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Reactions of Metals

Dr. Kendrew K. W. MakDepartment of Chemistry

The Chinese University of Hong Kong

IJSO Training (Phase 3)

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Structure and Bonding (鍵合 ) in Metals

Metal atoms are bonded to one another in a Giant Metallic Structure (巨型金屬結構 ) by Metallic Bonds (金屬鍵 ), which result from the attraction between a “sea” of delocalized electrons (離域電子 ) and metal ions.

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Conduction of Electricity by Metals

The delocalized electrons move towards the positive pole ( 正極 ) of the battery, leaving the metal.

At the same time, an equal number of electrons move into the other end of the metal from the negative pole ( 負極 ).

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• Good conductors of electricity and heat – electrons are delocalized

• Solids with high melting points ( 熔點 ) and boiling points ( 沸點 ) – strong giant lattice structure ( 巨型晶格結構 )

• Have high densities ( 密度 ) – close packing of atoms

• Malleable (can be rolled into sheets) and ductile (can be pulled out into wire) – metal atoms are held together by the non-directional metallic bonds

Properties of Metals Explained by Structure and Bonding

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Comparing Reactivity (活潑性 ) of Common Metals

How to compare:

1. The lowest temperature at which the reaction can be initiated.

2. The rate of reaction.

3. The amount of heat energy given out from the reaction.

For a fair comparison of reactivity:

1. Should be allowed to react with the same substance

2. Should be roughly in the same state of subdivision (e.g. all in large lumps or all in fine powder)

3. Should react under the same conditions of temperature and pressure.

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Reactions of Metals with Air

Different metals react with air at very different rates. Reactive metals like potassium ( 鉀 ) burn vigorously in air with characteristic flame and form metal oxides by combining with oxygen and form metal oxides. Some less reactive metals do not burn in air. They only react moderately even when are heated strongly. Some metals are very inert ( 惰性 ) and do not react with air/oxygen at all even under very strong heating. The trend of reactivity gives the Metal Reactivity Series (金屬活性序 ).

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Potassium 鉀(K)

They are ignited easily by gentle heating. They burn vigorously and give metal oxides (金屬氧化物 ).

Sodium 鈉 (Na)

Calcium 鈣 (Ca) Strong heating is required to ignite the metal. It burns vigorously.

Magnesium 鎂(Mg)

Strong heating is required to ignite the metals. They burn with dazzling (耀眼 ) white flame.

Aluminium 鋁(Al)

Zinc 鋅 (Zn) It burns moderately under strong heating.

Iron 鐵 (Fe) It does not burn but gives showery sparks under strong heating.

Lead 鉛 (Pb) They do not burn in air. They oxidize into oxides (氧化物 ) under strong heating.Copper 銅 (Cu)

Mercury 汞 (Hg) It does not burn in air but oxidizes under heating. Strong heating can decompose the metal oxide back into the metal.

Silver 銀 (Ag) They do not react with air under strong heating.

Gold 金 (Au)

Reactions of Metals with Air

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Reactions of Metals with Air

Burn in Air: Potassium 鉀 (K) 4 K(s) + O2(g) 2 K2O(s)

Sodium 鈉 (Na) 4 Na(s) + O2(g) 2 Na2O(s)Calcium 鈣 (Ca) 2 Ca(s) + O2(g) 2

CaO(s)Magnesium 鎂 (Mg) 2 Mg(s) + O2(g) 2 MgO(s)Aluminium 鋁 (Al) 4 Al(s) + 3 O2(g) 2

Al2O3(s)Zinc 鋅 (Zn) 2 Zn(s) + O2(g) 2 ZnO(s)Iron 鐵 (Fe) 3 Fe(s) + 2 O2(g) Fe3O4(s)

No Not Burn: Lead 鉛 (Pb) 2 Pb(s) + O2(g) 2 PbO(s)Copper 銅 (Cu) 2 Cu(s) + O2(g) 2 CuO(s)Mercury 汞 (Hg) 2 Hg(s) + O2(g) 2

HgO(s)

No Reaction: Silver 銀 (Ag)Gold 金 (Au)

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Appearance of Metals and Storage Methods

The shiny surface of very reactive metals (potassium, sodium) soon becomes dull when exposed to air.

The metal reacts with the oxygen in air to give the metal oxide.

Sodium and potassium are often protected from oxygen and moisture by storing under paraffin oil ( 石蠟油 ).

Calcium are usually stored in airtight containers.

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Reactions of Metals with Water

Alkali metals ( 鹼金屬 ) are so reactive that they react vigorously and exothermically ( 放熱 ) with water. In some cases the heat generated is sufficient to ignite the metal. In general the reactions of metals with water produce hydrogen gas (H2) and the hydroxides ( 氫氧化物 ) of the metals. Metals with moderate to low reactivity react very slowly with water. However, they react moderately with steam when heated and give hydrogen gas and metal oxides. Metals low in the metal reactivity series do not react with water even at high temperature.

