17.7-17.8Free
Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
Tap water leaves white scale in a kettle. Antacids calm the stomach. Cement holds our homes together. All of these use Group 2 compounds.
How are they made, and where are they used?
Make it, then use it.
The Dow process takes magnesium from the huge amount of Mg2+ in seawater.
Step 1. Seawater is treated with calcium hydroxide. It is made from limestone or oyster shells. Insoluble magnesium hydroxide forms.
Mg2+(aq) + Ca(OH)2(s) → Mg(OH)2(s) + Ca2+(aq)
Step 2. The magnesium hydroxide is filtered. Then it reacts with hydrochloric acid.
Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
Step 3. The magnesium chloride solution is concentrated and dried. The molten salt is electrolysed. Magnesium forms at the cathode. Chlorine forms at the anode.
Mg2+(l) + 2Cl−(l) → Mg(l) + Cl2(g)
The process gets a valuable metal from a very large, low-concentration source.
Limestone is mainly calcium carbonate. It is heated in a lime kiln at about 900 to 1000 °C.
This is called calcination or thermal decomposition.
CaCO3(s) → CaO(s) + CO2(g)
The reaction is reversible in a closed container. Removing CO2 all the time shifts the equilibrium to the right. So more product forms, faster.
Quicklime CaO is used in construction, agriculture, metallurgy and chemical industries.
Construction: CaCO3 is limestone, marble and chalk. It is used in building, cement, concrete, and as a filler in paper and plastics.
CaO and Ca(OH)2 are used in cement, mortar and plaster. Quicklime also removes impurities in steel making.
Gypsum CaSO4·2H2O is used for plasterboard and plaster of Paris.
Environment: CaO and CaCO3 remove acidic SO2 from power plant emissions. This helps prevent acid rain.
MgO lines furnaces because its melting point is high.
Magnesium metal is used in light alloys, fireworks and flares. Strontium compounds give red colour. Beryllium strengthens alloys and is used in X-ray tubes because it lets X-rays through.
Milk of magnesia, Mg(OH)2, neutralises excess stomach acid and is a mild laxative. Its low solubility makes it safe.
CaCO3 is an antacid in indigestion tablets. Calcium compounds give strong bones and teeth and prevent osteoporosis.
Epsom salt MgSO4·7H2O is used as a laxative and in baths. Magnesium compounds help muscles and nerves.
BaSO4 is used in barium meals for X-ray imaging of the digestive system. It is opaque to X-rays. It is safe because it is highly insoluble and is not absorbed.
Agriculture: CaCO3 and Ca(OH)2 are added to acidic soil. They neutralise the acid and raise the pH.
Calcium builds plant cell walls. Magnesium is the central atom in chlorophyll. Calcium nitrate and Epsom salt can be used as fertilizers.
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Tap water leaves white scale in a kettle. Antacids calm the stomach. Cement holds our homes together. All of these use Group 2 compounds.
How are they made, and where are they used?
Make it, then use it.
The Dow process takes magnesium from the huge amount of Mg2+ in seawater.
Step 1. Seawater is treated with calcium hydroxide. It is made from limestone or oyster shells. Insoluble magnesium hydroxide forms.
Mg2+(aq) + Ca(OH)2(s) → Mg(OH)2(s) + Ca2+(aq)
Step 2. The magnesium hydroxide is filtered. Then it reacts with hydrochloric acid.
Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
Step 3. The magnesium chloride solution is concentrated and dried. The molten salt is electrolysed. Magnesium forms at the cathode. Chlorine forms at the anode.
Mg2+(l) + 2Cl−(l) → Mg(l) + Cl2(g)
The process gets a valuable metal from a very large, low-concentration source.
Limestone is mainly calcium carbonate. It is heated in a lime kiln at about 900 to 1000 °C.
This is called calcination or thermal decomposition.
CaCO3(s) → CaO(s) + CO2(g)
The reaction is reversible in a closed container. Removing CO2 all the time shifts the equilibrium to the right. So more product forms, faster.
Quicklime CaO is used in construction, agriculture, metallurgy and chemical industries.
Construction: CaCO3 is limestone, marble and chalk. It is used in building, cement, concrete, and as a filler in paper and plastics.
CaO and Ca(OH)2 are used in cement, mortar and plaster. Quicklime also removes impurities in steel making.
Gypsum CaSO4·2H2O is used for plasterboard and plaster of Paris.
Environment: CaO and CaCO3 remove acidic SO2 from power plant emissions. This helps prevent acid rain.
MgO lines furnaces because its melting point is high.
Magnesium metal is used in light alloys, fireworks and flares. Strontium compounds give red colour. Beryllium strengthens alloys and is used in X-ray tubes because it lets X-rays through.
Milk of magnesia, Mg(OH)2, neutralises excess stomach acid and is a mild laxative. Its low solubility makes it safe.
CaCO3 is an antacid in indigestion tablets. Calcium compounds give strong bones and teeth and prevent osteoporosis.
Epsom salt MgSO4·7H2O is used as a laxative and in baths. Magnesium compounds help muscles and nerves.
BaSO4 is used in barium meals for X-ray imaging of the digestive system. It is opaque to X-rays. It is safe because it is highly insoluble and is not absorbed.
Agriculture: CaCO3 and Ca(OH)2 are added to acidic soil. They neutralise the acid and raise the pH.
Calcium builds plant cell walls. Magnesium is the central atom in chlorophyll. Calcium nitrate and Epsom salt can be used as fertilizers.
Q1. Quick Check 17.4 (a): write the balanced equation for the electrolysis step of the Dow process.
Mg2+(l) + 2Cl−(l) → Mg(l) + Cl2(g)
Q2. Quick Check 17.4 (b): write the balanced equation for making quicklime from limestone.
CaCO3(s) → CaO(s) + CO2(g)
Q3. Quick Check 17.4 (c): state one agricultural use of CaCO3 and the chemical principle.
It is added to acidic soil. It is a base, so it neutralises the acid and raises the pH. Crops then grow better.
Q4. Exercise Q2 (g): write the balanced equation for the precipitation of magnesium ions from seawater in the Dow process.
Mg2+(aq) + Ca(OH)2(s) → Mg(OH)2(s) + Ca2+(aq)
Q5. Exercise Q2 (h): state one medical use each of BaSO4 and Mg(OH)2.
BaSO4: barium meal for X-ray imaging of the digestive system. Mg(OH)2: milk of magnesia, neutralises excess stomach acid.
✗ “Magnesium is made by electrolysing seawater directly.”
✓ No. First Mg(OH)2 is precipitated. It becomes MgCl2. Only then is the molten MgCl2 electrolysed.
✗ “Barium sulphate is safe because it dissolves.”
✓ The opposite. It is safe because it is highly insoluble. It passes through the body without being absorbed.
1. Which Group 2 oxide forms the most alkaline solution in water? (Exercise Q1, part 1)
(a) Basic character of the oxides increases down the group. Strontium is the lowest in the list.
2. Which Group 2 element does not react with water or steam? (Exercise Q1 part 2)
(a) A protective oxide layer stops it.
3. What forms when a basic oxide such as CaO reacts with water? (Exercise Q1 part 4)
(a) The oxide ion makes OH− in water.
4. Which Group 2 carbonate is the most thermally stable? (Exercise Q1 part 5)
(a) Stability rises down the group.
5. Which Group 2 sulphate is the least soluble? (Exercise Q1 part 8)
(a) Sulphates get less soluble down the group.
6. What forms at the cathode in the Dow process?
(a) Chlorine forms at the anode.
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