CHEMICAL PROPERTIES OF METALS
Reaction of metals with oxygen : Almost all metals combine with oxygen to form metal oxides.
Metal + Oxygen → Metal oxide
But all metals do not react with oxygen at the same rate. They show different reactivity towards oxygen. Such as sodium and potassium react so vigorously that they catch fire if kept in the open. Hence, they are kept in kerosene for protection.
4Na(s) + O2(g) → 2Na2O
4K(s) + O2(g) → 2K2O
Sodium and potassium react with oxygen of air, burns with a golden yellow colour and form sodium and potassium oxides, which dissolve with water to form alkali called sodium hydroxide and potassium hydroxide. These turn red litmus blue.
Na2O(s) + H2O (ℓ) → 2NaOH(aq)
K2O(s) + H2O(ℓ) → 2KOH(aq)
Zinc react with oxygen only when strongly heated to form zinc oxide,
2Zn(s) + O2(g) → 2ZnO
Since it is less reactive than sodium and potassium.
Copper does not react with oxygen even on strong heating. It react very slowly on prolonged heating to form cupric oxide (black oxide). Reaction is given as:
2Cu(s) + O2(g) → 2CuO(s)
Similarly, Aluminium forms aluminium oxide.
4Al(s) + 3O2(g) → 2Al2O3.
|The order of reactivity of these metals towards oxygen is as follows:|
Na > K > Mg > Zn > Fe > Cu
At ordinary temperature, the surface of metals like magnesium (Mg), Aluminium (Al), Copper(Cu), Zinc(Zn), lead (Pb) etc. are covered with a thin layer of oxide. The protective oxide layer prevents the metals from further oxidation. Silver and gold does not react with oxygen due to this silver and gold are called noble metals.
DO YOU KNOW ?
Anodising is a process of forming a thick oxide layer of aluminium. Aluminium develops a thin oxide layer when it is exposed to air. This oxide coat of aluminium (Al) makes it resistant to further corrosion. During anodising, the resistance can be improved further by making the oxide layer thicker. In this process, a clean Al article is made the anode and dilute sulphuric acid (H2SO4) is used for electrolyte. The oxygen gas evolved at the anode react with Al to make a thicker protective oxide layer. This oxide layer can be dyed easily to give Al articles an attractive finishing.
NATURE OF METALLIC OXIDE
Generally, metallic oxides are basic in nature except aluminium and zinc oxides which are amphoteric in nature. That means these oxides (Al2O3, ZnO) react with base as well as acid. The basic oxide of metals react with acid to give salt.
For example :
Some oxide of metals dissolve in water and form alkalis.
Example for :
Reaction showing amphoteric in nature of Al2O3 and ZnO.
REACTION OF METALS WITH WATER
Metal react with water and produce a metal oxides and hydrogen gas. Metal oxides that are soluble in water dissolve in it to form metal hydroxide. But all metals do not react with water.
Metal + Water → Metal oxide + Hydrogen gas
Metal oxide + Water → Metal hydroxide
The following activity clears this reaction properly.
Activity : 1.7 :
Procedure : Take the samples of common metals such as copper, iron, calcium, aluminium, sodium and potassium. Put small amount of the samples separately in beakers which are half-filled with water.
Aim : To study about the reactivity of metals with water & zinc.
Observation : Sodium and Potassium calcium react with cold water. Sodium and Potassium react violently with cold water, making a hissing sound. These reactions are exothermic and evolved hydrogen gas which catches fire and leads to little explosion.
Calcium react less violently and the piece of calcium starts floating in water because the bubbles of hydrogen gas formed stick to the surface of the metal. Now put the metals which do not react with cold water in beaker which is half filled with hot water. Magnesium reacts rapidly with hot water. It also starts floating due to the bubbles of hydrogen sticking to its surface. [The rest samples of metals which do not react with cold and hot water, are made to react with steam].
To study its reactivity with steam, to set up an experiment as shown in figure as below.
A lump of wet glass wool is placed at the bottom of a boiling tube. Then place the metals samples in the middle of the horizontally kept boiling tube one by one. Now start at the bottom of a boiling tube with a help of burner. The water present in glass wool forms steam on heating. This steam then pass over the metal. Observe that aluminium react with steam and iron does not react under ordinary conditions but it reacts only when steam is passed over hot iron. Copper do not react with water either at ordinary temperature or at higher temperature. The following reaction occur from the above activity.
(i) Na and K metals react vigorously with cold water to form NaOH and H2 gas is liberated.
This reaction is so violent and exothermic that the H2 gas evolved catches fire.
(iii) Calcium reacts with cold water to form Ca(OH)2 and H2 gas. It is less violent.
(iv) Aluminium does not react either with cold or hot water. But it react only with steam to form aluminium oxide and hydrogen gas.
(v) Similarly, Zinc react with steam to form zinc oxide and H2 gas.
(vi) Magnesium react with hot boiling water to form MgO and H2 gas.
(vii) Iron metal react when steam is passed over red hot iron and form iron oxide and H2 gas.
(viii)Copper do not react with even under strong conditions. Similarly, gold and silver also do not react with water. The above reactions indicates that sodium and potassium are the most reactive metals while copper is least reactive.
|Result : The reactivity order of these metals with water are|
K > Na > Ca > Mg > Al > Zn >Fe > Cu
Reactivity with water decreases.
REACTION OF METALS WITH ACIDS
The highly reactive metals react with dilute acid to displace hydrogen from acid. The reactivity of different metals is different with same acid.
AQUA REGIA (ROYAL WATER)
Aqua regia is a Latin word it means " royal water". It is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3 : 1. It is a highly corrosive, fuming liquid and it used for dissolving gold and platinium.
REACTION OF METALS WITH SOLUTIONS OF OTHER METAL SALTS
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 . This reaction is also known as displacement reaction.
Aim : To compare the reactivity of the metals.
Procedure : Take a clean wire of copper and an iron nail, two test tube. Now dissolve copper sulphate in water in one test tube and ferrous sulphate in another test tube. Place iron nail in the blue coloured copper sulphate solution with the help of a thread and copper wire in the greenish colour ferrous sulphate solution as shown in figure as below.
Observation : The blue colour of copper sulphate has faded and becomes greenish. The green colour of the solution is due to the formation of iron (II) sulphate and copper is displaced. A reddish-brown coating is formed on the surface of iron nail. The reaction is represented by the chemical equation.
But the greenish colour of FeSO4 does not change. That means no reaction take place.
Conclusion : These activities show that iron metal is more reactive than copper.
Similarly, Reaction of copper with silver nitrate solution :
When a strip of copper metal is placed in a solution of AgNO3. The solution becomes gradually blue and a shining coating of silver metal gets deposited on the copper strip. The reaction may be written as :
However, if we place silver wire in a copper sulphate solution no reaction occurs. This means copper can displace silver from its salt solution but silver cannot displace copper from its solution. i.e. copper is more reactive metal than silver.