Detailed Discussion On Magnesium

ChemistryThe s-Block ElementsFor JEE aspirants

Magnesium is the Group 2 metal met most often in exams: a light, strongly reducing metal ( $E^{\circ} = - 2.36$ V) protected by a thin oxide film, made by electrolysis of fused magnesium chloride, and essential to life as the centre of chlorophyll and the partner of ATP. This page covers its sources and extraction, its reactions with air, $C O_{2}$ , water and acids, the Grignard reagent, its important compounds, and the biological roles of magnesium and calcium. JEE Advanced asks for the products, the reasons behind them and the links to organic chemistry.

On this page1Magnesium at a glance2Extraction3Burning in air and CO₂4Water and acids5Reducing agent6Grignard reagent7Compounds8Mg and Ca in the body9Solved examples

Key Formulas - Quick Reference

★ Must learnBurning in air: $2Mg + O_{2} \to 2MgO$ and $3Mg + N_{2} \to M g_{3} N_{2}$ ; $M g_{3} N_{2} + 6 H_{2} O \to 3Mg (OH)_{2} + 2N H_{3}$ .
★ Must learnReduces $C O_{2}$ : $2Mg + C O_{2} \to 2MgO + C$ ( $Δ H = - 810$ kJ); also $Si O_{2}$ → Si, $B_{2} O_{3}$ → B.
Water: very slow in the cold; $Mg + 2 H_{2} O \to Mg (OH)_{2} + H_{2}$ (hot); $Mg + H_{2} O \to MgO + H_{2}$ (steam).
★ Must learnAcids: $Mg + 2HCl \to MgC l_{2} + H_{2}$ ; with very dilute $HN O_{3}$ : $Mg + 2HN O_{3} \to Mg (N O_{3})_{2} + H_{2}$ .
★ Must learnDow process: $M g^{2 +}$ (sea water) + lime → $Mg (OH)_{2}$ → $MgC l_{2}$ ; hydrate dried in HCl gas; fused $MgC l_{2}$ electrolysed.
Hydrolysis on heating: $MgC l_{2} \cdot 6 H_{2} O \to MgO + 2HCl + 5 H_{2} O$ (so anhydrous $MgC l_{2}$ needs dry HCl).
★ Must learnGrignard: $RX + Mg \to RMgX$ (dry ether); $RMgX + H_{2} O \to RH + Mg (OH) X$ ; with $C O_{2}$ then $H_{3} O^{+}$ → RCOOH.
Compounds: MgO (refractory, m.p. 3125 K), $Mg (OH)_{2}$ (milk of magnesia), $MgS O_{4} \cdot 7 H_{2} O$ (Epsom salt), $MgC O_{3}$ (magnesite).

1. Magnesium at a Glance

Magnesium ([Ne] $3 s^{2}$ ) is the second member of Group 2. Calcium and magnesium rank fifth and sixth in abundance in the earth's crust. Magnesium occurs as magnesite ( $MgC O_{3}$ ), dolomite ( $CaC O_{3} \cdot MgC O_{3}$ ), carnallite ( $KCl \cdot MgC l_{2} \cdot 6 H_{2} O$ ) and Epsom salt ( $MgS O_{4} \cdot 7 H_{2} O$ ), and as $M g^{2 +}$ in sea water. It is a silvery white, somewhat greyish, light metal (1.74 g cm $^{- 3}$ ). Its ion, $M g^{2 +}$ (72 pm), is almost exactly the size of $L i^{+}$ (76 pm), which is why the two metals show a diagonal relationship.

Figure 1: Magnesium in numbers. It ranks sixth in abundance in the earth's crust and is the lightest structural metal (1.74 g cm

^{- 3}

, about two-thirds the density of aluminium).

