Friday, May 20, 2011

Class X Chemical properties of Metal and Nonmetal

Metals are Electropositive Elements
Metals are very reactive. Metals tend to loose electrons easily and form positively charged ions; therefore metals are called electropositive elements. Sodium metal forms sodium ions Na+, Mg forms positively charged Magnesium ions Mg2+and aluminium forms aluminium ions Al3+. The electropositive nature allows metals to form compounds with other elements easily.
Na    -----------à       Na+ + e-
(2, 8, 1)                 ( 2, 8)
Mg ---------------àMg2+ + 2e-
(2, 8, 2)             (2, 8)
Al   --------------- Al3+ + 3e-
(2, 8, 3)            (2, 8)
Reaction of Metals with Oxygen
Metals like sodium (Na) and potassium (K) are some of the most reactive metals. Potassium, sodium, lithium, calcium and magnesium react with oxygen and burn in air.
Metals from aluminium to copper in the activity series of metals, react slowly when heated in air to form the metal oxides. Aluminium is the fastest and copper is the slowest of them.
Sodium metal reacts with the oxygen of the air at room temperature to form sodium oxide. Hence, sodium is stored under kerosene to prevent its reaction with oxygen, moisture and carbon dioxide.

Sodium oxide is a basic oxide which reacts with water to form sodium hydroxide.

Mg does not react with oxygen at room temperature. On heating, Mg burns in air with intense light and heat to form MgO.
2Mg (s) + O2 (g) ---Heat------à 2 MgO(s)
Zinc metal burns in air only on strong heating to form zinc oxide.
2Zn (s) + O2 (g) ---Heat------à 2 ZnO(s)
Iron metal does not burn in dry air even on strong heating. In moist air, iron is oxidized to give rust.

On heating in air it burns with a brilliant flame forming triferric tetroxide

Copper is the least reactive metal and does not burn in air even on heating. However, on prolonged strong heating copper reacts with oxygen and forms copper (II) oxide (CuO) outside and copper (I) oxide (Cu2O) inside.

Gold and platinum do not react with oxygen in air.
Reaction of Metals with Water
Potassium, sodium, lithium and calcium react with cold water.
Sodium reacts vigorously with cold water forming sodium hydroxide and hydrogen.

Metals from magnesium to iron in the activity series of metals, react with steam (but not H2O) to form the metal oxide and hydrogen gas.

Red hot iron reacts with steam to form Iron (II, III) oxide.

Note: The reaction between iron and steam is irreversible. Tin, lead, copper, silver, gold and platinum do not react with water or steam.
Reaction of Metals with Acids
Potassium, sodium, lithium and calcium react violently with dilute H2SO4 and dilute HCl, forming the metal salt (either sulphate or chloride) and hydrogen gas. The reaction is similar to the reaction with water.

Magnesium, aluminium, zinc, iron, tin and lead react safely with dilute acid. Magnesium is the fastest and lead is the slowest of the six.

Zinc with dilute sulphuric acid is often used for the laboratory preparation of hydrogen. The reaction is slow at room temperature, but its rate can be increased by the addition of a little copper (II) sulphate. Zinc displaces copper metal, which acts as a catalyst.
Metals below hydrogen (copper, silver, gold and platinum), will not react with dilute acid. They cannot displace hydrogen from the non-metal anion.
Note:  Copper reacts with oxyacids like nitric acid and sulphuric acid because these acids are strong oxidizing agents.
In general,  Hydrochloric acid makes a metal chloride.   
Sulphuric acid makes a metal sulphate.
Reactions with nitric acid are more complex, the nitrate is formed but the gas is rarely hydrogen, and more often, an oxide of nitrogen.
Reaction of Metals with Salt Solutions
Reactive metals can displace any metal less reactive than itself, from the oxide, chloride or sulphate of the less reactive metal in solution or their molten state. If metal A displaces metal B from its solution, it is more reactive than B.
Copper (II) sulphate solution is blue; iron sulphate solution is almost colourless when dilute. 
During the displacement, the blue solution loses its colour and the iron metal is seen to turn pink-brown as the displaced copper becomes deposited on it.
On heating the mixture of magnesium powder and black copper (II) oxide, white magnesium oxide is formed with brown bits of copper

Adding magnesium to blue copper (II) sulphate solution, the blue colour fades as colourless magnesium sulphate is formed and brown bits of copper metal form a precipitate

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