LANGUAGE OF CHEMISTRY_CLASS 8

LANGUAGE OF CHEMISTRY

 

Chemical Symbols:

  It is the short hand representation of an atom of an element.

Each element is denoted by a symbol. Usually the first letter of its name is used as its symbol. However, when more than one element has the same first letter, two letters are used to denote the symbol.

 

Significance of symbol:

It represents:

1.      The name of an element.

2.      One atom of an element

Chemical Formula:

 A formula represents one molecule of a pure substance (elements and compounds). So it is also called molecular formula.

E.g. N₂ represents one molecule of nitrogen; HNO₃ represents one molecule of nitric acid.

 Radical:

It is an atom or group of atoms of same or different elements that behave as a single unit. They are charged particles. They also have a valency and chemical formula.

 If a radical has positive charge it is called basic radical (cation) and if it has negative charge it is called an acidic radical (anion).

E.g. NH₄⁺ is a basic radical and NO₃^- is an acidic radical.

Valency:

It is the combining capacity of an atom of an element or radical. The valency of an element or of a radical is defined as the number of hydrogen atoms that will combine with it or displace one atom of that element or radical.

E.g. 2 atoms of H combine with one atom of oxygen. So the valency of oxygen is 2.

Note: The elements having valency 1, 2, 3 and 4 are called monovalent, divalent, trivalent and tetravalent elements.

 Variable valency:

Certain elements have been found which show more than one valency.

 E.g. In ferric chloride, the valency of iron is +3 while the valency of iron in ferrous chloride is +2.

When an element exhibits two different valencies, for the lower valency we use the suffix ous and for the higher valency we use the suffix ic. Modern chemists have started using Roman numerals in place of these trivial names.

e.g. Instead of writing FeCl₂ as Ferrous chloride they are writing it as Iron (II) chloride and FeCl_3,as iron (III) chloride instead of ferric chloride.

Non metals can also exhibit variable valency. Sulphur has been found to show a valency of 2, 4 & 6 while phosphorus shows a valency of 3& 5.

Valency of radicals:

 

Radical                                                                          Formula

 Valency 1: Monovalent

Bromide                                                                                              Br ^-1    

Acetate                                                                                                CH₃COO^-1

Chloride                                                                                              Cl^-1

Ammonium                                                                                         NH₄⁺

Cuprous or copper (I)                                                                         Cu⁺¹

Bicarbonate (hydrogencarbonate)                                                       HCO₃^-

Fluoride                                                                                               F^-1       

Bisulphate (hydrogensulphate)                                                           HSO₄^-

Bisulphite (hydrogensulphite)                                                             HSO₃^-

Iodide                                                                                                 I^-1        

Chlorate                                                                                              ClO₃^-

Mercurous or mercury (I)                                                                    Hg⁺²   

Chlorite                                                                                               ClO₂^-

Potassium                                                                                            K¹⁺

Cyanide                                                                                               CN^-

Silver                                                                                                   Ag¹⁺   

Hydroxide                                                                                          OH^-

Sodium                                                                                                Na¹⁺

Hypochlorite                                                                                      ClO^-

Nitrate                                                                                                 NO₃^-

Nitrite                                                                                                  NO₂^-

Perchlorate                                                                                          ClO₄^-

Permanganate                                                                                     MnO₄^-

 

Valency 2: Divalent

Barium                                                                                                Ba⁺²    

Carbonate                                                                                            CO₃^2-

Cadmium                                                                                            Cd⁺²   

Chromate                                                                                            CrO₄^2-

Calcium                                                                                               Ca⁺²

Dichromate                                                                                         Cr₂O₇^2-

Cobalt                                                                                                 Co⁺²

Peroxide                                                                                              O₂^2-

Cupric or copper (II)                                                                           Cu⁺²    

Silicate                                                                                                Si₄O₃^2-

Ferrous or iron (II)                                                                              Fe⁺²    

Sulphate                                                                                              SO₄^2-

Plumbous or lead (II)                                                                          Pb⁺²

Sulphite                                                                                               SO₃^2-

Magnesium                                                                                          Mg⁺²               

Manganous or manganese (II)                                                            Mn⁺²               

Mercuric or mercury (II)                                                                     Hg⁺²               

Oxide                                                                                                  O^2-                             

Stannous or tin (II)                                                                             Sn⁺² 

Sulphide                                                                                             S^2-                   

Zinc                                                                                                     Zn⁺² 

                                                                                                                                   

Valency 3: Trivalent

Aluminium                                                                                          Al⁺³    

Ferricyanide                                                                                        [Fe(CN)₆]^3-

Auric or gold (III)                                                                               Au⁺³

Phosphate                                                                                            PO₄^3-

Chromic or chromium (III)                                                                 Cr⁺³    

Phosphite                                                                                           PO₃^3-

Ferric or iron (III)                                                                               Fe⁺³                

Nitride                                                                                                 N^-3                  

                       

Valency 4: Tetravalent

 Ferrocyanide                                                                                       [Fe(CN)₆]^4-

Plumbic or lead (IV)                                                                           Pb⁺⁴                

Stannic or tin (IV)                                                                               Sn⁺⁴                

 

Writing Chemical Formulae:

How to write a formula using criss cross method:

The following steps have to be followed to write a formula:

1. Write the symbol of the respective radicals. The positive radical is written on the left hand side and negative radical on the right hand side.

