The reactivity series , Corrosion

 

The reactivity series :--

The reactivity series is a list of metals arranged in the order of their decreasing activities.

The reactivity series is also called as activity series.

Reactivity series or activity series is an empirical calculated and structurally analytical progression of a series of a metals which are arranged by their reactivity from highest to lowest.

 

The following series known as reactivity series are given below :--

Potassium       K                                                        most reactive

Sodium            Na

Calcium           Ca

Magnesium     Mg                                                   (Reactivity decreases from top to bottom)

Aluminium      Al

Zinc                  Zn

Iron                 Fe

Lead                Pb

Hydrogen       [H]

Copper           Cu

Mercury         Hg

Silver              Ag

Gold                Au                                                        least reactive

 

Properties of Ionic compounds:--

 

Physical nature :--

                             Ionic compound are solids and hard due to strong force of attraction between the positive and negative ions.

They are generally brittle. And can be broken into pieces when the pressure is applied.

Melting and boiling point:--

                                   Ionic compound have high melting point and high boiling point. This is due to the large amount of energy is required to break the strong ionic bonds.

 

Solubility:--

                    Ionic compounds are generally soluble in water. Ionic compounds are insoluble in solvents such as kerosene, petrol.

Conduction of electricity :--

                                           The conduction of the electric current through a chemical solution involves the movement of the charged particles.

When the electric current is passed through the solution, a solution of an ionic compound in water contains ions, which move to the opposite electrodes.

Ionic compound do not conduct electric current in solid state because movement of the ions is not possible due to their rigid structure.

But in molten state, ionic compound conduct electric current because in the molten state, the electrostatic forces of attraction between oppositely charged ions overcome due to the heat. Thus, the ions move freely and conduct electric current.

 

 

Corrosion:--

             Corrosion is a process in which a refined metal is naturally converted in to a more stable form such as its oxide, hydroxide, carbonate and sulphate state.

It is destruction of the materials or metals by chemical reaction with their environment.

 

Examples :--

When Silver articles exposed to air, it becomes black after sometime. This is because when silver articles react with the sulphur in the air to form a coating of silver sulphide.

 

Carbon dioxide is present in the air. Copper reacts with the carbon dioxide and slowly loses its shiny brown surface and gains a green coat. This green substance is basic copper carbonate.

Iron when exposed to moist air for a long time acquires a coating of a brown flaky substances which is called rust.

 

Prevention of corrosion

 

There are various method to slow corrosion damage to metallic objects which are exposed to the salt water, weather, acids.

The rusting of iron can be prevented by painting, oiling, greasing, galvanising, chrome painting, anodising or making alloys.

Galvanisation is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc. The galvanised article is protected against rusting even if the zinc coating is broken.

The most common method of galvanisation is to hot dip the metal in a bath of molten zinc.

Here, zinc is more reactive metal than the iron. So when they react with the oxygen, it forms a protective oxide layer. These oxide layer prevent inner iron from getting in contact with oxygen.

Alloying is a good method of improving the properties of metal.

For example :--

The most widely used metal are Iron. But we can not used these metal in pure state. So that, when pure iron is in hot state, it can be easily stretched. And pure iron is very soft in hot condition.

When iron is mixed with the nickel and chromium, we get stainless steel. Stainless steel is very hard and does not rust.

Similarly, when iron mixes with the other substances, its properties also changes. The substances added  may be metal or non- metal.

 

Alloy :--

            An alloy is a homogeneous mixture of two or more metals or a metal and non-metal.

Alloy is prepared by first melting the primary metal and then dissolving the other elements in it with definite proportions.

Then it is kept for cooling at room temperature.

 

The electrical conductivity of an alloy is less than pure metal.

The melting point of an alloy is less than pure metal.

An alloy of mercury with another metal is known as amalgam.

For example:--

                    

Brass is an alloy of copper (Cu) and zinc (Zn).

Bronze is an alloy of copper (Cu) and tin (Sn). But bronze is not good conductors of electricity.

Solder is an alloy of lead (Pb) and tin (Sn). Solder has a low melting point.

Solder is used for welding wires.

24 carat gold is also known as pure gold. Pure gold is very soft. So it is not suitable for making jewellery. So pure gold is mixed with silver or copper to make it hard.

In India,  22 carat gold is used for making ornaments. 22 carat means 22 parts of pure gold is alloyed with 2 parts of copper or silver.