O Level Chemistry: Metals


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O Level Chemistry

Chap 14: Metals

 

 

Metals and Alloys

 

1.

Metals have a regular arrangement of closely packed positive ions surrounded by “a sea of mobile valence electrons”.  Atoms in a metal are packed tightly in layers and held by strong metallic bonds.

 

 

2.

 

 

 

Properties of metals and the relation to its structure

Physical Property

Relation to structure

Malleable (easily beaten into thin sheets) and ductile (can be drawn into wires without breaking)

Atoms (of the same size) are regularly arranged in layers. They can slide over each other easily when forced is applied.

High density

Metal atoms are closely packed.

High melting and boiling points

Metal atoms are packed tightly in layers by strong metallic bonds. A lot of energy is needed to break the strong metallic bonds.

Good conductors of heat and electricity

In metals, the valence electrons of the atoms are mobile and can move throughout the metal. They allow heat and electricity to be conducted.

 

 

3.

An alloy is a mixture of a metal with one or a few other elements.

·         Bronze = Copper + Tin

·         Brass = Copper + Zinc (Musical Instruments, decorative ornaments)

·         Stainless steel = Iron + Chromium + Nickel + Carbon (Cutlery, surgical instruments)

 

 

4.

Why metals are often used in the form of alloys

a)

Alloys are harder and stronger than pure metals.

Structures of Metals VS Alloys

Metals

Alloys

Pure metals are weak and soft. The atoms (of the same size) are regularly arranged in layers which can slide over each other easily when force is applied. Pure metals are thus soft and malleable.

- When a pure metal is alloyed, a different element is added to the pure metal.

- This disrupts the regular arrangement of atoms in the pure metal by the addition of other atoms of a different size.

- Therefore, it is difficult for the atoms in an alloy to slide over each other.

- This makes alloys harder, stronger and less malleable than pure metals.

 

 

 

 

 

 

b)

While pure metals corrode easily, alloys have a higher resistance to corrosion. E.g.

Copper-nickel alloy is used to make coins instead of pure copper.

 

 

c)

Alloying is used to lower the melting points of metals. E.g. Solder is an alloy of tin and lead and has a lower melting point than its constituent metals, thus can be used to join metals.

 

The Reactivity Series

4.

Chemical Properties of Metals:

·         Form positive ions (cations) by loss of electrons (metals are reducing agents)

·         Form ionic compounds e.g. metal chlorides/oxides

·         Usually (but not always) react with dilute HCl or H2SO4 to produce salt + hydrogen

·         React with oxygen to form basic oxides or amphoteric oxides

 

 

5.

Reactivity Series

·         The reactivity series is a measure of a metal’s tendency to lose electrons and form a positive ion. Metals high up in the reactivity series have a greater tendency to form its positive ions.

·         The order of reactivity can be deduced by reference to

-          the metal’s reaction with water, steam and dilute HCl

-          the reduction of their oxides by carbon and/or hydrogen, or any other form of decomposition.

·         Reactive metals are unstable and tend to react to form compounds (compounds are stable). Unreactive metals are more stable but their compounds tend to be less stable than those of the more reactive metals.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reactivity Series of 13 metals

Reactivity

Element

Extraction

Text Box: Heat oxide with cokeText Box: SteamText Box: Heating or physical extractionText Box: WaterText Box: Dilute HCl/H2SO4Text Box: Electrolysis

 

                         Carbon    Hydrogen     Heat

                         (oxide)      (oxide)        (carb)

 

Potassium

 

x

x

x

Sodium

x

x

x

Calcium

x

x

/

 

Magnesium

x

x

/

 

Aluminium

 

x

x

/

Zinc

 

/

x

/

Iron

 

/

/

/

 

Lead

 

/

/

/

 

Hydrogen

 

 

 

 

Copper

 

/

/

/

Manganese

Heat

Heat

 

Silver

Heat

Heat

/*

Gold

Heat

Heat

 

Platinum

Heat

Heat

 

Extraction method is from the metal oxide for Carbon and Hydrogen, and the metal carbonate for Heating.