2.    The valency of the radicals is written on top of the symbol.

3.    The valency of the radicals will be interchanged without the positive and negative signs.

4.    The interchanged valencies are written to the lower right of the radicals.

5.    If the radical contains more than one atom it should be enclosed within brackets.

 

Illustration:

Formula of sodium chloride:            Formula of magnesium chloride:

     Na⁺¹      Cl^-1                                                                 Mg⁺²    Cl^-1

  

     Na₁      Cl₁                                                                     Mg₁       Cl₂   

         NaCl                                                           MgCl₂

Formula of calcium hydroxide:         Formula of ammonium sulphate:

 Ca⁺²       OH^-1                                                                  NH₄⁺       SO₄^2-

 

 Ca₁      (OH)₂                                                                   (NH₄)₂       (SO₄)₁

    Ca(OH)₂                                                                          (NH₄)₂ SO₄

 

NAMING OF COMPOUNDS:

1. A metal and non- metal: When we write the name of a binary compound consisting of a metal and non- metal we write the name of the metal first and add a suffix ide to the non metallic part.

E.g. MgCl₂ is named as magnesium chloride.

2.    Two non metals: If a compound contains two non metals prefix mono, di, tri, tetra etc are used to indicate the number of atoms present.

E.g. CO₂ is named as carbon dioxide; PCl₃ is named as phosphorus trichloride.

3.    Two elements and oxygen: Oxygen is the last element in the formula.

a.    If only one compound is possible then the suffix ate is added.

b.      If two compounds are possible for the compound containing less number of oxygen, suffix ite is used and for the compound containing more number of oxygen suffixes ate is used.

c.  If any compound contains less number of oxygen than the compound which ends with ite then the prefix hypo is added and if it contains more number of oxygen then the compound with suffix ate, then the prefix per is used.

E.g.

KClO – Potassium hypochlorite

KClO₂ – Potassium chlorite

KClO₃ – Potassium chlorate

KClO₄ – Potassium perchlorate                           

 Acids:

a. While writing the names of binary acids the prefix hydro is added and the suffix ic is added to the name of the non-metal.

E.g. HCl – hydrochloric acid, HBr - Hydrobromic  acid

b. Acids containing polyatomic radicals are named on the basis of the second element. The prefix hydro is not used for them.

E.g. H₂SO₄ is called sulphuric acid.

 Chemical Equation:

A shorthand representation of a chemical reaction in terms of symbols and formulae of the substances involved in the reaction is called chemical equation. A reaction can be represented by a word equation or a chemical equation.

E.g. Word Equation:  Hydrogen + Oxygen → Water

Chemical Equation: H₂ + O₂ → H₂O

Reactant and Product:

The substances that react among themselves in a chemical reaction are known as reactants, whereas the substances that are produced as a result of the chemical reaction are known as products.

 How to write a Chemical Equation:

1. The reactants are written on the right hand side and the products on the left hand side.

2. A plus sign (+) is placed between any 2 reactants and any 2 products.

3. The reactants and products are separated by an arrow (→) pointing towards the product. The ‘→’ means to produce or give.

 

Skeletal Equation: An unbalanced chemical equation in which only the symbols and formula of the reactants and products are written.

E.g. Fe   + H₂O → Fe₃O₄ + H₂

 Balanced Chemical Equation: It is a chemical equation in which the number of atoms of each kind on the reactant side and the product side are equal.

E.g. 2H₂ + O₂ → 2H₂O

 

BALANCING A CHEMICAL EQUATION BY HIT AND TRIAL METHOD:

1.      Count the number of times each element occurs on either side of the equation.

2.      The element which occur the least number of times should be balanced first.

3.      If two or more elements occur the same number of times, then balance the metal atom first.

 

Illustration:

Balance the following skeletal equation.

Fe   + H₂O → Fe₃O₄ + H₂

Step 1: Find the number of atoms on both sides of the equation:

Reactant side            Product side

Fe = 1                                      Fe = 3

H = 2                                       H = 2

O = 1                                       O = 4

Step 2: To balance Fe atoms, Fe on the reactant side is multiplied by 3.

Step 3: Now to balance O atoms, write 4 before water on the reactant side.

3Fe   + 4H₂O → Fe₃O₄ + H₂

 

Step 4: Hydrogen atoms are not yet balanced. So to balance hydrogen, write 4 before hydrogen on the product side.

3Fe   + 4H₂O → Fe₃O₄ + 4H₂

      The equation is finally balanced.

Exercise:  Balance the following equations: 1.      Zn + HCl → ZnCl2 + H2 2.      Pb(NO3)2 → PbO + NO2 + O2   ASSIGNMENTS   1.     Write the chemical formula of the following: i.       Sodium oxide ii.     Potassium sulphate iii.  Zinc hydroxide iv.  Ferrous sulphate v.     Ferric chloride vi.  Ammonium phosphate vii.                        Calcium bicarbonate viii.                        Cupric sulphide ix.      Lead(II) nitrate x.       Aluminium Nitride   2.     An element Y shows variable valency of +2 and +3. Write the formula of its: i.                   Nitrite ii.                 Bisulphide iii.              Zincate iv.              Permanganate v.                 Dichromate                     Note:  Write the answer in a tabular form as shown below:   S. No. Radical Valency 2 Valency 3 1. Nitrite  Y(NO2)2               3. Write the name of the following compounds:         i. HNO3                          ii. H2SO4                        iii. HCl         iv. K2SO4                      iv. CuCO3   4. Identify the basic and acidic radical in the compound Potassium nitrate. 5.       Balance the following equations: i.                    Mg + N2→ Mg3N2 ii.                  S + HNO3 → H2SO4 + H2O +NO2        conc iii.                P2O5    + H2O → H3PO4 iv.                P + conc HNO3→ H3PO4 + H2O +NO2 v.                  ZnCO3 + HCl  →  ZnCl2 + H2O + CO2