Language of Chemistry

(a) Radical is an atom of an element or a group of atoms of the same or different elements that behave as a single unit and has positive or negative charge.

(b) Valency is the combining capacity of an atom of an element or of a radical with the atoms of other elements or radicals to form molecules.

(c) A molecular formula of a compound is the symbolic representation of its one molecule which shows the number of atoms of each element present in it.

Some elements exhibit more than one valency and they are said to have variable valency.

Examples: Iron and copper.

(a) A chemical equation is the symbolic representation of a chemical reaction using symbols and formulae of the substances involved in the reaction.

(b) It is necessary to balance a chemical equation so as to make the number of atoms of the reactants equal to the number of atoms of the products because a chemical reaction is just a rearrangement of atoms and atoms are neither created nor destroyed during the chemical reaction.

(c) The limitations of a chemical equation are:

It does not inform about:

1. the physical states of the reactants and the products i.e. whether they are solids, liquids or gases.
2. the concentration of reactants and products.
3. the time taken for the completion of reaction.
4. the rate at which a reaction proceeds.
5. the heat changes during the reaction i.e. whether heat is given out or absorbed.
6. the conditions such as temperature, pressure, catalyst, etc. which affect the reaction.
7. the nature of the reaction i.e. whether it is reversible or irreversible.

A chemical equation can be made more informative by the following ways:

1. The physical state of the reactants and products can be indicated by putting s for solid, l for liquid, g for gas, aq. for aqueous state besides the symbols for the reactants and products.
2. Evolution or absorption of heat during the reaction can be denoted by adding or subtracting a heat term on the product side.
3. Temperature, pressure and catalyst can be indicated above the arrow (⟶) separating the reactants and products.
4. Concentration of reactants and products are indicated by adding the words (dil.) for dilute and (conc.) for concentrated before their formulae.
5. By the sign ⟶ or ⇌ information about irreversible and reversible reactions can be known.
6. Upward arrow (↑) indicates a gas is evolved, downward arrow (↓) indicates a precipitate is formed, sign delta (Δ) indicates heat is evolved or absorbed.

Law of conservation of mass states that matter can neither be created nor be destroyed, it can only be transformed from one form to another.

(a) Differences between Reactants and Products

(b) Differences between a balanced and an unbalanced chemical equation

1. N₂ + 3H₂ ⟶ 2NH₃

2. 2H₂ + O₂ ⟶ 2H₂O

3. Na₂O + H₂O ⟶ 2NaOH

4. 2CO + O₂ ⟶ 2CO₂

5. Zn + 2HCl ⟶ ZnCl₂ + H₂

1. 2Fe + 3Cl₂ ⟶ 2FeCl₃

2. Mg + H₂SO₄ ⟶ MgSO₄ + H₂

3. 2Mg + O₂ ⟶ 2MgO

4. CaO + H₂O ⟶ Ca(OH)₂

5. 2Na + Cl₂ ⟶ 2NaCl

We get the following information from the given chemical equation:

1. Zinc and Hydrochloric acid are reactants. Zinc is in solid state that reacts with dilute HCl.
2. The reaction is irreversible.
3. The products formed are aqueous solution of zinc chloride and hydrogen gas.
4. The equation is balanced i.e. number of atoms of each element on reactants and products sides are equal.