Chemical properties of metals

CHEMICAL PROPERTIES OF METALS

The atoms of the metals have usually 1, 2 or 3 electrons in their outermost shells. These outermost electrons are loosely held by their nuclei. Therefore, the metal atoms can easily lose their outermost electrons to form positively charged ions. For example, sodium metal can lose one electron from its outermost shell to form positively charged ion, Na+. After losing the outermost electron, it gets stable electronic configuration of the noble gas (Ne : 2, 8). Similarly, magnesium can lose two electrons from its outermost shell to form Mg2+ ion and aluminium can lose its three outermost electrons to form AI3+ ion.

Na Na+ + e-

(2, 8, 1) (2,8)

Mg Mg2+ + 2e-

(2, 8, 2) (2, 8)

Al Al3+ + 3e-

(2, 8, 3) (2, 8)

Since the metal atoms lose electrons and form positively charged ions, therefore, the metals are called electropositive elements.

Some of the important chemical properties of metals are discussed below :

REACTION WITH AIR:

Oxygen : Metals react with oxygen to form oxides.These oxides are basic in nature. When these oxides are dissolved in water, they give alkaline solutions. For example, sodium metal reacts with oxygen of the air and form sodium oxide.

4 Na + O2 2 Na2O 

Sodium oxide Sodium oxide reacts with water to form, an alkali called sodium hydroxide. Therefore, sodium oxide is a basic oxide.

Na2O + H2O 2NaOH 

Sodium  hydroxide 

Due to the formation of sodium hydroxide (which is an alkali), the solution of sodium oxide in water turns red litmus blue (common property of all alkaline solutions).

Similarly, magnesium is a metal and it reacts on heating with oxygen, catches fire and burns with a brilliant white flame to form magnesium oxide.

2 Mg + O 2 MgO 

Magnesium oxide

The magnesium oxide reacts with acids (say sulphuric acid) to form magnesium sulphate (a salt) and water as the only products. Thus, magnesium oxide is a basic oxide in nature.

MgO + H2SO4 (dil.) MgSO4 + H2O

Magnesium oxide Sulphuric acid Magnesium sulphate Water

Metals act as reducing agents-i.e they donate electrons.

Test for basic oxides is that they turn red litmus blue. 

REACTION WITH WATER:

Different metals behave differently in their chemical reaction with water.

(i) Metals like sodium and potassium react with water as follows -

When a small piece of sodium or potassium is dropped in cold water -

• (A) It floats and at the same time melts to form a silvery ball of metal.
• (B) The silvery ball of the metal darts over the surface of water with a hissing noise producing tiny bubbles of the hydrogen gas.
• (C) The silvery ball becomes smaller in size and catches fire.

In case of sodium, it burns with a golden yellow flame, whereas in case of potassium, it burns with a lilac flame.

2Na + 2H2O 2NaOH + H2

Sodium Cold water Sodium hydraoxide Hydrogen

2K + 2H2O 2KOH + H2

Potassium Water Potassium hydroxide Hydrogen

When calcium metal is dropped in water, the following observations are made :

•  It sinks in water and reacts vigourously to liberate tiny bubbles of hydrogen. 
•  It gradually dissolves in water and the colour of water becomes milky due to the formation of partially soluble calcium hydroxide.

Ca + 2H2O Ca(OH)2 + H2

Calcium Water Calcium hydroxide Hydrogen

(ii) Magensium reacts mildly with cold water, but reacts vigorously with boiling water or steam so as to form magensium hydroxide and hydrogen gas.

Mg + 2H2O Mg(OH)2 + H2

Magnesium boiling water Magnesium hydroxide Hydrogen 

(iii) Hot zinc and iron, mildly react with steam to form their respective oxides and hydrogen gas.