2. Extraction: the Dow Process

Magnesium is too strong a reducing agent to be obtained by chemical reduction on a large scale, so, like sodium, it is made by electrolysis of its fused chloride. In the Dow process, sea water is treated with slaked lime (from limestone or sea shells) to precipitate magnesium hydroxide, which is dissolved in hydrochloric acid. The solution is evaporated to $MgC l_{2} \cdot 6 H_{2} O$ , dried to the anhydrous chloride, and electrolysed in the molten state (with NaCl or $CaC l_{2}$ as flux, about 973 K). Liquid magnesium collects at the cathode and floats; chlorine at the anode is turned back into HCl.

M g^{2 +} + Ca (OH)_{2} Mg (OH)_{2} ↓ + C a^{2 +}

Mg (OH)_{2} + 2HCl MgC l_{2} + 2 H_{2} O

MgC l_{2} electrolysis, fused Mg + C l_{2}

Figure 2: The Dow process. Mg, like Na, is too strong a reducing agent to be made by chemical reduction, so its fused chloride is electrolysed. The hydrate must be dried in HCl gas, or it hydrolyses to MgO.

2.1 Why the hydrate must be dried in HCl

Heating $MgC l_{2} \cdot 6 H_{2} O$ in air does not give anhydrous $MgC l_{2}$ . The small, highly charged $M g^{2 +}$ polarises the water molecules bound to it, and the hydrate hydrolyses, first to basic chloride and finally to MgO. Heating it in a stream of dry HCl pushes the hydrolysis equilibrium back, and the anhydrous salt is obtained.

MgC l_{2} \cdot 6 H_{2} O heat in air MgO + 2HCl + 5 H_{2} O

Figure 3: Hydrated

MgC l_{2}

cannot be dried simply by heating; the small

M g^{2 +}

ion hydrolyses water. A stream of dry HCl shifts the equilibrium back (Le Chatelier) and gives anhydrous

MgC l_{2}

Key idea

Magnesium, like sodium, is made by electrolysis of the fused chloride; its hydrated chloride must be dehydrated in HCl gas to stop hydrolysis.

3. Chemical Properties

Figure 4: Everything magnesium does comes from its strong reducing power (

E^{\circ} = - 2.36

V) and its oxide film. Two favourites: very dilute

HN O_{3}

gives

H_{2}

, and alkyl halides give Grignard reagents.

3.1 Burning in air and in carbon dioxide

Magnesium is kinetically inert to oxygen in the cold because of its oxide film, but once ignited it burns with dazzling brilliance in air, giving MgO and some magnesium nitride. Moistened ash gives ammonia, a test for the nitride. Burning magnesium even continues in $C O_{2}$ , taking oxygen from it and leaving black carbon, so a $C O_{2}$ extinguisher must never be used on a magnesium fire (dry sand is used).

2Mg + O_{2} 2MgO

3Mg + N_{2} M g_{3} N_{2}

M g_{3} N_{2} + 6 H_{2} O 3Mg (OH)_{2} + 2N H_{3}

2Mg + C O_{2} 2MgO + C

Figure 5: Magnesium is such a strong reducing agent that it takes oxygen even from

C O_{2}

. Burning in air gives MgO and some

M g_{3} N_{2}

(NCERT 10.17 i).

JEE Advanced

Why magnesium can strip oxygen from $C O_{2}$ , $Si O_{2}$ and $B_{2} O_{3}$ . MgO has one of the most negative enthalpies of formation per mole of O ( $Δ_{f} H^{\circ} = - 601.6$ kJ mol $^{- 1}$ ). With $Δ_{f} H^{\circ}$ of $C O_{2}$ $- 393.5$ , $Si O_{2}$ $- 910.9$ and $B_{2} O_{3}$ $- 1273.5$ kJ mol $^{- 1}$ :

Reaction	$Δ H^{\circ}$ / kJ	Use
$2Mg + C O_{2} \to 2MgO + C$	$2 (- 601.6) - (- 393.5) = - 809.7$	Mg fires cannot be put out with $C O_{2}$
$2Mg + Si O_{2} \to 2MgO + Si$	$2 (- 601.6) - (- 910.9) = - 292.3$	silicon from sand
$3Mg + B_{2} O_{3} \to 3MgO + 2B$	$3 (- 601.6) - (- 1273.5) = - 531.3$	impure boron

The same strength makes magnesium the reducing agent in the Kroll process for titanium, $TiC l_{4} + 2Mg \to Ti + 2MgC l_{2}$ . Only the more electropositive metals (and electrolysis) can in turn reduce $M g^{2 +}$ .

