CHEMISTRY FORM 2
- 1.1 Structure of the atom
- 1.2 Atomic Number and Mass Number
- 1.3 Isotopes
- 1.4 Energy levels and electron arrangement
- 1.5 Development of the Periodic Table
- 1.6 Relative Atomic Mass and Isotopes
- 1.7 Ion Formation
- 1.8 Chemical Formulae
- 1.9 Chemical Equations
- 2.1 Alkali metals (Group I elements)
- 2.2 Alkali Earth Metals (Group II elements)
- 2.3 Halogens (Group VII elements)
- 2.4 Noble gases (Group VIII elements)
- 2.5 Properties and Trends Across the Periodic Table
- 3.1 Bond
- 3.2 Ionic bond
- 3.3 Giant ionic structure
- 3.4 Covalent bond
- 3.5 Co-ordinate bond
- 3.6 Molecular structures
- 3.7 Giant covalent structures
- 3.8 Metallic Bond
- 3.9 Types of bond across a period
- 3.10 Oxides of elements in Period 3
- 3.11 Chlorides of Period 3 elements
- 4.1 What is a salt?
- 4.2 Types of salt
- 4.3 Solubility of salts in water
- 4.4 Methods of preparing salts
- 4.4.1 Reacting a Metal with an Acid
- 4.4.2 Reacting an Acid with a Base (Neutralization)
- 4.4.3 Reacting an Acid with a Carbonate (or hydrogencarbonate of metal)
- 4.4.4 Combining elements Directly (Direct Combination of elements)
- 4.4.5 Precipitation (Double decomposition)
- 4.5 Action of heat on salts
- 4.6 Uses of salts
- 5.1 Electrical conduction
- 5.2 Electrical conductivity of molten substances
- 5.3 Electrical conductivity of substances in aqueous state
- 5.4 Electrolysis
- 5.5 Applications of electrolysis
- 6.1 Allotropes of carbon
- 6.2 Chemical properties of carbon
- 6.3 Carbon (IV) oxide
- 6.4 Carbon (II) oxide (CO)
- 6.5 Large scale production of sodium carbonate and sodium hydrogencarbonate
- 6.6 Effect of carbon (II) oxide and carbon (IV) oxide on the environment
- 6.7 Carbon cycle
Effect of an electric current on substances: Electrolysis
5.0 Effect of an electric current on substances
So far we have learnt about the movement of electrons and ions towards electrodes. What happens to ions as they arrive at the electrodes?
Figure 5.4(a) Set-up for electrolysis of various substances
Electrolysis of molten lead (II) bromide
Observe what happens at the electrodes as molten lead (II) bromide conducts electricity.
(courtesy Youtube-The electrolysis of lead bromide by David Read)
- What are the charges on the cathode, and which particles carry them?
- When lead (II) ions (Pb2+) reach the cathode, they combine with the charges there and become neutralized (or discharged). Write the ionic equation for the reaction.
- The product deposits on the cathode. What is observed on the cathode? Name the substance responsible for this observation.
- When bromide (Br-) ions reach the anode, they lose their negative charge to the electrode (anode). Write the ionic equation for the reaction.
- What is observed on the anode?
- In this process, called electrolysis, molten lead (II) bromide decomposes to new substances as it conducts electricity. Define electrolysis.
- A substance that behaves in a similar manner as molten lead (II) bromide is called an electrolyte. Define electrolyte.
Answers to Questions 5.4a
Electric cell, as a power source, urges electrons in the solid parts of the circuit (wires and anode) to move to the cathode. This makes the cathode negative. The anode automatically becomes positive, having lost some electrons to the cathode. This flow of electrons is anticlockwise in Figure 5.4 and only occurs along the solid conductors.
Positive ions in the electrolyte are then attracted to the cathode, where they gain (combine with) the negative electrons and become neutralized (discharged). Remember, unlike charges attract and combine with each other.
Example at the cathode: Pb2+(l) + 2e ⟶ Pb(s)
Negative ions are attracted to the positive anode where they lose their negative charge and become neutralized.
Example at the anode: 2Br-(l) ⟶ Br2(g) + 2e
Overall, conduction in solids is by electrons; but in electrolytes, it is by ions (Figure 5.4(b).
Figure 5.4(b) Movement of ions and electrons during electrolysis
AnalogyElectrons (-) are like cars, which need a ferry to cross a water channel (likened to electrolyte) but move freely once on land (likened to solid conductor). In Figure 5.4(b) Br- is like a ferry-with-a-car.
Electrolysis of molten sodium chloride, copper (II) oxide and aluminium oxide
- Complete the following table for electrolysis of molten sodium chloride, copper (II) oxide and aluminium oxide.
- For the electrolysis of molten sodium chloride, describe the observations made at the anode and cathode separately.
- For the electrolysis of molten aluminium oxide, describe the observations made at the anode and cathode separately.
Answers to Questions 5.4b
In electrolysis, the electrode used in electrolysis should be a good conductor which does not react with the electrolyte, because this would interfere with the process. For that reason, graphite is more commonly used. Such an electrode is said to be inert.
Electrolysis of aqueous solutions
Observe the demonstration of substances which conduct electricity in aqueous solutions.
- Which of the following aqueous solutions conduct electricity?
Solution of ethanol in water
Copper (II) sulphate solution
Sodium chloride solution
Sodium hydroxide solution
Sulphuric acid solution
- As a conclusion, which types or groups of substances conduct electricity in aqueous solutions?
- Does pure water conduct electricity?
Answers to Questions 5.4c
All soluble acids, bases, and salts dissociate in water to form ions; so their aqueous solutions are electrolytes. But, unlike molten electrolytes, they have more than one type of positive and negative ions (at least four) which compete for discharge at the electrodes. These will be discussed in later modules.