 

Refer to TB for details on the reactions of metals. Note:

-          Metals above hydrogen will react with dilute acids to produce hydrogen gas (they are more reactive and hence can displace hydrogen from solutions of acids).

-          Lead appears not to react with dilute HCl/H2SO4 which is an exception to the acid + metal reaction (refer to chap on acids and bases). However, just remember that metals above hydrogen will react with dilute acids.

-          Dilute H2SO4 reacts with metals similarly to HCl, the salt formed is a metal sulfate.

 

-          K to Ca reacts with steam too, but is similar to reacting with water.

-          There is some ambiguity at the borderlines between the groups. Mg is considered not to react with water (although it can) as the reaction is too slow unless the surface layer of oxide protecting the metal is specially removed.

-          Al appears relatively unreactive despite its high position in the reactivity series.

This is because Al is reactive and reacts with oxygen, forming a thin and protective layer of Al2O3 which coats the metal and prevents further reaction. This can be used to prevent corrosion (see Alloys)

 

 

Applications of the Reactivity Series

 

6.

Reduction of Metal Oxides by Reducing Agents

·         The reactivity of metals can be studied by how easily metal oxides decompose. The more reactive a metal is, the more difficult it is to decompose its oxides (reduce the oxide to the metal)

·         Above zinc = not reduced by reducing agents (too stable, use passing electricity).

·         Below copper = oxide will decompose simply by heating without needing a reducing agent.

2Ag2O (s)  4Ag (s) + O2 (g)

 

 

a) with Carbon (Zn & below) [smelting]

·         E.g.     2CuO (s) + C (s)  Cu (s) + CO2 (g)

Cu2+­ ions from copper (II) oxide are reduced to copper; carbon is oxidised to carbon dioxide.

·         Metals below magnesium are often extracted from their ores (industrially) by reduction with carbon. [?!]

 

b) with Hydrogen (Fe & below)

·         metal oxide + hydrogen  metal + steam

·         CuO (s) + H2 (g)  Cu (s) + H2O (g)

 

 

7.

Decomposition of Metal Carbonates

 

·         Compounds of reactive metals are stable and not easily reduced and decomposed by heat (more stable to heat/thermally stable). The more reactive a metal is, the more difficult it is to decompose its compounds (e.g. it takes a lot of heat and time to decompose CaCO3, but CuCO3 decomposes quickly and easily).

·         Carbonates- Ca & below:

metal carbonate  metal oxide + carbon dioxide

·         Silver carbonate: the silver oxide produced further decomposes to form silver + oxygen (unstable)

2Ag2CO3 ® 4Ag + 2CO2 + O2

 

 

8.

Displacement of Metals

 

·         More reactive metals can displace a less reactive metal from its salt solution or oxide. This is because more reactive metals have a greater tendency to form its positive ions. E.g. Mg can displace ZnSO4 but Cu, Pb and Fe cannot.

·         The more reactive a metal is, the more readily it forms compounds; unreactive metals tend to stay uncombined.

 

a) from solution

·         E.g. iron displaces copper ions in CuSO4 to form copper metal and FeSO4

Fe (s) + CuSO4 (aq) ® FeSo4 (aq) + Cu (s)

·         Metal displacement reactions are redox reactions. The more reactive metal is oxidised (by becoming ions and forming compounds) while the less reactive metal is reduced (by changing back to atoms).

E.g.     Fe (s) + Cu2+ (aq) ® Fe2+ (aq) + Cu (s)

Copper ions are reduced to copper atoms; iron is oxidised to iron (II) ions.

 

b) from oxide

·         More reactive metals can reduce the oxide of a less reactive metal.

·         E.g. Thermite reaction- Al displaces Fe from Fe2O3

Fe2O3 (s) + 2Al (s) ® Al2O3 (s) + 2Fe (l)

Al atoms react with O2- ions from Fe2O3 to form Al2O3, Fe2O3 is reduced to molten Fe.

