Chemical Reactions And Equations

Chemical Reactions: A chemical reaction is a process in which one or more substances undergo a chemical change to form a new substance with new chemical properties.

            When a chemical reaction takes place, the following changes usually occur:

                    (1) Change of states

                    (2) Change in colour

                    (3) Evolution of gas

                    (4) Changes in temperature

                    (5) Formation of precipitation etc.

    Example- (1) Formation of curd from milk.

                    (2) Cooking of food

                    (3) Combustion of magnesium ribbon in air

                    (4) Rusting of iron etc.

Chemical Equation: Description of a chemical reaction in a shorter form using word or symbol is called a chemical equation. 

                                     Magnesium + Oxygen → Magnesium oxide ------- is a word-equation. 

                                            (Reactants)                    (Product)

                                Mg + O₂--------→ MgO ----- is a skeletal chemical equation for the above reaction.

Balanced Chemical Equation: According to law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, a chemical reaction where the number of atoms in each elements are equal in numbers before and after the reaction is called a balanced chemical reaction.

    E.g.                             2Mg + O₂--------→ 2MgO ---------- is a balanced chemical reaction.

Writing Symbols of Physical States: To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. 

   E.g.                            2H₂(g) + O₂(g) -------→ 2H₂O (l)     

Types of Chemical Reactions: In general there are 5 different types of reactions-

                                                        (1) Combination Reaction

                                                        (2) Decomposition Reaction

                                                        (3) Displacement Reaction

                                                        (4) Double Displacement Reaction

                                                        (5) Oxidation And Reduction Reaction

1. Combination Reaction: A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

        E.g.         CaO (s) + H₂O (l) ---→ Ca(OH)₂( aq)

                        2H₂(g) + O₂(g) -------→ 2H₂O (l)

     In the above mentioned reactions, two reactants combine to form a single product.

2. Decomposition Reaction: If in a reaction a single reactant breaks down to give two or more  simpler products the reaction is called a decomposition reaction.

E.g.                     CaCO₃(s) -------→ CaO(s) + CO₂(g)

                        (Limestone)                 (Quick lime)

    In the above reaction, calcium carbonate is broken down to calcium oxide and carbon di oxide.

3. Displacement Reaction: In a reaction, if one element is displaced or removed by another element, then such reaction is known as displacement reaction.

E.g.                     Fe(s) + CuSO₄(aq) ---→FeSO₄ (aq) + Cu(s)

    Here, Fe displaces Cu from CuSO4 solution and occupied its place.

4. Double Displacement Reaction: A reaction in which there is an exchange of ions between the reactants is called double displacement reactions.

E.g.                     Na₂SO₄(aq) + BaCl₂(aq) ---→ BaSO₄(s) + 2NaCl(aq)

5. Oxidation And Reduction Reaction: If a substance gains oxygen during a reaction, it is said to be oxidized. 

E.g.                     2Cu(s) + O₂(g) ---→ 2CuO(s)

    On the other hand, if a substance loses oxygen during a reaction, it is said to be reduced.

E.g.                 CuO(s) + H₂(g) ---→Cu + H₂O(l)

Some Special types of reactions:

1. Exothermic And Endothermic Reaction: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. Examples of exothermic reactions is Burning of natural gas

                            CH₄ (g) + 2O₂ (g) ---→ CO₂ (g) + 2H₂O(g) + Heat

     Reactions in which energy is absorbed in the form of heat, light or electricity for breaking down the reactants, are known as endothermic reactions.

                    2AgCl (s) -----------→ 2Ag(s) + Cl₂(g) (in presence of sunlight)

2. Thermal Decomposition Reaction: When a decomposition reaction is carried out by heating, it is called thermal decomposition reaction.

E.g.             CaCO₃(s) -------→ CaO(s) + CO₂(g) (in presence of heat)

                        (Limestone)                 (Quick lime)

3. Precipitation Reaction: If in a reaction a substance which is insoluble in water (precipitate) is formed, then the reaction is called a precipitation reaction

E.g.             Na₂SO₄(aq) + BaCl₂(aq) ---→ BaSO₄(s) + 2NaCl(aq)

                                                                      (Precipitate)

4. Redox Reaction:  Generally it is seen that in a reaction if one reactant gets oxidized then the other gets reduced. Such reactions where oxidation and reduction takes place side by side in the same reaction, are called oxidation-reduction reactions or redox reactions.

E.g.                 CuO + H₂ ---→ Cu + H₂O

        In the above reaction, hydrogen is oxidized by gaining the oxygen atom and CuO is reduced by loosing oxygen.

Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc. then its shine is reduced and a coloured coating is formed on it. This is called corrosion of metals. Reddish brown coating on iron (rusting), black coating on silver and the green coating on copper are some examples of corrosion. Corrosion causes damage to the metal.

Prevention of corrosion: Corrosion can be prevented by painting metal surfaces, by galvanization, applying grease etc.

Rancidity:  When fat/oil containing food materials are kept for a long time, it develop some sort of smell. It is because, when fats and oils are open in air they are oxidized and their smell and taste change. This is called rancidity. 

Prevention: Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidized.