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Metals and their Properties

 

What is a Metal?

A metal may be defined as an element which ionizes by electron loss to form positive ion. Hydrogen sometimes loses an electron to form a unipositive ion.

However, this does not make hydrogen a metal, which is the only exception to the above definition.

General Properties of Metals

1. They are electron donors.

Hence, they act as reducing agents during chemical reactions.

2. They ionize in solution, producing positive ions.

3. Reactive metals (i.e. metals above hydrogen in the reactivity series) will displace hydrogen from acids, to form salts.

4. They form basic oxides. The water soluble ones form alkalis.

5. Their chlorides are electrovalent.

6. They combine with hydrogen (with difficulty) to form hydrides, which are electrovalent.

7. They are good conductors of heat and electricity.

8. They have high boiling and melting points.

9. They are hard or soft depending on the degree of covalency.

10. They are malleable and ductile.

11. They have lustre.

Principles of Extraction of Metals

Metals generally are found in nature in their ores as: oxides or salts, such as sulphides, chlorides and carbonates.

In these compounds, the metals exist as positive ions, and to obtain them as free elements, it will be necessary to reduce them (that is, for them to acquire electrons).

Note: the process of extraction of metals from their ores is basically a reduction process which requires an electron donor to provide the electrons for the metallic ions.

There are three main methods by which the reduction can be effected . Whatever method applied depends on the stability of the ore:

By Electrolysis:

This method is used to reduce the ores of very reactive metals (e.g. ores of K, Na, Ca, and Mg). These ores are very stable and cannot be reduced by chemical method, but by very powerful oxidation and reduction processes, which electrolysis is about.

Recall that during electrolysis, the metal ions migrate to the cathode where they acquire electrons and become reduced - reduction occurs at the cathode.

A disadvantage of this method is that it is expensive to install and maintain, especially where electric power supply is expensive and inconsistent.

By Chemical Process:

This involves the use of carbon or carbon(II) oxide to reduce the ores (which are mainly oxides) of less reactive metals, such as Pb, Sn, Fe and Zn.

Coke or carbon(II) oxide are used to reduce them and to obtain the metals. In the instance where the ores is a sulphide, it will first be converted to the metallic oxide by roasting, before it is heated in coke or carbon(II) oxide.

By Thermal Reduction:

Some ores are reduced to obtain the metals by the application of heat. Example, when the ore, mercury(II) sulphide is heated in air, mercury is obtained; platinum is also obtained by firstly converting its ore to ammonium hexachloroplatinate(IV) and then applying heat.

Summary of Reactivity of Some Metals; Their Common Ores and Methods of Extracting Them


Reactivity

Potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), and aluminium (Al) are very reactive, with potassium (K) the most reactive while aluminium (Al) the least in that order.

Zinc (Zn), iron (Fe), tin (Sn), and lead (Pb) are moderately reactive, with lead (Pb) the least in that order.

Copper (Cu), mercury (Hg), silver (Ag), and gold (Au) are least reactive, with gold (Au) the least in that order.

Action With Water

K, Na, and Ca
The metals displace hydrogen from cold water to form oxides.

Mg, Al, Zn, and Fe
The metals displace hydrogen from steam at red heat to form oxides.

Sn, Pb, Cu, Hg, Ag, and Au
The metals do not react with either cold water or steam.

Action With Dilute Acids

K, Na, Ca, Mg, Al, Zn, and Fe
The metals displace hydrogen to form salts. However, Al will only react with dilute HCl.

Sn, Pb, Cu, Hg, Ag, and Au
These metals do not react with dilute acids. However, Pb, and Sn will react with dilute HNO3 to form nitrates.

Most Common Ore

K and Na
The metals occur as chlorides.

Ca and Mg
Occur as chlorides and carbonates.

Al
Occurs as oxides.

Zn, Fe, and Sn
Occur as oxides, carbonates, and sulphides.

Pb, Cu, and Hg
Occur as sulphides.

Ag and Au
Occur as free elements.

Method of Extraction

K and Na
The metals are extracted from their ores by electrolysis of fused hydroxides or chlorides.

Ca and Mg
Extracted by the electrolysis of fused chlorides.

Al
Extracted by the electrolysis of oxides.

Zn, Fe, Sn, and Pb
Extracted through roasting of carbonates and sulphides to form oxides, then reduction of oxides by carbon or carbon(II) oxide.

Cu
Extracted through roasting the ore in air.

Hg
Extracted by heating the ore in air.

Ag and Au
Mined as free elements.


  

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