Alkaline Earth Metals, Aluminium and Tins – 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

1. List the alkaline earth metals and state their general properties.
2. Describe how calcium is extracted, and state its properties, compounds and uses.
3. Describe how aluminium is extracted, and state its properties and uses.
4. Describe how tin is extracted, and state its properties and uses.

What are the alkaline earth metals?

The alkaline earth metals are the six elements of Group 2A (IIA), placed in the second column of the periodic table.

What are the general properties of alkaline earth metals?

Alkaline earth metals are reactive, silvery metals whose atoms each have two electrons in the outer shell.

• Electron arrangement: two outer electrons, which the atom loses to form a 2+ ion (for example Ca²⁺).
• Appearance: silvery-white metals when freshly cut.
• Melting points and density: higher melting points and densities than Group 1 metals, but still low for metals.
• Conductivity: good conductors of heat and electricity.
• Reactivity: increases down the group, so barium is more reactive than magnesium.
• Reaction with oxygen: burn in air to form oxides, for example 2Mg + O₂ → 2MgO.
• Hydroxides: basic, and their solubility in water increases down the group.
• Compounds: mostly ionic, and the metals show an oxidation state of +2.
• Flame colours: calcium gives a brick-red flame, strontium crimson and barium apple-green.
• Radium: radioactive, so it is rare and handled with great care.

How is calcium extracted?

Calcium is extracted mainly by the electrolysis of molten calcium chloride.

Ores of calcium

• Limestone: calcium carbonate, CaCO₃.
• Gypsum and anhydrite: hydrated and anhydrous calcium sulphate.
• Dolomite: a carbonate of calcium and magnesium.
• Fluorspar: calcium fluoride.

Method 1: electrolysis of molten calcium chloride

1. Obtain calcium chloride from the calcium ore.
2. Melt the calcium chloride so that its ions can move freely.
3. Pass an electric current 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 released.

Method 2: reduction of calcium oxide

1. Heat calcium oxide (lime) strongly with aluminium.
2. Aluminium, the stronger reducing agent here, removes oxygen from the lime.
3. Calcium metal and aluminium oxide are produced.

What are the properties of calcium?

Calcium is a silvery-white metal that reacts readily with water, oxygen and nitrogen.

Physical properties

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

Chemical properties

• With water: reacts steadily with cold water to give calcium hydroxide and hydrogen gas: Ca + 2H₂O → Ca(OH)₂ + H₂.
• With air: readily reacts with oxygen and nitrogen, forming calcium oxide and calcium nitride.
• On heating: burns with a brick-red flame to form calcium oxide: 2Ca + O₂ → 2CaO.
• Compounds: forms compounds with many other elements.

Biological importance

• Bones and teeth: calcium gives them their structure and strength.
• Body functions: essential for nerve transmission, muscle function and blood clotting.

What are the compounds of calcium?

Calcium forms many compounds, and four common ones are listed below.

• Calcium carbonate (CaCO₃): abundant in nature as limestone, marble and chalk.
• Calcium chloride (CaCl₂): used as a drying agent, a de-icer and in food preservation.
• Calcium hydroxide (Ca(OH)₂): known as slaked lime, used in mortar and as a pH regulator.
• Calcium oxide (CaO): known as quicklime, used in cement production and other industrial processes.

What are the uses of calcium?

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

• Construction: limestone and gypsum are used to make cement, mortar and plaster.
• Bones and teeth: calcium is needed to form and maintain them.
• Body functions: needed for nerve transmission, muscle function and blood clotting.
• Metallurgy: calcium is used as a reducing agent in extracting some metals from their ores.
• Drying: calcium chloride is a desiccant that absorbs moisture.
• Water treatment: calcium hydroxide adjusts pH and removes impurities.
• Medicine: calcium supplements treat calcium deficiency in the body.
• Agriculture: calcium is needed for plant growth, and calcium-containing fertilisers improve soil quality.

How is aluminium extracted?

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

Steps in the extraction

1. Bauxite mining: bauxite, the aluminium ore, is mined. It contains impurities.
2. Bauxite refining: the Bayer process removes the impurities to give pure alumina (aluminium oxide, Al₂O₃).
3. Dissolving the alumina: alumina is dissolved in molten cryolite at about 950°C.
4. Electrolysis: current passes through the molten mixture in a carbon-lined cell. Aluminium ions are reduced to aluminium at the cathode.
5. Collection: molten aluminium is denser than the cryolite mixture, so it sinks to the bottom and is tapped off.

Why is cryolite used?

• Lower melting point: pure alumina melts at a very high temperature, and cryolite lets the cell work at about 950°C.
• Conduction: the molten mixture conducts electricity so that electrolysis can take place.

Reactions in the cell

• Cathode: Al³⁺ + 3e⁻ → Al.
• Anode: oxide ions lose electrons to form oxygen.
• Anode wear: the oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes must be replaced regularly.

What are the properties of aluminium?

Aluminium is a light, silvery-white metal that resists corrosion because of a thin oxide layer on its surface.

• Density: low, so it is a light metal.
• Conductivity: a good conductor of heat and electricity.
• Malleable and ductile: it can be rolled into foil and drawn into wire.
• Corrosion resistance: a thin layer of aluminium oxide protects the metal underneath.
• Reaction with acids and alkalis: it reacts with both, and its oxide is amphoteric.

What are the uses of aluminium?

Aluminium is used where a strong, light, corrosion-resistant metal is needed.

• Aerospace: used in aircraft construction.
• Automotive: used for lightweight vehicle parts.
• Packaging: foil and containers are light and resist corrosion.
• Construction: doors, windows and roofing sheets use its strength and corrosion resistance.
• Cookware: pots and utensils use its excellent heat conductivity.
• Electrical cables: overhead power lines use its good conductivity and low density.

What is tin and how is it extracted?

Tin (Sn) is a soft, silvery-white metal extracted mainly from its ore, cassiterite (tin(IV) oxide, SnO₂).

• Source in Nigeria: cassiterite is mined mainly on the Jos Plateau.

Steps in the extraction

1. Crush the ore and wash it with water to remove lighter waste rock.
2. Roast the concentrated ore to remove impurities such as sulphur.
3. Heat the ore with carbon (coke) in a furnace: SnO₂ + 2C → Sn + 2CO.
4. Tap off the molten tin and refine it to make it purer.

Properties of tin

• Physical: soft, malleable, silvery-white metal with a low melting point.
• Chemical: resists corrosion in air and water, so it protects other metals.

Uses of tin

• Tin plating: a thin coat of tin protects steel food cans from rusting.
• Solder: an alloy of tin and lead used to join metal parts and wires.
• Bronze: an alloy of tin and copper.

Evaluation

1. Name the six alkaline earth metals.
2. Mention three compounds of calcium and one use of each.
3. Describe the extraction of aluminium from bauxite.
4. State four uses of aluminium.
5. Name the ore of tin and state two uses of tin.

Assignment

1. Describe how calcium is extracted from its ores and state four uses of calcium.