ELECTROCHEMICAL CELL

CONDUCTOR

A conductor is a substance which conducts electricity but is not chemically changed during the conduction.

INSULATORS

A non-conductor is a substance which does not allow the passage of electricity. Sometimes these non-conductors are used to protect something from electricity. They are then called insulators.

ELECTROLYTES

Electrolytes are compounds which when molten or dissolved in water conduct electric current and are decomposed in the process.

NON-ELECTROLYTES

A non-electrolyte is a liquid which does not allow the passage of electricity.

FACTOR AFFECTING ELECTROLYSIS

There are three main factors that influence the electrolysis products. There are:

1. Position of the electrochemical series2. The concentrations3. Types of electrode

ELECTROCHEMICAL SERIES

The chart above lists the ions in order of difficulty of discharge The ions at the top of the list is more difficult to be discharged, but as we go down the

table, their become easier to be discharged. For example, Cu+ easier to be discharged compare with G+ and OH- is easier to be discharged compare with I-.

This series is called Electrochemical series

The lower the ion in the electrochemical series, the easier the ion to be discharged during electrolysis.

ELECTROLYSIS OF AQUEOUS SULPHURIC ACID

As sulphuric acid is aqueous, it is composed not only of hydrogen ions (H+) and sulphate ions (SO4

2-), but also of hydroxide ions (OH-) from the water.

H2SO4 + H2O 2H+ + SO42- + H+ + OH-

The apparatus used to carry out this electrolysis and collect the gases given off is shown in Figure 9 .8.

When we have more than one type of ion moving to an electrode, selective discharge (or preferential discharge) takes place.

This means that the ion which can lose or gain electrons with the greatest ease is discharged, and the other ions, which are harder to discharge, remain in solution.

With the electrolyte aqueous sulphuric acid, migration of ions to the electrodes also occurs.

At the Cathode At the Anode

Here we have only one ion, the hydrogen, H+ (aq), and each ion gains an electron to become a hydrogen atom.

Two of these newly formed atoms then combine to form a hydrogen gas molecule.

Here we have a choice of either sulphate, SO4

2- (aq), or hydroxide OH- (aq) ions.

Hydroxide is easier to discharge, so oxygen gas is given off at the anode.

Equation:2H+ + 2e H2

Equation:OH- + 4e O2 + H2O

Notes

With electrolysis of aqueous solutions of dilute acids or alkalis, the volume of hydrogen given off at the cathode is roughly twice that of the oxygen gas at the anode.

Accordingly, the elements of water are lost and as the electrolysis

continue the concentration of the acid or alkali increases.

Essentially, the electrolysis of aqueous sulphuric acid is the electrolysis of water, with hydrogen and oxygen gas being given off in a ratio of 2: 1.

CONCENTRATION

If the concentration of a particular ion is high, it will be selected to be discharged even though it is higher in the electrochemical series compares with another ion present in the solution.

For example, if dilute hydrochloric acid is electrolysed, hydrogen gas is given off at the cathode and oxygen gas at the anode.

However, when concentrated hydrochloric acid is electrolysed, hydrogen gas is still given off at the cathode, but chlorine rather than oxygen gas will be released at the anode, even though chloride is in a higher position in electrochemical series.

ELECTROLYSIS IN THE CONCENTRATED HYDROCHLORIC ACID

TYPE OF ELECTRODE

This is best shown if we consider the electrolysis of aqueous copper (II) sulphate solution.

Electrolysis of Copper (II) Sulphate by Using Carbon Electrode

Anode

1. If we use carbon electrodes, they are inert electrodes and do not affect the electrolysis. 2. Therefore, at the anode, we have a choice of sulphate or hydroxide ions.

3. The hydroxide ions are easier to discharge, so oxygen gas is given at the anode :

Partial equation

O2 (g) + 2H2O (l) + 4e-40H- (aq) (oxygen gas given off)

Cathode

At the cathode, we have a choice of copper or hydrogen ions. The copper ions are easier to discharge, so we see a pink deposit of copper metal on the

carbon electrode.

Partial equation

Cu (s)Cu2+ (aq) + 2e- (copper metal deposited)

Electrolysis of Copper (II) Sulphate by Using Copper Electrode

However, if we use copper electrodes, these are active electrodes and do affect the electrolysis.

Anode

At the anode, the copper electrode dissolves into solution:

Partial equation

Cu 2+ (aq) + 2eCu(s) (copper electrode dissolves )

Cathode

At the cathode, the copper ions are deposited as pink copper metal:

Partial equation

Cu (s)Cu2+ (aq) + 2e- (copper metal deposited)

ENGAGE

Figure 1

1. What do you understand about Figure 1 diagram?

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2. Give 3 factors that can affect light in the bulb

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3. What the different between electrolyte and non electrolyte?

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Answer

1. The bulb is blink because it has a good electrolyte where have high concentration of electrolyte and good electrode.

2. 3 factors that can affect light in the bulb Position of the electrochemical series The concentrations Types of electrode

3. Electrolytes are compounds which when molten or dissolved in water conduct electric current and are decomposed in the process while a non-electrolyte is a liquid which does not allow the passage of electricity.

EMPOWER

1. Electrolysis is the dissociation of an electrolyte into ions at the electrodes by the passage of electric current.

2. In electrolysis, the circuit must contain with:

a) electrode

b) Aqueous solution

c) Electric current

d) Wires

3. Log on the web side and observe the animation to answer the question.

http://www.tutorvista.com/content/chemistry/chemistry-ii/electrolysis/electrolysis-animation.php

Electrolysis

Electrolysis is the dissociation of an electrolyte into ions at the electrodes by the passage of electric current.

This diagram shows that in this solution it is contain an anions and cations movement.

This diagram show that when we use electrode and have electric current, the ions move. We used copper electrode and copper sulfate solution.

When we give electric current, the ions move to cathode and anode electrodes.

Device in which electrolysis is carried out is called a Voltmeter or an electrolytic cell.

Question

1. What is the meaning of conductor?2. What do you understand about electrochemical series?3. Give three factor that affect electrolysis.

Answer

1. A conductor is a substance which conducts electricity but is not chemically changed

during the conduction.

2.

The chart above lists the ions in order of difficulty of discharge

The ions at the top of the list is more difficult to be discharged, but as we go down the table, their become easier to be discharged. For example, Cu+ easier to be discharged compare with G+ and OH- is easier to be discharged compare with I-.

This series is called Electrochemical series

The lower the ion in the electrochemical series, the easier the ion to be discharged during electrolysis.

3. There are three main factors that influence the electrolysis products. There are: Position of the electrochemical series The concentrations Types of electrode

ENHANCE

Based on the diagram, please answer it with true or false.

1. Electron flow from cathode to anode ____________

2. The electrons flow from the anode through the external circuit to the cathode _______3. Anode is labeled as a negative sign while cathode is label as a positive sign ________4. Electrons as being attracted to the negative cathode from the positive anode through the

external circuit ___________5. The chemical process occurring in a voltaic cell, the direction of the migration of ions in

solution, and the motion of electrons between electrodes in the external circuit. ________6. Oxidation occur at cathode, while reduction occur at anode _________7. The cell compartments can be separated by a salt bridge _____________

Answer

1. False2. True3. True4. False5. True6. False7. True