Seawater holds only a small concentration of magnesium ions, yet it is one of the largest sources of magnesium metal. Limestone, on the other hand, is everywhere, and heating it gives one of the oldest industrial chemicals. Section 17.7 of the book explains both processes.
Chapter 17 of the PECTAA Chemistry 12 book is Group 2 Elements. After complex ion formation (section 17.6), section 17.7 covers extraction and purification of Group 2 elements and their compounds: magnesium from seawater (the Dow process) and quicklime from limestone. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Magnesium from seawater: the Dow process (section 17.7.1). The book calls it perhaps the most famous and industrially significant of these processes. It is a multi-step chemical and electrochemical route from the Mg²⁺ ions in seawater to magnesium metal.
Step 1: precipitation. Seawater is treated with calcium hydroxide, made from limestone or oyster shells. Magnesium comes out as insoluble magnesium hydroxide:
Mg²⁺(aq) + Ca(OH)₂(s) → Mg(OH)₂(s) + Ca²⁺(aq)
This works because Mg(OH)₂ is far less soluble than Ca(OH)₂, the trend you met in the solubility of Group 2 hydroxides.
Step 2: conversion to chloride. The Mg(OH)₂ is filtered off and reacted with hydrochloric acid:
Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)
Step 3: electrolysis. The MgCl₂ solution is concentrated and dried. Molten MgCl₂ is then electrolysed in large cells:
Mg²⁺(l) + 2Cl⁻(l) → Mg(l) + Cl₂(g)
Magnesium forms at the cathode and chlorine at the anode. The book praises the process for drawing a valuable metal from an abundant but dilute source on a massive scale.
Quicklime from limestone (section 17.7.2). Limestone is an abundant sedimentary rock made mainly of calcium carbonate. Quicklime is calcium oxide, used in construction (cement, mortar), agriculture (soil treatment), metallurgy and other chemical industries.
- Crushed limestone is heated in a lime kiln, typically at 900 to 1000 °C.
- The process is called calcination, a thermal decomposition:
CaCO₃(s) → CaO(s) + CO₂(g) (heat)
- Modern kilns heat the charge with circulating hot gases at the reaction temperature.
- In a closed container the reaction is reversible. Removing CO₂ continuously shifts the equilibrium to the right, so the reaction goes on to give a large amount of CaO.
Comparing the two processes.
| Feature | Dow process | Quicklime |
|---|---|---|
| Raw material | Mg²⁺ ions in seawater | Limestone (CaCO₃) |
| Main idea | precipitation, then electrolysis | thermal decomposition |
| Product | magnesium metal | calcium oxide |
| By-product | chlorine gas | carbon dioxide |
Both start from Group 2 compounds that are plentiful in nature. The Dow process needs electrical energy for the final step, because magnesium is a reactive metal and its ions hold on to their electrons. The lime kiln needs heat energy, because Group 2 carbonates decompose only at high temperature, and calcium carbonate needs about 900 °C.
Key points
- Dow process order: precipitate Mg(OH)₂ → make MgCl₂ → electrolyse molten MgCl₂.
- Cathode: Mg. Anode: Cl₂.
- Calcination: CaCO₃ → CaO + CO₂ at about 900 to 1000 °C.
- Removing CO₂ drives the decomposition forward.
Worked examples
Example 1 (balancing the steps). Check that each Dow process equation is balanced.
- Mg²⁺ + Ca(OH)₂ → Mg(OH)₂ + Ca²⁺: one Mg, one Ca, two O and two H on each side; charge +2 on each side.
- Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O: one Mg, two Cl, four H and two O on each side.
- Mg²⁺ + 2Cl⁻ → Mg + Cl₂: charge 0 on each side; magnesium gains 2 electrons at the cathode, two chloride ions lose 2 electrons at the anode.
Example 2 (calcination). How much CaO forms from 100 g of pure CaCO₃?
- Molar masses: CaCO₃ = 40 + 12 + 48 = 100 g mol⁻¹; CaO = 40 + 16 = 56 g mol⁻¹.
- 100 g CaCO₃ is 1 mol, giving 1 mol CaO = 56 g, and 44 g of CO₂ is released.
Example 3 (equilibrium reasoning). Why does the kiln let CO₂ escape?
- CaCO₃ ⇌ CaO + CO₂ is reversible in a closed space.
- Removing CO₂ lowers its concentration, so the equilibrium shifts to the right and more CaO forms.
Common mistakes in the exam
- Electrolysing aqueous MgCl₂. The book uses molten, dried MgCl₂.
- Swapping electrodes. Metal at the cathode, chlorine at the anode.
- Writing CaO + CO₂ → CaCO₃ as the kiln reaction. That is the reverse.
- Forgetting the Ca(OH)₂ source. The book names limestone or oyster shells.
- Leaving out state symbols in equations where the question asks for them.
Exam technique
- Present the Dow process as three numbered steps, each with its equation.
- Link step 1 to the solubility trend of Group 2 hydroxides for an extra mark.
- For quicklime, give the equation, the temperature range and one use.
- Check your board's notice for the sections in your paper.
Practice questions
- Name the product formed at the anode in the Dow process. Answer: Chlorine gas.
- Write the equation for converting Mg(OH)₂ to MgCl₂. Answer: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
- What is calcination? Answer: Strong heating causing thermal decomposition, here CaCO₃ → CaO + CO₂.
- Give two uses of quicklime from the book. Answer: Construction (cement, mortar) and agriculture (soil treatment).
- How much CO₂ is released from 50 g of CaCO₃? Answer: 0.5 mol, which is 22 g.
Ready to practise this? Continue in 2nd Year Chemistry.