CBSE Tutorials

Matter-Its Nature and Behaviour | Part 2

Compounds

· A compound is a substance made up of two or more elements chemically combined in a fixed proportion by weight e.g., water, NaCl, NH₄Cl and CO₂, etc.,

Properties of Compounds

· A compound cannot be separated into its components by physical methods.

· The properties of a compound are entirely different from those of its constituent elements.

· The composition of a compound is fixed i.e. the constituents are present in fixed proportion by weight.

· Compounds have fixed melting point, boiling point etc.

· A compound is a homogeneous substance.

Chemical Formulae

· The chemical formula of a compound represents the composition of a molecule of the compound in term of the symbols of the elements present in it. E.g., a formula of H₂O represents that water have two atoms of hydrogen element and one atom of oxygen element.

Method of writing the formula :

If we want to write the formula of a compound, which is made up of two elements have valencies x and y respectively, it can be done as follows :

1. Write the symbols or formulae of the basic radicals to the left and acidic radicals to the right.
2. The valency of each radicals is written on the radicals.
3. the criss-cross rule is applied by exchange of numerical value of valency of each radical.
4. a complex radical (or compound radical) is enclosed in brackets and the subscript is written outside the lower right hand corner.
5. A common factor is eliminated and unit subscript is omitted.

For example : (i) calcium chlorite

(a) symbols are written with their valencies Ca²⁺ (ClO₂) ^–

(b) exchange the numerical value of valency of each radical.

(c) Omitting the unit subscript, the formula of calcium chlorite is Ca (CIO₂)₂

Empirical formula :

The empirical or simple formula of a compound shows the simplest whole number ratio of the atoms of various elements present in one molecule of the compound for instance; the empirical formula of acetylene is CH and that of glucose is CH₂O.

Molecule :

It gives the actual number of atoms of component elements in one molecule of the compound. For instance the molecular formula of fructose is C₆H₁₂O₆.

Calculation of empirical and molecular formula :

The empirical formula of a compound is calculated from the percentage composition of compound as follows :

(a) The percentage of each element is divided by its atomic weight to get the relative number of atoms of various elements present in the compound.

(b) The relative number of atoms is divided by the minimum value to have the simplest ratio of the number of atoms.

(c) If the simplest ratio is not a whole number then it is made a whole number by either changing the value to the nearest whole number or by multiplying with a suitable integer throughout.

(d) The whole number is written as a subscript at the lower right hand corner of the symbol of the element to give the empirical formula.

(e) The empirical formula weight is calculated by adding the atomic weight of atoms of all elements present in empirical formula.

(f) The molecular weight of the compound is calculated from the given data.

(g) ‘n’ is calculated by using the relation.

n =

(h) Molecular formula = (Empirical formula) n

Chemical Equations

· A chemical equation is the symbolic representation of an actual chemical change e.g.,

Zn + H₂SO₄ ZnSO₄ + H₂

· A chemical equation represents :

– Compounds taking part in the reaction.

– Products or compounds formed during the reaction.

– Catalysts used for the reaction (if any).

Notes

– Silicon (Si) is an element and silica (SiO₂) is a compound.

– Oxides of metals are basic and non metals are acidic.

– Crystallization is based on difference in solubility.

– Mixture which boil at definite boiling point are called azeotropic mixtures.

– NH₄Cl sumblimes due to its decomposition.

– I₂ is the only halogen which sublimes.

– A compound is always homogeneous.

– Ants contain formic acid (HCOOH).

– Urea is the first organic compound prepared in laboratory by Wohler in 1928.

– Acetic acid is the first organic compound which is extracted from neutral material by Kolbe.

Methane is the first organic compound which is synthesized in laboratory from its elements by Berthelot.