Class 10 Science Notes

Chapter 1: Chemical Reactions and Equations

CBSE Class 10 Science Notes Chapter 1 Chemical Reactions and Equations provides a concise summary for quick revision. Key points include balanced and unbalanced equations, characteristics of chemical reactions, and their types.

What is a Chemical Reaction?

A chemical reaction involves the transformation of one chemical substance into another. For example, rusting of iron, digestion of food, and respiration are all examples of chemical reactions.

• Reactants: Substances participating in a reaction.
• Products: New substances formed during a reaction.
• For example there is reaction of NaOH(Sodium hydroxide) and HCl(Hydrogen chloride) we get NaCl and H₂O. In this reaction NaOH and HCl are reactant whereas NaCl and Water are products.

Characteristics of Chemical Reactions

• Evolution of gas (e.g., zinc with sulfuric acid).
• Change in color (e.g., reaction of citric acid with potassium permanganate).
• Change in state (e.g., combustion of candle wax).
• Change in temperature (e.g., quicklime with water).
• Formation of precipitate (e.g., barium chloride with sulfuric acid).

What is a Chemical Equation?

A chemical equation represents a reaction using symbols of substances. A balanced chemical equation has equal atoms of each element on both sides. It should follow the law of Conservation of mass.

For example there is a chemical reaction below

CH₄ + O₂ → CO₂ + H₂O

Now on the Reactant side, there is one atom of Carbon, 4 atoms of hydrogen and 2 atoms of Oxygen. But on the other side, there are 3 atoms of oxygen. So we need the same no. of atoms on that side. So we will use hit and trial method.

Here is our balanced equation.

CH₄ + O₂ → CO₂ + 2H₂O

Types of Chemical Reactions

Combination Reaction

• In this type of chemical reaction two reactants react with each other to form a product. Generally these reactions are exothermic as they release the energy in surrounding. For example when we mix quick lime(Cao) in water it reacts vigorously and produces Slaked lime with the release of heat. So this is a combination reaction.
• Cao + H₂O → Ca(OH)₂ + Heat. Ca(OH)₂ is the substance which was used extensively in older homes in the place of paint
• It is that due to which the clothes got whitened
• When it is kept in open air especially CO₂ it gets harder.
• Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Decomposition Reaction

• In this type of reaction, one reactant is considered under different conditions and then it breaks into two or more products. In brief it's opposite of combination reaction. The different conditions or factors are - - -

Thermal Decomposition

• In this, a compound is decomposed with the help of heat. For example
• 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ In this reaction the ferrous sulphate was green but now it has turned reddish brown.
• CaCO₃ → Cao + COO₂

Electrolytic Decomposition

• In this recation, the substance is decomposed with the help of water and electricity.
• H₂O → H₂ + O₂

• Oxygen is collected in test tube connected with anode and hydrogen gas is collected at cathode. As we can see that hydrogen gas volume is twice the volume of oxygen because in one molecule of water two atoms are of hydrogen and one is of oxygen. IMPORTANT!!!

Photolytic Decomposition

• In this type of decomposition reaction, the substance is decomposed with the help of sunlight.
• For example, when silver chloride is exposed in sunlight it turns Grey from white.
• 2AgCl → 2Ag + Cl₂(g)
• 2AgBr → 2Ag + Br₂(g)
• Now all the decomposition reactions are endothermic as they require energy to react. But there is an exception that decomposition of vegetable matter into compost is an exothermic recation. IMPORTANT!!!!.
• Displacement Reaction: A more reactive element displaces a less reactive one.
• Double Displacement Reaction: Ions are exchanged between reactants.
• Oxidation-Reduction (Redox) Reactions: Oxidation and reduction occur simultaneously.
• Exothermic Reactions: Release heat, e.g., combustion.
• Endothermic Reactions: Absorb heat, e.g., photosynthesis.

Effects of Oxidation in Everyday Life

• Corrosion: Slow degradation of metals (e.g., rusting of iron).
• Rancidity: Oxidation of fats and oils in food items.

Preventive measures for corrosion and rancidity include painting, greasing, airtight storage, and adding antioxidants.

Practice Questions

• Define a chemical reaction and list its characteristics.
• What is a balanced chemical equation? Why should chemical equations be balanced?
• Explain oxidation and reduction with examples.
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