·         Note: Highly exothermic reaction, molten iron (which melted due to heat) is formed. This reaction is used to weld railway lines together.

 

 

9.

Using the Reactivity Series

Refer to TB/242

 

 

 

Extraction of Metals

10.

Metals are found in the ground in rocks called ores. E.g. iron is found in the ore haematite.

An ore is a compound of the metal (oxides, sulphides, chlorides or carbonates) mixed with large amounts of earth and rock.

 

 

11.

Metal

Method of Extraction

K to Al

From ore: Using electricity to decompose the molten metal compounds (electrolysis)

Zn to Pb

From metal oxide: Reduce the metal oxide by heating with coke (smelting)

Cu to Pt

Found free in the ground naturally as uncombined metals / physical extraction

 

·         The more reactive the metal is, the harder it is to extract the metal from its ore. E.g. compounds of reactive metals are stable and difficult to be reduced.

·         Whenever possible, reduction with coke is used. Electrolysis is only used for reactive metals because carbon cannot take oxygen away from the metal oxide, as the bonds in the metal oxide are too strong.

·         Smelting is cheap while electrolysis is expensive.

 

 

Extraction of Iron from Haematite

12.

Refer to TB/244-246

·         The iron that is extracted from the blast furnace is known as cast iron, and is mainly used to produce steel.

 

 

Steel

13.

·         Steel is an alloy of iron with carbon and/or other metals.

·         Different types of steel are made by varying the amount of carbon and by adding different metals to iron. Each type has different amounts of carbon and other metals added, hence its own unique properties and uses.

 

 

14.

·         Carbon steels: Iron + Carbon

·         Alloy steels: Iron + Carbon + One or more of: Mn, Ni, Cr, W, V. These metals are added to change the properties of the steel.

 

Category

Type of Steel

Uses

Special Properties

Carbon Steels

Mild Steel (low carbon)

0.25% Carbon

Car bodies and machinery

Hard, strong and malleable

High Carbon Steel

0.45 – 1.5% Carbon

Cutting and boring tools, e.g. knives, hammers

Strong but brittle (more carbon atoms to prevent sliding)

Alloy Steels

Stainless Steel

Alloy of iron, chromium, nickel & carbon.

Equipments in chemical plants, cutlery, surgical instruments

Extremely durable, resistant to rust and corrosion even when heated

 

 

Qn: Explain how the properties of low carbon and high carbon steel differ.

Low carbon steel is softer as it is more malleable. High carbon steel contains more carbon atoms which prevent sliding of the iron atoms. Hence, high carbon steel is harder but brittle.

 

Rusting

15.

·         Rusting is the oxidation of iron to form hydrated iron (III) oxide.

·         When an object made of iron (e.g. iron, mild steel) is exposed to moist air for some time, a reddish-brown substance (rust) slowly forms on the surface, through rusting/ corrosion of iron.

·         Rusting occurs only in the presence of both oxygen and water. The presence of sodium chloride and acidic substances (e.g. sulphur dioxide, CO2) speeds up the rusting process.

·         A simplified equation is

iron + oxygen + water ® hydrated iron (III) oxide [rust]

4Fe (s) + 3O2 (g) + 2xH2O ® 2Fe2O3.xH2O (s)

·         Note: 

-          Mild steel will rust faster than high carbon steel due to higher iron composition

-          Only iron (and alloys containing iron) will rust. Magnesium corrodes.

 

 

16.

Rust Prevention

Method

Description

Using a protective layer

-          The metal may be coated with a layer of substance that stops water and air from reaching the metal.

-          This is done by painting, greasing, and coating with another metal.

-          E.g. In galvanising (or zinc-plating), a thin film of zinc is used to cover the iron. Note: even if the zinc layer is damaged, the iron will not rust because zinc is more reactive than iron and it will corrode in place of iron (the more reactive metal corrodes preferentially).

