looking at the electrolysis of water. Water does conduct electricity, but only poorly.
By observation and testing we have determined that the products are H₂ and O₂.
In your workbook you have a diagram something like this in exercise 1. Take notes on that while we use it to look at what happens at each electrode.

Cathode

Electrons from the negative end of the power supply are forced into water at the cathode. This process is called reduction because the negatively charged electrons reduce the charge (oxidation state) of the hydrogen.

In the reaction, hydrogen is freed from the water and released as a gas. The unbalanced equation for this reaction can be written this way. H₂O + e^- H₂ + OH^-

(Note: This equation is not yet balanced.)

Let me elaborate on what is happening in this reaction using electron dot diagrams. An electron with a negative charge is forced into the water molecule letting an H atom leave with its own electron. Or perhaps the presence of the electron allows the H atom to move away from the water molecule and leave its own electron behind (with O).

H
ЗЗ
: O :
ЗЗ
H

ЗЏ

H
ЗЗ
: O :
ЗЗ
Џ

H З

As you know, a hydrogen atom with a single electron doesn't just sit around doing nothing. It likes to pair up with another hydrogen atom to form a hydrogen molecule. The process shown here occurs twice and the two hydrogen atoms combine to form a hydrogen molecule.

H ЗЗ : O : ЗЗ H	ЗЏ		H ЗЗ : O : ЗЗ Џ	H З		H:H
H ЗЗ : O : ЗЗ H	ЗЏ		H ЗЗ : O : ЗЗ Џ	H З		H:H

Overall, this is the reduction reaction that takes place at the cathode.

2 H₂O + 2 e^- 2 OH^- + H₂

You may have noticed that this reaction alters the H⁺/OH^- balance around the cathode making the solution more basic.

Anode

The anode is connected to the positive end of the power supply. Electrons are forced out of the water molecules. This process is called oxidation because the loss of electrons increases the oxidation state of an element (in this case the element is oxygen).

In the reaction, oxygen is freed from the water and released as a gas. The unbalanced equation for this reaction can be written this way. H₂O O₂ + H⁺ + e^-

Diagram of electrolysis of water showing anode reaction.

(Note: These equations are not yet balanced.)

Again, let me elaborate on what is happening in this reaction using electron dot diagrams. In the first step, an electron is removed from the water molecule. Removing the electron from the bond between H and O releases the H (without its electron) from its bond with O. In the second step, a second electron is removed, which also frees a second hydrogen ion. The oxygen has now lost two electrons and is an oxygen atom with two unpaired electrons.

H
ЗЗ
: O :
ЗЗ
H

ЗЏ H⁺

З
: O :
ЗЗ
H

2 ЗЏ

2 H⁺

З
: O :
З

If this reaction occurs twice, two oxygen atoms are formed, which combine to make an oxygen molecule.

З
: O :
З

: O :
ЗЗ
ЗЗ
: O :

Overall, this is the oxidation reaction that takes place at the anode.

2 H₂O O₂ + 4 H⁺ + 4 e^-

Again, you may have noticed that this reaction alters the H⁺/OH^- balance around the anode making the solution more acidic.

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E-mail instructor: Eden Francis

Clackamas Community College
Љ1998, 1999 Clackamas Community College, Hal Bender