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Reactions of Metals with Water

Metal Reactivity (活潑性 )

Potassium (K)

They react vigorously with cold water. The heat generated from the reaction can ignite the metals.

Sodium (Na)

Calcium (Ca)

It reacts slowly with cold water.

Magnesium (Mg)

It reacts very slowly with hot water. It reacts with steam when being heated and gives hydrogen gas and metal oxide.

Aluminium (Al)

They do not react with cold or hot water. They react with steam when being heated and give hydrogen gas and metal oxides.(Iron oxidizes to hydrated iron oxide (水合氧化鐵 ) (rust) when oxygen is present in the water.)

Zinc (Zn)

Iron (Fe)

Lead (Pb) They do not react with water or steam.

Copper (Cu)

Mercury (Hg)

Silver (Ag)

Gold (Au)

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Actions of Potassium, Sodium and Calcium on Water

Action of potassium on water

Potassium melts to a silvery ball, moves quickly on the water surface with a hissing sound, and burns with a lilac ( 淡紫色 ) flame.

Potassium + Water Potassium hydroxide solution ( 氫氧化鉀溶液 ) + Hydrogen gas

The resulting alkaline solution will turn red litmus ( 石蕊試紙 ) blue because of the potassium hydroxide formed.

Action of sodium on water

The reaction of sodium with water is similar to that of potassium, except it is less vigorous.

Sodium + Water Sodium hydroxide solution ( 氫氧化鈉溶液 ) + Hydrogen gas

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Actions of Potassium, Sodium and Calcium on Water

Action of calcium on water

Calcium reacts moderately with water, forming hydrogen gas and the slightly soluble calcium hydroxide ( 氫氧化鈣 ) (milky suspension).Calcium + Water Calcium hydroxide + Hydrogen gas

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Actions of Magnesium, Aluminium, Zinc and Iron on Water

Action of magnesium on water

Magnesium has almost no reaction with cold water. It reacts slowly with hot water to give the slightly soluble magnesium hydroxide ( 氫氧化鎂 ) and hydrogen.

Magnesium + Water Magnesium hydroxide + Hydrogen gas

Action of magnesium on steam

Magnesium reacts with steam vigorously when heated, and produce an intense white light

Magnesium + Steam Magnesium oxide ( 氧化鎂 ) + Hydrogen gas

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Actions of Magnesium, Aluminium, Zinc and Iron on Water

Action of zinc and iron on water/steam

Zinc and iron do not react with cold or hot water. They react with steam slowly to give the respective oxides

Zinc + Steam Zinc oxide ( 氧化鋅 ) + Hydrogen gas

Action of aluminium on steam

Aluminium apparently does not react with steam. It is because aluminium metal is usually coated with a very inert protective layer of Al2O3.

Iron + Steam Iron(II)iron(III) oxide ( 氧化鐵 (III) 鐵 (II)) + Hydrogen gas

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Reactions of Metals with Dilute Hydrochloric Acid(稀氫氯酸 )

Metal Reactivity

Potassium (K)

They react explosively with dilute hydrochloric acid.

Sodium (Na)

Calcium (Ca)

It reacts readily with dilute hydrochloric acid.

Magnesium (Mg)

It reacts moderately with dilute hydrochloric acid.

Aluminium (Al)

They react slowly with dilute hydrochloric acid.

Zinc (Zn)

Iron (Fe)

Lead (Pb) It reacts very slowly with dilute hydrochloric acid.

Copper (Cu) They do not react with dilute hydrochloric acid.

Mercury (Hg)

Silver (Ag)

Gold (Au)

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The Metal Reactivity Series ( 金屬活性序 )

By comparing their reactions with air, water and dilute hydrochloric acid, we can arrange common metals in order of reactivity.

Potassium, KSodium, NaCalcium, CaMagnesium, AlZinc, ZnIron, Fe

Lead, PbCopper, CuMercury, HgSilver, AgGold, Au

No reaction with water/steam

Most reactive

Least reactive

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Applications of the Reactivity Series

Reduction of Metal Oxides

A metal lower in the reactivity series has compounds with lower stability ( 穩定性 ), so its oxide can be more easily reduced ( 還原 ).

1. Heating the metal oxide alone.2 HgO(s) 2 Hg(l) + O2(g)2 Ag2O(s) 4 Ag(s) + O2(g)

2. Reducing the metal oxide by another metal.CuO(s) + Mg(s) Cu(s) + MgO(s)Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(s) + heat

3. Reducing the metal oxide by carbon.2 PbO(s) + C(s) 2 Pb(s) + CO2(g)2 CuO(s) + C(s) 2 Cu(s) + CO2(g)