Using a sacrificial metal

(sacrificial protection)

-          Involves attaching a more reactive metal (e.g. zinc) to iron. Since zinc is more reactive than iron, it gets corroded in place of the iron because zinc loses its valence electrons more readily. As long as a more reactive metal is present, iron will not rust.

-          E.g. fixing bars of zinc to a ship’s hull prevents the ship’s steel body from rusting, attaching magnesium blocks to underground pipes

Using alloys

-          Stainless steel, which is a rust resistant alloy, can be used. On exposure to air and moisture, a very hard coating of chromium (III) oxide forms on the surface of stainless steel, preventing it from further corrosion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Recycling of metals

17.

·         Metals are finite resources and need to be conserved.

·         The amount of metal ores in the Earth is limited. If metal extraction continues at present rates, the supplies of many metals will run out. Hence, there is a need to recycle metals.

 

 

Advantages

Disadvantages

Economic

-          It saves the costs of extracting new metals from their ores.

-          Fewer landfills to dispose used metal objects will need to be built. This saves the cost of building landfill sites.

-          It is very expensive, considering other processing costs like collecting, transporting and separating the scrap metals.

Social

-          It helps to conserve the limited amount of metals in the Earth.

-          With the increasing world population, more land will be available if ore mining is reduced.

-          It takes time and effort for the world to practise recycling as a way of life.

Environmental

-          Reduces the environmental problems related to extracting metals from ores (e.g. waste gases including carbon monoxide which are formed in the blast furnace through iron extraction will be reduced)

-          The recycling process may cause pollution if not done properly (e.g. metal fumes from the recycling process)

 

 

 

 

Sample Questions

 

 

Q1

Some alloys are coated with a layer of aluminium. Explain how this prevents the alloy from corroding.

Al, being a reactive metal, reacts with oxygen, forming a thin and protective layer of aluminium oxide which coats the alloy and protects it from further corrosion.

 

 

Q2

Explain why magnesium blocks are often attached to underground steel pipes.

 

Magnesium, being more reactive than iron, acts as a sacrificial metal to corrode in place of iron. It provides sacrificial protection against rusting for the steel pipes.

 

 

Q3

A block of iron is coated with copper. The block was accidentally scratched and the iron beneath was exposed.

a)      Explain what would happen to the iron.

b)      Will the copper in contact with the iron corrode faster or slower than normal? Explain your answer.

 

a)      The iron will undergo rusting because it is now exposed to oxygen and water (moisture in air). In addition, the iron will rust faster than normal. This is because iron, being more reactive than copper, acts as a sacrificial metal to corrode in place of iron (when itself already undergoes rusting).

b)      The copper will corrode slower than normal, because iron provides sacrificial protection for the copper against rusting by corroding in place of copper.





The Reactivity Series

Metal

Reactivity

 

Extraction

K

Potassium

 

React with water

 

 

 

 

 

 

 

 

Electrolysis

Na

Sodium

Li

Lithium

Sr

Strontium

Ca

Calcium

Mg

Magnesium

 

Al

Aluminum

Carbon

included for comparison

Zn

Zinc

 

 

 

React with acids

 

 

 

 

 

 

 

 

 

Smelting with

coke

 

Cr

Chromium

Fe

Iron

Cd

Cadmium

Co

Cobalt

Ni

Nickel

Sn

Tin

Pb

Lead

Hydrogen

included for comparison

Cu

Copper

 

Highly unreactive

 

 

 

 

 

Found naturally

uncombined

 

Heat or physical

extraction

Ag

Silver

Hg

Mercury

Au

Gold

Pt

Platinum

 

Going from bottom to top, the metals:

·         increase in reactivity;

·         lose electrons more readily to form positive ions;

·         corrode or tarnish more readily;

·         require more energy (and different methods) to be separated from their ores;

·         become stronger reducing agents.

Notes:

Carbon is able to reduce metal oxides Zn to Pb because it is “higher up” in the reactivity series than them.

 

 

 

 

 

 

200310

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