Electrolysis

Electrolysis of Water

Learning Objectives

• At the end of this lesson, students should be able to:

• Describe electrolysis as the conduction of electricity by an electrolyte when molten or aqueous leading to the decomposition of the electrolyte

• Predict the likely products of the electrolysis of a specified binary compound in the molten state or in aqueous solution

• Describe the uses of electrolysis in electroplating of metals

Previous Knowledge

• From atomic structure and bonding, we know electrons play an important part in the chemical properties of substances.

• So, electricity, in the form of an electric current, will have some effect on chemical substances through which it is able to pass.

Imagine you are the captain of a submarine. Your crew tells you that oxygen supply is running low. You feel helpless because you are 1000m under the sea. You have nothing but sea water, lots of platinum, wires, excess electricity and glass containers. What can you do to save yourself and the crew?

What is Electrolysis

It is a chemical process whereby

An electrolyte is decomposed by the passage of electric current.

An electrical energy is used. Why?

Why is an electrical energy used?

• It is to cause a non-spontaneous chemical reaction to occur.

• For e.g., water in a beaker under atmospheric condition of 25°C and 1 atm. Can it decompose without electricity?

• 2H2O(l) 2H2(g) + O2(g)

Answer: No

Electrolysis

An ionic compound

It can be an aqueous solution or a molten compound

It conducts electric current with decomposition at the electrodes

E.g., dilute sulfuric acid, H2SO4 (aq) and copper (II) sulfate solution, CuSO4 (aq)

What is an electrolyte?

Electrolysis

Carbon graphite or metal by which the electron enters or leaves the electrodes

They can be inert or reactive

If it is connected to the positive terminal of a power supply, it is called the anode

If it is connected to the negative terminal of a power supply, it is called the cathode

What are Electrodes?

How does Electrolysis Works?

1. When power is switched on, electrons move from anode to cathode

2. Electrolytes dissociate to form negatively charged anions and positively charged cations

3. The ions conduct electricity through the electrolyte

4. Cations are attracted towards the negative electrode(cathode) and they take excess electrons and neutralize themselves

5. Anions are attracted to the positive electrode (anode) and they give up the excess electrons to the anode and neutralize themselves

6. Eventually, salts and gases are liberated at these electrodes.

Electrolysis

Electrolysis of a Molten Compound

Anode (oxidation): 2Cl- (l) --> Cl2 (g) +2e

Cathode (reduction): 2Na+ (l) + 2e --> 2Na (l)

Thus, sodium chloride can be electrolyzed to form sodium metal and chlorine gas

What is the cation?

What is the anion?

Electrolysis of an Aqueous Solution

For aqueous solution, it is not so straightforward as for a molten compound.

This is because of the presence of additional ions in the aqueous solution.

So, which cations or anions are released or discharged?

For e.g., at anode: Is it OH- from H2O or SO42- from

H2SO4 ?

Electrolysis

When 2 or more ions of similar charge are present under similar conditions in a solution, one is preferentially selected for discharge. This depends on:-

1. Position of the metal or group in the reactivity series

2. Concentration

3. Nature of the electrode

Electrolysis

1. Position of the metal or group in the reactivity series

AnionsCations

To Cathode To Anode

K+

Na+

Ca2+

Mg2+

Zn2+

Fe2+

Pb2+

H+

Cu2+

Ag+ OH-

I-

Br-

Cl-No3

-

SO42-

Any ion will be discharged from solution in preference to those above it

Electrolysis

2. Concentration

Increase of concentration of an ion tends to promote its discharge

3. Nature of the Electrode

Inert electrodes like carbon graphite and platinum are not affected by ions surrounding them.

Reactive electrodes like copper or silver can affect the order of discharge.

For e.g., in the electrolysis of copper (II) sulfate solution, if inert electrodes (graphite) are used, oxygen is evolved at the anode. But if active electrode like copper is used, copper is dissolved at the anode.

Electrolysis

Electrodes: Platinum

2 moles of hydrogen gas liberated at cathode, 1 mole of oxygen gas liberated at anode

SO42- or OH-? H+

OH-(aq) – e- OH(g)

OH(g) + OH(g) H2O + O(g)

O(g) + O(g) O2(g)

H+(aq) + e- H(g)

H(g) + H(g) H2(g)

Questions

• Predict the products which will be obtained at the anode (+ve) and at the cathode (-ve), when an electric current is passed through aqueous sodium

sulfate, Na2SO4. Electrodes: Graphite

Anode: H+ or Na+

Cathode:OH- or SO4

2-

Answer: H+ Answer: OH-

Na+ and SO42- will not be released as they are too

reactive.

Electrolysis of Copper Sulfate Solution

Cathode Anode

Cu2+ (aq)

H+ (aq)

SO42- (aq)

OH- (aq)

From Copper (II) Sulfate: Cu2+(aq) SO4

2-

(aq)

From water : H+(aq) OH-

(aq)

Cu2+(aq) + 2e-

Cu(s)OH- discharged. So, O2 is given

off.Cu(s) – 2e- Cu2+

(aq)

Pt or carbon

electrode

Copper electrode

A pupil wishes to plate a steel spoon with silver using electrolysis. What should be the electrolyte,

cathode and anode?

AgNO3(aq)

Silver spoon

(cathode)

Ag (anode

)

+ -

http://www.youtube.com/watch?v=_uYWf6fVa4I&feature=related

Questions on Electroplating

Aqueous copper (II) sulfate is using copper electrodes.

(a) The positive electrode decreases in mass. Why?

(b) The negative electrode increases in mass. Why?

Write an ionic equation, including state symbols, for the reaction at the positive & negative electrodes.

Anode: Impure Copper

Cathode: Pure Copper

Cu (s) Cu2+(aq) +

2e-

Cu2+(aq) + 2e-

Cu(s)

The End