3.2 Water and acids

The oxide film makes magnesium react only very slowly with cold water. With hot water it gives the hydroxide and hydrogen, and with steam it gives the oxide and hydrogen. It dissolves readily in dilute hydrochloric and sulphuric acids, liberating hydrogen. With nitric acid most metals give oxides of nitrogen, because $N O_{3}^{-}$ is reduced instead of $H^{+}$ ; magnesium (and manganese) are the exceptions that give hydrogen with very dilute (about 2%) nitric acid.

Mg + 2 H_{2} O hot water Mg (OH)_{2} + H_{2}

Mg + H_{2} O steam MgO + H_{2}

Mg + 2HN O_{3} very dilute Mg (N O_{3})_{2} + H_{2}

Exam Trick Cold, hot, steam: nothing, hydroxide, oxide. Magnesium with water gives almost nothing in the cold (oxide film),

Mg (OH)_{2}

H_{2}

with hot water, and MgO +

H_{2}

with steam. With nitric acid, only very dilute acid gives

H_{2}

3.3 Grignard reagents

An alkyl or aryl halide reacts with magnesium turnings in dry ether to give an organomagnesium halide, the Grignard reagent. Two ether molecules coordinate to Mg through their oxygen lone pairs, making it tetrahedral. The C-Mg bond is highly polar ( $C^{δ -}$ - $M g^{δ +}$ ), so the reagent behaves as a source of the carbanion $R^{-}$ : water destroys it to the alkane, and $C O_{2}$ or carbonyl compounds build new C-C bonds.

RX + Mg dry ether RMgX

RMgX + H_{2} O RH + Mg (OH) X

Figure 6: Magnesium's link to organic chemistry. The C-Mg bond is polar (

C^{δ -}

), so RMgX acts as a source of

R^{-}

; any water destroys it, so ether must be dry.

3.4 Predicting any magnesium reaction

Figure 7: Three questions settle most magnesium reaction questions: is it an oxide Mg can reduce, is it an acid (watch

HN O_{3}

), or is it water (cold, hot or steam)?

Key idea

Magnesium is a strong reducing agent held back by an oxide film: sluggish in the cold, vigorous once started, and able to take oxygen even from

C O_{2}

Quick Recall: tap to check

What products form when magnesium is burnt in air?

MgO (major) and

M g_{3} N_{2}

(minor); the nitride gives

N H_{3}

with water.

Why can a

C O_{2}

extinguisher not be used on burning magnesium?

2Mg + C O_{2} \to 2MgO + C

is strongly exothermic, so Mg keeps burning.

Why must a Grignard reaction be carried out in dry ether?

Water protonates the carbanion-like R group:

RMgX + H_{2} O \to RH + Mg (OH) X

4. Important Compounds of Magnesium

Figure 8: Six magnesium compounds and their one-line uses. Note the pattern of a small

M g^{2 +}

: hydrated salts, a sparingly soluble weak-base hydroxide and an easily decomposed carbonate.

Magnesium oxide, MgO (magnesia): made by heating $MgC O_{3}$ or $Mg (OH)_{2}$ ; a white, very high-melting (3125 K) basic oxide used as a refractory lining for furnaces.
Magnesium hydroxide: a sparingly soluble weak base; its suspension in water, milk of magnesia, is used as an antacid: $Mg (OH)_{2} + 2HCl \to MgC l_{2} + 2 H_{2} O$ .
Magnesium sulphate heptahydrate (Epsom salt): used as a purgative. $MgS O_{4}$ is soluble because the large hydration enthalpy of $M g^{2 +}$ beats the lattice enthalpy.
Magnesium carbonate: decomposes easily on heating, $MgC O_{3} \to MgO + C O_{2}$ ; basic magnesium carbonate is an ingredient of toothpaste.
Magnesium chloride: deliquescent $MgC l_{2} \cdot 6 H_{2} O$ ; a paste of MgO with $MgC l_{2}$ solution sets hard (Sorel cement).

