Electrochemistry

Electrochemistry

The study of the relationship between chemical reactions and electrical energy

Electrochemical series of the common metals

(ease of ionization increases going upwards)

1. K+

2. Ca²+

3. Na+

4. Mg²+

5. Al³+

6. Zn²+

7. Fe²+

8. Pb²+

9. H+

10. Cu²+

11. Ag+

Discharged ions

Higher metals ionises and the ions of the lower metal are converted to atoms

Electrochemical series of some non-metals

1. Fluorine (F-)

2. Chlorine (Cl-)

3. Bromine (Br-)

4. Iodine (I-)

Conductors

Substances which allow electricity to pass through

Non-conductors

Substances which do not allow an electric current to pass through

Electrolyte

A compound that forms ions when molten or in aqueous solution

Definition of decomposed

To be chemically unchanged

Metallic conduction

1. Mobile electrons carry the electric current through the metal

2. The metal remains chemically unchanged

Electrolytic Conduction

1. Mobile ions carry the electric current through the electrolyte

2. The electrolyte decomposes

Strength of an electrolyte

We can distinguish between strong and weak electrolytes based on the concentration of ions in the electrolyte

To be fully ionised

All hydrogen ions have to be removed from the compound

(If not it was partially ionised)

Pure water

An extremely weak electrolyte

Electrolysis

The chemical change which occurs when an electric current is passed through an electrolyte

Electrolytic cell has 3 main components:

1. A battery or other d. c power supply

2. Two electrodes connected via a wire

3. The electrolyte

Anode

The positive electrode

Cathode

The negative electrodes

At the anode (OIL)

General formula:

A*n- —> A + ne-

(anion are attracted to the anode)

At the cathode (RIG)

General formula:

C*n+ + ne- —> C

(cations are attached to the cathode )

Molten electrolytes

Contain only two different ions, one cation and one anion both discharged during electrolysis

Preferential discharge

In the electrolysis of an aqueous solution, one type of ion of each charge will be discharged in preference to the other

Electrochemical series of anions

The ease of discharge increases downwards

Electric charge on 1 electron

=1.6 × 10^-¹⁹C

1C of electric charge is equivalent to (1/(1.6x10^-¹⁹) electrons

Faraday constant

The size of the electrical charge on one mole of electrons (96 500Cmol^-1)

Quantitative electrolysis

States that the mass of a substance produced at an electrode during electrolysis is directly proportional to the quantity of electricity passing through the electrolytic cell.

During electrolysis

Electrons flow from the anode to the cathode through electrical wires

Electron size

1C = 6.25x10¹⁸

Quantity of electrical charge flowing through an electrolytic cell during electrolysis depends on 2 factors:

1. The rate of flow of electrical charge

2. The length of time that current flows for (formula:Q=It)

Considering moles

1mol of electrons is equivalent to 6.0×10²³

Uses of electrolysis

1. Extracting metals from their ores

2. Purifying metals(electrorefining)

3. Plating one metal with another(electroplating)

4. Coating a metal with its oxide(anodising)

Electrolysis of the molten ore

A powerful method of reduction used to extract aluminium and metals above aluminium in the electrochemical series from their ores.

Principles of electrorefining

1. The impure metal is made the anode

2. The cathode is is a very thin sample of the pure metal

3. Electrolyte is an aqueous solution containing ions of the metal being purified

4. The anode is active and ionises and ions of the metal enters the electrolyte where they join the other metal ions

5. Metal ions are discharged at the cathode where they form pure metals which build up around the cathode

6. The electrolyte remains unchanged

7. Any insoluble impurities present in the anode fall to the bottom of the cell forming a sludge which can be removed

What is electrorefining suitable for

Only suitable for purifying metals whose ions are below hydrogen in the electrochemical series