CBSE Class 10 Science Chemical Reactions and Equations Notes Set 07

1. Chemical change and chemical equation

In a chemical change a new substance with new properties is formed. Signs of a chemical reaction are a change in state, a change in colour, evolution of a gas, or a change in temperature. A chemical equation is the shorthand way of writing a reaction using symbols and formulae. Reactants are written on the left and products on the right, joined by an arrow.

Because atoms are neither created nor destroyed (law of conservation of mass), the number of atoms of each element must be the same on both sides. An equation that satisfies this is called a balanced equation. To balance one, we change only the coefficients in front of the formulae and never the subscripts inside a formula. Physical states can be shown as (s), (l), (g) and (aq).

2. Steps to balance an equation

1. Write the skeleton equation with correct formulae.

2. List the number of atoms of each element on both sides.

3. Start with the compound containing the most atoms and balance one element at a time using coefficients.

4. Balance hydrogen and oxygen last.

5. Recount every element to check.

3. Types of chemical reactions

1. Combination reaction: two or more reactants form a single product. Example: 2Mg + O₂ → 2MgO and CaO + H₂O → Ca(OH)₂ (slaking of lime, which gives out a lot of heat).

2. Decomposition reaction: a single reactant breaks into two or more products. It needs energy, so there are three kinds. Thermal decomposition uses heat: CaCO₃ → CaO + CO₂. Electrolytic decomposition uses electricity: 2H₂O → 2H₂ + O₂. Photolytic decomposition uses light: 2AgBr → 2Ag + Br₂, which is why silver salts are used in black and white photography.

3. Displacement reaction: a more reactive element displaces a less reactive element from its compound. Example: Fe + CuSO₄ → FeSO₄ + Cu. The blue colour of copper sulphate fades and a brown deposit of copper forms on the iron.

4. Double displacement reaction: two compounds exchange their ions. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. When an insoluble solid forms, it is also called a precipitation reaction.

4. Heat changes

Exothermic reactions release heat, for example burning of natural gas, respiration and decomposition of vegetable matter into compost. Endothermic reactions absorb heat, for example ZnCO₃ → ZnO + CO₂ on heating.

5. Oxidation and reduction (redox)

Oxidation is gain of oxygen or loss of hydrogen. Reduction is loss of oxygen or gain of hydrogen. In CuO + H₂ → Cu + H₂O, copper oxide is reduced and hydrogen is oxidised. The substance that gives oxygen (or removes hydrogen) is the oxidising agent, and the other is the reducing agent. Because both processes occur together, such reactions are called redox reactions.

6. Effects of oxidation in daily life

Corrosion is the slow eating away of metals by air, moisture or chemicals. Rusting of iron needs both air and water and gives hydrated iron(III) oxide, Fe₂O₃.xH₂O. Silver turns black (silver sulphide) and copper turns green (basic copper carbonate). Rusting can be prevented by painting, oiling, galvanising and alloying.

Rancidity is the oxidation of fats and oils in food, which gives a bad smell and taste. It is prevented by adding antioxidants, using air-tight containers, flushing packs with nitrogen and refrigeration.

Worked examples

Example 1. Balance: Fe + H₂O → Fe₃O₄ + H₂.

Fe₃O₄ has 3 Fe and 4 O, so put 3 before Fe and 4 before H₂O. That gives 8 H on the left, so put 4 before H₂. Balanced equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Example 2. Identify the type: Zn + 2HCl → ZnCl₂ + H₂.

Zinc is more reactive than hydrogen and displaces it from the acid, so this is a displacement reaction. It is also a redox reaction because zinc is oxidised and hydrogen ions are reduced.

Example 3. A shiny brown element X turns black when heated in air. Name X and the black product.

X is copper. It combines with oxygen to form black copper(II) oxide: 2Cu + O₂ → 2CuO. Passing hydrogen over the hot CuO turns it back to brown copper, showing reduction.

Quick Check Questions

Answer these five questions to test your understanding of the chapter.