4.1 Uses of magnesium

Light alloys with Al, Zn, Mn and Sn; Mg-Al alloys in aircraft construction.
Powder and ribbon in flash powders and bulbs, incendiary bombs and signals (the dazzling white light).
Reducing agent in the extraction of Ti (Kroll), B and Si; Grignard reagents in organic synthesis.
Milk of magnesia as an antacid; $MgC O_{3}$ in toothpaste.

MgOBasic, very high melting (3125 K), refractory; from burning Mg or heating

MgC O_{3}

M g_{3} N_{2}

Ionic nitride formed with MgO when Mg burns in air; hydrolysed by water to

Mg (OH)_{2}

and 2

N H_{3}

5. Biological Importance of Magnesium and Calcium

An adult body contains about 25 g of Mg and 1200 g of Ca, compared with only 5 g of iron and 0.06 g of copper. All enzymes that utilise ATP in phosphate transfer require magnesium as the cofactor. The main pigment for the absorption of light in plants, chlorophyll, contains magnesium.

Figure 9: Magnesium's two big biological jobs: the light-absorbing pigment chlorophyll has Mg at its centre, and every ATP-using (phosphate transfer) enzyme needs

M g^{2 +}

(NCERT 10.10).

About 99% of body calcium is present in bones and teeth. Calcium also plays important roles in neuromuscular function, interneuronal transmission, cell membrane integrity and blood coagulation. The calcium concentration in plasma is regulated at about 100 mg L $^{- 1}$ by two hormones, calcitonin and parathyroid hormone. Bone is not an inert, unchanging substance: it is continuously dissolved and redeposited, to the extent of about 400 mg of calcium per day in a human, and all of it passes through the plasma.

Figure 10: Calcium is the most abundant metal in the body (1200 g), 99% of it in bones and teeth; bone is not inert but is dissolved and redeposited at about 400 mg per day (NCERT 10.10).

Exam Trick 'Mg makes it green, Ca makes it firm.' Magnesium: chlorophyll and every ATP enzyme. Calcium: bones and teeth (99%), muscle, nerves and blood clotting, held at about 100 mg L

^{- 1}

by calcitonin (down) and parathyroid hormone (up).

Key idea

Mg is the cofactor of ATP and the centre of chlorophyll; Ca builds bone and carries signals, with its plasma level held constant by two hormones.

Figure 11: Magnesium on one page: a light, strongly reducing metal protected by an oxide film, made by electrolysis, and essential to plants (chlorophyll) and to every cell (ATP).

Quick Recall: tap to check

Which metal ion is present in chlorophyll?

M g^{2 +}

, held by the four nitrogen atoms of the porphyrin ring.

Which two hormones control plasma calcium?

Calcitonin (lowers it) and parathyroid hormone (raises it); the level is kept near 100 mg L

^{- 1}

6. Solved Examples

Solved Example 1

What happens when magnesium is burnt in air? How can one product be identified?

Solution:

$2Mg + O_{2} \to 2MgO$ (major) and $3Mg + N_{2} \to M g_{3} N_{2}$ (minor). On adding water, the nitride gives ammonia, which turns moist red litmus blue: $M g_{3} N_{2} + 6 H_{2} O \to 3Mg (OH)_{2} + 2N H_{3}$ .

Solved Example 2

Calculate

Δ H^{\circ}

for

2Mg + C O_{2} \to 2MgO + C

, given

Δ_{f} H^{\circ}

(MgO)

= - 601.6

and

Δ_{f} H^{\circ} (C O_{2}) = - 393.5

kJ mol

^{- 1}

. What does it say about fighting a magnesium fire?

Solution:

$Δ H^{\circ} = 2 (- 601.6) - (- 393.5) =$ $- 809.7$ kJ.

