Metallurgy Science Notes
Physical properties of metals :
Physical properties of nonmetals :
Properties of metals :
- Solid state (Exception : Mercury and gallium)
- Typical lustre
- Malleability and ductility
- Hardness (Exception : Lithium, sodium and potassium)
- Good conductors of heat and electricity
- High melting and boiling points (On the other hand, the melting and boiling points of the metals sodium, potassium, mercury and galium are very low.)
- Sonorous and produce sound on striking a hard surface.
Properties of nonmetals:
- Gaseous or solid state (Exception : Bromine in liquid state)
- Lack of any typical lustre (Exception : lodine and Diamond)
- Brittleness in the solid state (Exception : Diamond is the hardest natural substance)
- Bad conductors of heat and electricity (Exception: Graphite) (Diamond is good conductor of heat)
- Low melting and boiling points.
Chemical properties of metals :
→ Reaction of metals with oxygen : Metals combine with oxygen on heating in air and metal oxides are formed. Sodium and potassium are highly reactive metals. Sodium metal combines with oxygen in the air at room temperature to form sodium oxide.
4Na(s) + O2(g) → 2Na2O(s)
→ Sodium readily catches fire on keeping exposed to air producing a lot of heat. Therefore, to prevent accident in the laboratory or elsewhere it is kept in kerosene.
→ Sodium oxide reacts with water to form sodium hydroxide (alkali)
Na2 O(s) + H2O(l) → 2NaOH(aq)
→ Magnesium ribbon burns in air form magnesium oxide.
→ Reaction of metals with water : Sodium and potassium react vigorously with water to evolve hydrogen. Calcium reacts with water less vigorously to evolve hydrogen. Magnesium reacts with hot water to evolve hydrogen. Aluminium, iron and zinc do not react with cold or hot water but they react with steam to evolve their oxides and hydrogen.
→ Reaction of metals with acids : Metals react with dilute hydrochloric acid or dilute sulphuric acid to form metal chloride or metal sulphate and hydrogen gas. The rate of evolution of Hy is maximum in case of magnesium. The reactivity decreases in the order Mg > Al > Zn > Fe.
→ Reaction of metals with nitric acid : Metals react with nitric acid to form nitrate salts. Depending on the concentration of nitric acid, various oxides of nitrogen (NO, NO2,) are formed.
→ Aqua Regia: Aqua Regia is freshly prepared by mixing concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1. Aqua Regia is a highly corrosive and fuming liquid. It is one of the few reagents which can dissolve the noble metals like gold and platinum.
→ Reactions of metals with salts of other metals : The reactivity of all metals is not the same. All metals do not react with oxygen, water and acids. As a result, the relative reactivity of metals cannot be determined using these reagents. If a metal A displaces another metal B from the solution of its salt, it means that the metal A is more reactive than the metal B. Metal A+ Salt solution of metal B → Salt solution of metal A + Metal B.
Reactivity series of metals :
→ Reactivity series of metals : The arrangement of metals in the increasing or decreasing order of reactivity is called the reactivity series of metals. Metals are divided into the following groups according to their reactivity.
- Highly reactive metal.
- Moderately reactive metals.
- Less reactive metals.
Chemical properties of nonmetals :
→ Reaction of metals with nonmetals : The ionic compound is formed when metal reacts with nonmetal.
→ Ionic compound is formed as sodium loses one electron while chlorine accepts one electron. Reaction of nonmetals with oxygen: Nonmetals combine with oxygen to form acidic oxides. In some cases, neutral oxides are formed.
→ Reaction of nonmetals with water : Non metals do not react with water, (Exception-Halogen). Chlorine dissolves in water giving the following reaction.
→ Reaction of dilute acid with nonmetals: Nonmetals do not react with dilute acids, (Exception-Halogen). Chlorine reacts with dilute hydrobromic acid to form bromine and HCI.
Cl2(g) + 2HBr(aq) → 2HCl(aq) + Br2(aq)
→ Reaction of nonmetals with hydrogen : Nonmetals react with hydrogen under certain conditions such as proper temperature, pressure and use of catalyst.
S + H2 → H2S(g)
N2 + 3H2 → 2NH3(g)
Ionic compounds :
→ Ionic compounds : The compounds formed from two units, i.e. cation and anion are called ionic compounds. An electrostatic force of attraction between oppositely charged ions (cations and anions) is called an ionic bond.
General properties of ionic compounds:
- Ionic compounds are crystalline solids have a definite shape and hard due to strong electrostatic force of attraction between oppositely charged ions.
- They are generally brittle. When pressure is applied they break into pieces.
- They have high melting and boiling points.
- They are soluble in water and insoluble in solvents such as kerosene and petrol.
- They conduct electricity in the molten state and also in an aqueous solution.
