Polymers, Giants and Macro Molecules II – SS3 Chemistry Lesson Note (First Term)

Alkaline earth metals are the Group 2A (IIA) elements of the periodic table: beryllium, magnesium, calcium, strontium, barium and radium.

Behavioural objectives

By the end of the lesson, students should be able to:

1. list and explain the general properties of alkaline earth metals.
2. discuss the extraction, properties and uses of calcium.
3. name four compounds of calcium and state one use of each.
4. describe the extraction of aluminium from bauxite.
5. state the properties and uses of aluminium.

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos from source

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Periodic table of elements

What are the alkaline earth metals?

The alkaline earth metals are the six metals in Group 2A (IIA): beryllium, magnesium, calcium, strontium, barium and radium.

• Beryllium (Be): the lightest member, at the top of the group.
• Magnesium (Mg): used in alloys, flash bulbs and indigestion medicine.
• Calcium (Ca): found in limestone, bones and teeth.
• Strontium (Sr): gives a crimson flame.
• Barium (Ba): gives an apple-green flame.
• Radium (Ra): a radioactive element at the bottom of the group.

What are the general properties of alkaline earth metals?

Alkaline earth metals are silvery, reactive metals with two outer electrons, which form ions with a 2+ charge.

• Electronic structure: each atom has two electrons in its outer shell.
• Ion formed: each atom loses two electrons to form M²⁺, so the valency is 2.
• Appearance: they are silvery-white and shiny when freshly cut.
• Melting points and densities: they have fairly high melting points and low densities.
• Hardness: they are harder than the Group 1 metals.
• Conductivity: they conduct heat and electricity.
• Reactivity: they are reactive, and reactivity increases down the group.
• Oxides and hydroxides: the oxides are basic, and the hydroxides of calcium, strontium and barium are alkaline.
• Compounds: they form mainly ionic compounds.
• Flame colours: some give coloured flames, as shown below.

Flame colours of some alkaline earth metals

How is calcium extracted?

Calcium is extracted by the electrolysis of molten calcium chloride, obtained from ores such as limestone.

• Ores: limestone (CaCO₃), gypsum and anhydrite, dolomite and fluorspar.

Method 1: Electrolysis of molten calcium chloride

1. Calcium chloride is prepared from the ore.
2. The calcium chloride is melted so that its ions can move.
3. An electric current is passed through the molten salt.
4. Calcium ions gain electrons at the cathode: Ca²⁺ + 2e^– → Ca.
5. Chloride ions lose electrons at the anode, and chlorine gas is given off.

Method 2: Reduction of calcium oxide

1. Calcium oxide (lime) is heated with aluminium.
2. Aluminium reduces the calcium oxide.
3. Calcium and aluminium oxide are produced.

What are the properties of calcium?

Physical properties

• Appearance: it is a silvery-white, somewhat malleable metal.
• Hardness and density: it is fairly hard and has a low density compared with most metals.
• Conductivity: it is a good conductor of electricity.

Chemical properties

• With water: it reacts vigorously to give calcium hydroxide and hydrogen gas: Ca + 2H₂O → Ca(OH)₂ + H₂.
• With air: it reacts readily with oxygen and nitrogen in the air.
• With oxygen: 2Ca + O₂ → 2CaO.
• Flame test: it burns with a brick-red flame when heated.
• Compounds: it forms compounds with various elements.

Biological importance

• Bones and teeth: calcium is vital for their structure.
• Body functions: it is essential for nerve transmission, muscle function and blood clotting.

What are the compounds of calcium?

Calcium forms many compounds, and the notable ones are carbonate, chloride, hydroxide and oxide.

What are the uses of calcium?

Calcium and its compounds are used in building, industry, medicine and farming.

1. Construction: limestone and gypsum are used for making cement, mortar and plaster.
2. Bones and teeth: calcium is needed to form and maintain them.
3. Body functions: it is needed for nerve transmission, muscle function and blood clotting.
4. Metal extraction: calcium is a reducing agent for some metals in their ores.
5. Drying: calcium chloride is a desiccant that absorbs moisture.
6. Water treatment: calcium hydroxide adjusts pH and removes impurities.
7. Supplements: calcium tablets treat calcium deficiency in humans.
8. Agriculture: calcium is essential for plant growth, and calcium-containing fertilisers improve soil.

How is aluminium extracted?

Aluminium is extracted from bauxite by the Hall-Héroult process, which is the electrolysis of alumina dissolved in molten cryolite.

Steps in the extraction

1. Bauxite mining: bauxite, an aluminium ore, is mined from the Earth. It contains impurities.
2. Bauxite refining: the Bayer process removes impurities and gives pure alumina (aluminium oxide).
3. Dissolving the alumina: alumina is dissolved in molten cryolite at about 950°C.
4. Electrolysis: current is passed through the molten mixture in a carbon-lined cell.
5. Reduction at the cathode: aluminium ions gain electrons to form molten aluminium: Al³⁺ + 3e^– → Al.
6. Collection: molten aluminium is denser than the molten cryolite, so it sinks and is tapped from the bottom.

Key points about the process

• Role of cryolite: it lowers the melting point of alumina, so less energy is needed.
• Cathode: the carbon lining of the cell.
• Anodes: carbon blocks, where oxygen is released.
• Anode wear: the oxygen reacts with the hot carbon to form carbon dioxide, so the anodes burn away and are replaced.

What are the properties of aluminium?

Physical properties

• Appearance: it is a silvery-white metal.
• Density: it is light compared with iron and copper.
• Working: it is malleable and ductile.
• Conductivity: it conducts heat and electricity well.

Chemical properties

• Oxide layer: in air it forms a thin layer of aluminium oxide, which protects the metal from corrosion.
• With acids: it reacts with dilute acids to give a salt and hydrogen.
• With alkalis: it reacts with sodium hydroxide solution, because aluminium oxide is amphoteric.

What are the uses of aluminium?

Aluminium is used in transport, packaging, building, cookware and electrical work because it is light, strong and resists corrosion.

1. Aerospace: it is used for aircraft construction.
2. Automotive: it is used for lightweight vehicle parts.
3. Packaging: foil and containers are lightweight and resist corrosion.
4. Construction: it is used for doors, windows and structural parts because of its strength and corrosion resistance.
5. Cookware: pots and utensils use its excellent heat conductivity.
6. Medical equipment: it is used in some equipment because it resists corrosion.
7. Electrical cables: it is a good conductor and is light, so it is used for overhead power lines.

Evaluation

1. Identify the alkaline earth metals.
2. Mention three compounds of calcium.
3. Discuss the extraction of aluminium.
4. Enumerate four uses of aluminium.
5. State the role of cryolite in the extraction of aluminium.

Classwork: students answer the evaluation questions above.

Assignment

1. List four uses of calcium, then state two physical properties and two chemical properties of calcium.