All chemical reactions are represented by chemical equations. A chemical equation is a shorthand representation of a chemical reaction using the symbols and formulae of substance involved in the chemical reaction.
The symbols and formulae of the substances (elements or compounds) are arranged to show the reactants and products of a chemical reaction.
- When potassium nitrate is heated, it gives potassium nitrite and oxygen. This reaction may be represented in the form of a chemical equation as follows.
- Zinc and dilute sulphuric acid react to form zinc sulphate and hydrogen. This reaction is represented by a chemical equation as
Zn + H2SO4 → ZnSO4 + H2
Rules for writing chemical equation
Certain rules have to be followed while writing a chemical equation.
- The reactants taking part in the reaction are written in terms of their symbols or molecular formulae on the left-hand side of the equation.
- A plus (+) sign is added between the formulae of the reactants.
- The products of reaction are written in terms of their symbols or molecular formulae on the right-hand side of the equation.
- A plus (+) sign is added between the formulae of the products.
- In between the reactants and the products an arrow sign (→) is inserted to show which way the reaction is occurring.
A + B → C + D
In this chemical equation, A and B are the reactants, and C and D are the products. The arrow indicates that the reaction proceeds towards the formation of C and D.
Balancing of Chemical Equation
Observe the following two chemical equations :
Zn + H2SO4 → ZnSO4 + H2 …..(i)
Na + H2O → NaOH + H2 …..(ii)
In equation (i), the number of atoms of Zn, H, S and O are equal on both sides, i.e., the equation is balanced
The equations in which atoms of various elements on the reactants’ and the products side are equal.
Equation (ii) is not balanced because the number of hydrogen atoms in not equal on both sides. It is called a skeleton chemical equation.
Why Do We Balanc Equations?
The number of atoms of elements on both sides of a chemical equation should be equal in accordance with the law of conservation of mass.
The process of making atoms of various elements equal in an equation on either side is called balancing.
Steps in Balancing of Chemical Equations
A number of steps are involved in balancing a chemical equation, e.g.,
Na + H2O → NaOH + H2
Step-1 : Examine the number of atoms of different elements present in unbalanced equations.
Step-2 : Pick an element to balance the equation. In the above equation Na and O are balanced, Hydrogen is not.
Step : To balance Hydrogen on both sides we need to multiply H2O by 2 which makes Hydrogen atoms equal to 4 on the reactants’ side. To make Hydrogen 4 on the products’ side, multiply NaOH by 2. Now oxygen has become 2 on both side. But Sodium atoms has become two on the products’ side. Multiply Na by 2 on the reactants side so that they become equal on both side. The steps are as follows :
(i) Na + 2 H2O → NaOH + H2
(ii) Na + 2 H2O → 2NaOH + H2
(iii) 2 Na + 2 H2O → 2NaOH + H2
The equation is now balanced.
Example: Fe + H2O → Fe3O4 + H2
|Element||Number of atoms in reactants||Number of atoms in products|
Step-2: Pick up the compound which has the maximum number of atoms whether a reactant or a product, and in that compound select the elements which has the highest number of atoms, e.g., we select Fe3O4 in the above equation :
To balance oxygen atoms,
|In reactants||In products|
|1 (in H2O)
1 × 4
|4 (in Fe3O4)
4 × 1
To equalise the number of atoms, we put the coefficient on the left side of the formula.
A coefficient is a small whole number, like coefficients used in algebraic equations.
You must keep in mind that we can put coefficients but we cannot change the subscripts in the formula, i.e., to balance Oxygen atoms, we can put the coefficient 4 as 4 H2O and not H2O4 or (H2O)4. Now the partly balanced equation becomes as follows :
Fe(s) + 4 H2O(g) → Fe3O4(s) + H2(g)
Step-3: Pick up the second element to balance this partly balanced equation. Let us try to balance hydrogen atoms.
In partly balanced equation. Atoms of Hydrogen.
|In reactants||In products|
|8 (in 8 H2O)
8 × 1
|2 (in H4)
2 × 4
To equalise the number of Hydrogen atoms, we use 4 as the coefficient of H2 in the products.
Fe(s) + 4 H2O(g) → Fe3O4(s) + 4 H2
Step-4: Pick up third element to be balanced. The element which is left to be balanced is Fe.
|In reactants||In products|
|1 (in Fe)
1 × 3
|3 (in Fe3O4)
3 × 1
To equalise, we use 3 as coefficient of Fe in reactants.
3Fe + 4H2O → Fe3O4 + 4H2
|Atoms||In reactants||In products|
The equation is balanced because atoms of all the elements are equal on both sides.
This method of balancing equation is known as hit and trial method.
Balancing of Ionic Equations :
In these equations, charge balancing of atoms on both sides of the equation, e.g.,
Initial Cu2+(aq.) + H2S → CuS (s) + H+ (aq)
Balanced Cu2+ (aq.) + H2S → CuS (s) + 2H+ (aq)
We have balanced the charges. It was + 2 on LHS and we have made + 2 on RHS. Number of Hydrogen atoms, Cu and Sulphur atoms are also balanced on both sides.
|Sodium chloride||NaCl||Na+ and Cl–|
|Magnesium chloride||MgCl2||Mg2+ and Cl–|
|Magnesium oxide||MgO||Mg2+ and O2–|
|Calcium chloride||CaCl2||Ca2+ and Cl–|
|Calcium oxide||CaO||Ca2+ and O2–|
|Ammonium chloride||NH4Cl||and Cl-|
|Barium chloride||BaCl2||Ba2+ and Cr|
|Potassium nitrate||KNO3||K+ and|
|Cupric sulphate||CuSO4||Cu2+ and|
|Cupric chloride||CuCl2||Cu2+ and Cl–|
When an element forms electrovalent bond, its valency is known as electrovalency.
The number of electrovalent or ionic bonds an atom can form is called its electrovalency. The electrovalency of an element is, therefore, equal to the number of electrons lost or gained by the atom to form an ion.
Elements which lose electrons show positive electrovalency and those which gain electrons show negative electrovalency. For example, in the formation of sodium chloride (Na+Cl–), the electrovalency of sodium (Na) is +1, while that of chlorine (Cl) is – l.
Elements which lose or gain one, two, three, … , etc., electrons are said to be monovalent (orunivalent), divalent (or bivalent), trivalent, … , etc., respectively.
Monovalent elements : Na, CI, F
Divalent elements : Mg, Ca, Ba, O
Trivalent elements : Al, B
Characteristics of electrovalent or ionic compounds
- Electrovalent compounds are made up of positively and negatively charged ions. For example, sodium chloride (NaCl) is made up of Na+ and Cl– ions arranged in a definite order in three dimensions to form crystals.
- Electrovalent compounds have high melting and boiling points. This is due to the presence of strong electrostatic forces of attraction between the positive and negative ions. A large amount of heat energy is required to break this force of attraction. Hence, the melting and boiling points of electrovalent compounds are high.
- Electrovalent compounds are usually soluble in water but insoluble in organic solvents such as benzene, acetone, carbon disulphide and carbon tetrachloride.
- Electrovalent compounds conduct electricity in molten state and in their aqueous solutions.
In solid electrovalent compounds the ions are held together in fixed positions and cannot move. Hence, such compounds in the solid state do not conduct electricity.
When an electrovalent compound is dissolved in water or is melted, the crystal structure breaks down. The ions now become free to move and can, therefore, conduct electricity.
That the ionic compounds in molten state or in solution become conductors of electricity can be shown by the some specific activity.