The reaction is strongly exothermic, so magnesium keeps burning in $C O_{2}$ ; a $C O_{2}$ extinguisher would feed the fire. Dry sand is used instead.

Solved Example 3

Magnesium liberates hydrogen with
(A) concentrated

HN O_{3}

(B) very dilute

HN O_{3}

H_{2} S O_{4}

(D) cold water, rapidly

Solution:

Answer: (B). Very dilute (about 2%) $HN O_{3}$ gives $Mg + 2HN O_{3} \to Mg (N O_{3})_{2} + H_{2}$ . Concentrated $HN O_{3}$ gives oxides of nitrogen, hot concentrated $H_{2} S O_{4}$ gives $S O_{2}$ , and cold water reacts only very slowly because of the oxide film.

Solved Example 4

Which statements about the preparation of anhydrous

MgC l_{2}

are correct? (One or more options.)
(A) Heating

MgC l_{2} \cdot 6 H_{2} O

in air gives anhydrous

MgC l_{2}

.
(B) Heating it in dry HCl gas suppresses hydrolysis.
(C) Hydrolysis happens because

M g^{2 +}

is small and highly charged.
(D) Anhydrous

MgC l_{2}

is needed for the electrolytic extraction of Mg.

Solution:

Answer: (B), (C), (D). In air the hydrate hydrolyses to Mg(OH)Cl and finally MgO with loss of HCl, so (A) is wrong.

Solved Example 5

C H_{3} MgBr

is treated separately with (i) water and (ii) dry

C O_{2}

followed by dilute acid. Identify the organic products.

Solution:

(i) $C H_{3} MgBr + H_{2} O \to C H_{4} + Mg (OH) Br$ : methane.

(ii) $C H_{3} MgBr + C O_{2} \to C H_{3} COOMgBr$ , then $H_{3} O^{+}$ gives ethanoic acid, $C H_{3} COOH$ (one carbon added).

Solved Example 6

What mass of ammonia is obtained when 10.0 g of

M g_{3} N_{2}

is hydrolysed completely? (Mg = 24.3, N = 14.0, H = 1.0)

Solution:

$M (M g_{3} N_{2}) = 3 (24.3) + 2 (14.0) = 100.9$ g mol $^{- 1}$ ; $n = 0.0991$ mol.

1 mol nitride gives 2 mol $N H_{3}$ : $0.198 \times 17.0 =$ 3.37 g.

Solved Example 7

How many grams of

Mg (OH)_{2}

are needed to neutralise 0.100 mol of HCl in the stomach? (Mg = 24.3, O = 16.0, H = 1.0)

Solution:

$Mg (OH)_{2} + 2HCl \to MgC l_{2} + 2 H_{2} O$ : 0.050 mol $Mg (OH)_{2}$ $\times$ 58.3 g mol $^{- 1}$ = 2.92 g.

Practice Questions

What happens when magnesium is burnt in air? (NCERT 10.17 i)Answer: $2Mg + O_{2} \to 2MgO$ and $3Mg + N_{2} \to M g_{3} N_{2}$ , with a dazzling white light.
Explain the significance of sodium, potassium, magnesium and calcium in biological fluids. (NCERT 10.24)Answer: $N a^{+}$ (outside cells) and $K^{+}$ (inside) carry nerve signals and control water and ion balance ( $K^{+}$ also activates enzymes); $M g^{2 +}$ is the cofactor of ATP enzymes and the centre of chlorophyll; $C a^{2 +}$ builds bones and teeth and controls muscle, nerves and clotting.
Write the equation for the reaction of magnesium with steam.Answer: $Mg + H_{2} O \to MgO + H_{2}$ .
What is the role of slaked lime in the Dow process?Answer: It precipitates $M g^{2 +}$ from sea water as $Mg (OH)_{2}$ .
Give the formula and one use each of Epsom salt and milk of magnesia.Answer: $MgS O_{4} \cdot 7 H_{2} O$ , purgative; $Mg (OH)_{2}$ suspension, antacid.
Write the equation for the preparation of silicon from silica using magnesium.Answer: $2Mg + Si O_{2} \to 2MgO + Si$ .
Name the product when $C_{2} H_{5} MgBr$ reacts with water.Answer: Ethane, $C_{2} H_{6}$ (and Mg(OH)Br).

