Chemical Reaction and Equations Class 10 Notes Science Chapter 1

CHAPTER AT A GLANCE

  1. Chemical reaction: The process in which a substance undergoes change to produce new substances with new properties is known as a chemical reaction, e.g., magnesium carbonate when heated produces magnesium oxide and carbon dioxide (i.e. new substances with new properties).

A chemical change is generally accompanied by a change of state, change of color, evolution of a gas, change of temperature, etc.

  1. Chemical equation: The qualitative representation of a chemical reaction in a shorthand or concise form, in terms of symbols and formulae, is called a chemical
  2. A skeletal chemical equation or symbol equation: A chemical equation written in the form of symbols and formulae is called a skeletal chemical equation.
  3. Balanced chemical equation: A chemical equation in which the number of atoms of each element on L.H.S. (i.e. reactants) and R.H.S. (i.e. products) is equal is called a balanced chemical equation.
  4. Balancing of chemical equations: The process of making the number of different elements equal on both sides of the equation is known as the balancing of chemical equations.
  5. Types of chemical reactions:
    • Combination reactions: Combination reactions are those in which one element reacts with another to form a These types of reactions are also known as synthesis reactions, e.g., hydrogen combines with oxygen to give water.

2H2 (g) + 02 (g)             2H20 (l)

  • Decomposition reactions: Decomposition reactions are those reactions in which a compound breaks down into simpler compounds (or substances). This type of reaction is simply the reverse of combination These reactions require energy in the form of heat, light, electricity, etc.

E.g., Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.

Single displacement reactions or single substitution reactions:

A displacement reaction is a reaction in which an atom, or group of atoms, present in a molecule is displaced by another atom. This type of reaction can be represented as follows:

AB + C ——-+      AC + B

(Compound)            (new compound)

For example, iron when added to a solution of copper sulphate displaces copper metal.

CuS04 + Fe           FeS04 + Cu

Double displacement reactions or double decomposition reactions:

The reactions in which the mutual exchange of radicals takes place are known as double decomposition reactions. As a result of double decomposition reactions two new substances are formed.

E.g., sodium chloride reacts with sulphuric acid.

2NaCl + H2S04              Na2S04 + 2HC1

The double-displacement reactions have two major features in common:

  • First, two compounds exchange ions or elements to form new compounds.
  • Second, one of the products is either a compound that will separate from the reaction mixture in some way (commonly as a solid or gas) or a stable covalent compound, often water.

Double-displacement reactions can be further classified as precipitation, gas formation, and acid-base neutralisation reactions.

Oxidation-Reduction reactions

Oxidation reaction: Oxidation is defined as a process that involves the addition of oxygen or the removal of hydrogen.

Reduction reaction: The term reduction is defined as a process that involves the removal of oxygen or the addition of hydrogen.

  • Redox reactions: The reactions in which oxidation and reduction takes place simultaneously, are known as redox reactions, e.g.,

addition of oxygen (oxidation)

  • Exothermic and endothermic reactions: Chemical reactions usually proceed with either the liberation of heat or the absorption of heat.

When a chemical reaction liberates heat to the surroundings, it is said to be an ‘exothermic reaction’ and when it absorbs heat from the surroundings, it is said to be an ‘endothermic reaction’.

N2 (g) + 3H2 (g)   2NH3 (g) + Energy (Exothermic)

CaC03 (s)            CaO(s) + CO2 (g) (Endothermic)

  • Corrosion (erosion by chemical action): Corrosion is the degradation of metals and is generally called rust. Corrosion causes damage to car bodies, iron railings, ships, and all objects made of metals, especially those of iron. Corrosion of iron is a serious problem.
  • Rancidity: The most important cause of deterioration in fats and fatty foods is the oxidation of fats when exposed to air, light, or moisture or by bacterial action resulting in an unpleasant change in the flavor and odor of a food. The whole process is called rancidity

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