Chemical Reactions and Equations-Notes

1.  Chemical reactions- The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.

2.  Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of color of substance, evolution of heat, absorption of heat, evolution of gas and evolution of light.

3.  Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - H₂ + O₂ ⇒ H₂O

4.  Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.

      *Note- According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

                      I.  Combination- When two elements or one element and one compound or two compounds combines to give one single product.

                     II.  Decomposition- Splitting of a compound into two or more simple products.

                    III.  Displacement- It takes place when a more reactive metal displaces a less reactive metal.

                    IV.  Double displacement- Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

                     V.  Precipitation- The insoluble compound called precipitate forms in this reaction.

                    VI.  Exothermic- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic.

                   VII.  Endothermic- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic.

                  VIII.  Oxidation: Gain of oxygen or removal of hydrogen or metallic element from a compound is known as oxidation.

                     IX.  Reduction: Addition of hydrogen or removal of oxygen from a compound is called reduction.

                       X.  Redox- A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction.

                             Eg - NaOH + HCl ⇒ NaCl + H₂O

6.  Rusting- When iron reacts with oxygen and moisture forms a red substance called rust.

7.  Rancidity- Oils and fats when get oxidized on exposure to air show a change in taste and smell.

8.  Corrosion- Metals when attacked by oxygen, water, acids, gases, present in air changes its surface which is called corrosion.