Common Mistakes to Avoid

Watch out

Writing only MgO when Mg burns in air. Some $M g_{3} N_{2}$ always forms; it gives $N H_{3}$ with water.
Using a $C O_{2}$ extinguisher on burning Mg: $2Mg + C O_{2} \to 2MgO + C$ keeps it burning.
Saying Mg reacts rapidly with cold water. The oxide film makes it very slow; hot water or steam is needed.
Writing $H_{2}$ for Mg with ordinary dilute or concentrated $HN O_{3}$ . Only very dilute $HN O_{3}$ gives $H_{2}$ .
Getting anhydrous $MgC l_{2}$ by heating the hydrate in air. It hydrolyses; dry HCl gas is needed.
Writing the product of steam with Mg as $Mg (OH)_{2}$ . Steam gives MgO; hot water gives $Mg (OH)_{2}$ .
Preparing a Grignard reagent in ordinary (moist) ether or alcohol. Any O-H compound destroys RMgX.
Swapping the roles of calcitonin and parathyroid hormone: calcitonin lowers plasma calcium, parathyroid hormone raises it.

Frequently Asked Questions

How is magnesium extracted?

Magnesium is extracted mainly from sea water by the Dow process. Slaked lime precipitates magnesium hydroxide, which is dissolved in hydrochloric acid to give magnesium chloride. The hydrated chloride is dried in hydrogen chloride gas and the fused anhydrous chloride is electrolysed near 973 K, giving liquid magnesium at the cathode and chlorine at the anode.

Why does magnesium burn in carbon dioxide?

Magnesium oxide has a very large negative enthalpy of formation, so magnesium can take oxygen away from carbon dioxide. The reaction of two moles of magnesium with carbon dioxide gives magnesium oxide and carbon and releases about 810 kJ. This is why carbon dioxide extinguishers must not be used on magnesium fires.

Why does magnesium not react with cold water?

Magnesium is a strong reducing agent, but a thin, tightly bound film of magnesium oxide on its surface protects it, so it is kinetically inert to cold water. With hot water it slowly gives magnesium hydroxide and hydrogen, and with steam it gives magnesium oxide and hydrogen.

Why can anhydrous magnesium chloride not be made by heating its hydrate?

The small, doubly charged magnesium ion polarises the water molecules attached to it, so on heating the hydrate hydrolyses, losing hydrogen chloride and forming basic chloride and finally magnesium oxide. Heating in a stream of dry hydrogen chloride pushes this equilibrium back and gives the anhydrous salt.

What is the biological role of magnesium?

Magnesium ions are the cofactor for every enzyme that uses ATP to transfer phosphate groups, and magnesium is the central metal of chlorophyll, the light absorbing pigment of plants. An adult human body contains about 25 g of magnesium.

How is calcium in the blood controlled?

About 99 percent of body calcium is in bones and teeth, but plasma calcium is held near 100 mg per litre by two hormones: calcitonin lowers it and parathyroid hormone raises it. Bone is constantly dissolved and redeposited, exchanging about 400 mg of calcium with the plasma every day.

How is magnesium chemistry tested in JEE Advanced?

JEE Advanced asks the products of magnesium burning in air or carbon dioxide, the hydrolysis of magnesium nitride to ammonia, why very dilute nitric acid gives hydrogen, why hydrated magnesium chloride hydrolyses on heating, and Grignard reagents in organic conversion questions, often as multiple correct statements.

Which magnesium equations should I learn for JEE Advanced?

For JEE Advanced learn magnesium burning in oxygen and nitrogen, nitride hydrolysis giving two moles of ammonia, the reduction of carbon dioxide and silica, reactions with hot water, steam and very dilute nitric acid, the hydrolysis of hydrated magnesium chloride, and the formation and hydrolysis of a Grignard reagent.

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