Zn + H2O ZnO + H2

Zinc Water Zinc oxide Hydrogen

3 Fe + 4H2O Fe3O4 + 4 H2

Iron Steam ← Ferro-ferric oxide Hydrogen

Or iron (II, III) oxide

REDUCTION WITH DILUTE ACIDS (DISPLACEMENT REACTIONS):

Sulphuric acid, hydrochloric acid and nitric acid are called mineral acids. These acids in dilute form react with metals to form their respective salts and hydrogen gas. Many metals react with dilute acids and liberate hydrogen gas. Only less reactive metals such as copper, silver, gold etc. do not liberate hydrogen from dilute acids. The reactions of metals with dilute hydrochloric acid (HCI) and dilute sulphuric acid (H2SO4) are similar. With dil. HCI, they give metal chlorides and hydrogen, whereas with dil. H2SO4, they give metal sulphates and hydrogen.

For example :

(i) Sodium, magnesium and calcium. react violently with dilute hydrochloric acid (HCI) or dilute sulphuric acid liberating hydrogen gas and corresponding metal salt.

2Na (s) + 2HCl (aq) 2NaCl (aq) + H2 (g)

Sodium Sodium Chloride

2Na (s) + H2SO4 (aq) Na2SO4 (aq) + H2 (g)

Sodium Sodium sulphate

Similarly,

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

Magnesium Magnesium chloride

Mg (s) + H2SO4 (aq) MgSO4 (aq) + H2

Magnesium Sulphate

Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2

Zinc Zinc Chloride

Zn (s) + H2SO4 (aq) ZnSO4 (aq) + H2

Zinc Sulphate

Similarly,

2Al (s) + 6HCl (aq) 2AlCl3 (aq) + 3H2

Aluminium Aluminium chloride

Al (s) + 3H2SO4 (aq) Al2(SO)3 (aq) + 3H2 (g)

Aluminium Aluminium Sulphate

(ii) Iron reacts slowly with dilute HCI or dil. H2SO4 and therefore, it is less reactive than zinc and aluminium.

Fe(s) + 2HCI(aq) FeCI2(aq) + H2(g)

Iron           Ferrous chloride

Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g) 

Ferrous sulphate

Copper does not react with dil. HCl or dil H2SO4 .

Cu(s) + HCl (aq) No reaction Cu(s) + H2SO4(aq) No reaction 

Therefore copper is even less reactive than iron.

Dilute nitric acid (HNO3) is an oxidising agent. Although it oxidises metals, but does not produce hydrogen gas.

(d) Reactions of Metals with Bases : Some metals react with alkalies -

2Al +        2NaOH + 2H2O      2NaAlO2    + 3H2

Aluminium   Sodium   Water         Sodium Hydrogen

          Hydroxide meta aluminate

Zn     + 2NaOH Na2ZnO2 + H2

  Sodium Sodium Hydrogen

Hydroxide zincate

Sn + 2NaOH  + H2O Na2SnO3 + 2H2

Sodium Water Sodium Hydrogen

Hydroxidestannate

REACTIONS OF METALS WITH SALT SOLUTIONS :

When a more reactive metal is placed in a salt solution of less reactive metal, then the more reactive metal displaces the less reactive metal from its salt solution.

e.g. When a strip of zinc metal is put in the blue colour solution of copper sulphate, it is observed that the blue colour of the solution fades gradually and copper metal is deposited on the zinc strip. This means that the following reaction occurs: 

Zn(s) + CuSO4 (aq) ZnSO4(aq) + Cu(s)

Copper sulphate Zinc sulphate

(Blue solution) (Colourless solution)

In other words, we can say that zinc displaces copper from its solution.

However, if we take zinc sulphate solution and put a strip of copper metal in this solution, no reaction occurs.

ZnSO4 (aq) + Cu(s) No reaction 

Zinc sulphate

This means that copper cannot displace zinc metal from its solution. Thus, we can conclude that zinc is more reactive than copper. However, if we put gold or platinum strip in the copper sulphate solution, then copper is not displaced by gold or platinum. Thus, gold and platinum are less reactive than copper.