Metallurgy : Various concepts :
→ Minerals and ores : The compounds of metals that occur in nature along with the impurities are called minerals. The minerals from which metals are extracted economically are called ores.
→ Metallurgy: The process of extraction of metal in pure form from the ores. The metals are further purified by different methods of purification. All this process is called metallurgy.
→ Concentration of ores : The process of separating gangue from the ores is called concentration of ores. Some general methods of concentration of ores as follows:
→ Separation based on gravitation : The gravitational method is used to separate the heavy particles of ores from the light particles of gangue. The processes to do this separation are as follows:
- Wilfey table method (Separation based on gravitation)
- Hydraulic separation method.
- Magnetic separation method
- Froth floatation method
→ When a metal loses electrons the process is called an oxidation while when a nonmetal gains electrons, it is called a reduction,
Na → Na+ + e– (oxidation)
Cl + e– → Cl– (reduction)
→ Extraction of reactive metals : The extraction of highly reactive metals has to be done by electrolytic reduction.
→ Extraction of aluminium : Atomic number: 13, Electronic configuration : 2, 8, 3, Valency: 3. Aluminium is extracted from its main ore bauxite (Al2O3nH2O).
Extraction of aluminium involves the following steps:
- Concentration of bauxite ore (Hayer’s process)
- Electrolytic reduction of pure alumina
→ Concentration of bauxite ore : Powdered bauxite is heated with NaOH under high pressure and at 140 °C to 150 °C for 2 to 8 hours in a digester Al2O3 being amphoteric in nature forms sodium aluminate (NaAlO2) which is water soluble. The iron oxide impurities does not dissolve in aqueous NaOH and are separated by filtration.
→ However, silica reacts with sodium hydroxide to form soluble sodium silicate. NaAlO2 is hydrolysed to form insoluble Al(OH)– by diluting it with water and cooling to 50 °C. Aluminium hydroxide precipitate is then filtered, dried and calcinated by heating at 1000 °C to obtain pure alumina.
→ Electrolytic reduction of alumina : Cell – A steel tank with graphite lining: Electrolyte – Alumina dissolved in fused cryolite; Cathode – The graphite (carbon) in the form of lining of the steel tank; Anode – Graphite (carbon) rods dipped in the electrolyte; cryolite (Na3AlF6) and fluorspar (CaF2) are added in the mixture to lower its melting point up to 1000°C.
→ Electrolysis products: Cathode – Aluminium metal, Anode – Oxygen gas.
At the anode : 2O2- → O2 + 4e–
At the cathode : Al3+ + 3e– → Al
→ Extraction of moderately reactive metals : In metals, in the middles of the reactivity series such as iron, zinc, lead, copper are moderately reactive. These metals occur in the form of their sulphide salts or carbonate salts.
→ The sulphides ores are strongly heated in air to convert them into oxides. This process is called roasting. Carbonate ores are strongly heated in a limited supply of air to convert them into oxides. This process is called calcination.
→ The following reaction occur during roasting and cacination of zinc ore.
Roasting : 2ZnS +3O2 → 2ZnO + 2SO2↑
Calcination: ZnSCO3 → ZnO + CO2↑
The zinc oxide is reduced to zinc by using suitable reductant such as carbon.
ZnO + C → Zn + CO↑
→ Extraction of less reactive metals : The less reactive metals are at the bottom of reactivity series of metal. These metals are found in free state in nature. For example, gold, silver, platinum. The reserves of copper in free state are exhausted long ago. Copper is found mainly in the form of Cu,s. Copper is obtained from Cu S ore just by heating in air.
2Cu2S +3O2 → 2Cu2O + 2SO2↑
2Cu2O + Cu2S → 6Cu + SO2↑
→ Refining of metals : Metals obtained by various reduction processes contain impurities Electrolysis method is used to remove impurities to obtain pure metals.
→ Corrosion is degradation of a material due to reaction with its environment.
When old iron bars in the buildings are exposed to moist air for a long time, they acquire a coating of brown flaky substance called rust. (Fe2O3.H2O)
→ If copper vessels are not cleaned for a long time, they react with moist carbon dioxide in the air, lose their shine and gain a green coat of copper carbonate. (CuCO3)
→ When silver ornaments or idols are kept exposed to air for a long time, silver reacts with sulphur in the air to form a coating of black silver sulphide. (Ag2S)
→ The metallic parts of the body of old cars are corroded, eaten up and sometimes become perforated. The old cars also lose the original colour due to formation of flakes of rust.
Prevention of corrosion :
→ By coating with some substance on the metal surface so that the contact of the metal with moisture and oxygen in the air is prevented and no reaction would occur between them.
→ By applying a layer of paint, oil, grease or varnish on the surface of a metal to prevent corrosion. For example, corrosion of iron can be prevented by this method.
→ Corrosion is prevented by coating a corrosive metals with a noncorrosive metal. This can be done in many ways: