Chemical Reactions and Equations Notes

A chemical reaction is a process in which one or more substances, called reactants, change into new substances, called products, with different properties. Everyday life is full of chemical reactions: milk turning into curd, food being digested, iron rusting, wood burning, a cake rising in the oven and plants making food through photosynthesis. A chemical equation is the short, symbolic way of writing a reaction. This chapter is one of the most important in general science and appears regularly in SSC CGL, CHSL, MTS, railway, defence and state PCS exams, as well as in the science section of UPSC Prelims, where questions often link reactions to daily-life phenomena like corrosion, rancidity and the use of antacids.

Quick Facts at a Glance

• Law of conservation of mass (Antoine Lavoisier): Mass is neither created nor destroyed in a chemical reaction, so equations must be balanced.
• Main types: Combination, decomposition, displacement, double displacement and redox reactions.
• Exothermic reactions release heat; endothermic reactions absorb heat.
• Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons.
• Corrosion (like rusting) and rancidity (spoiling of fats) are everyday effects of oxidation.

1. Signs of a Chemical Reaction

We can usually tell that a chemical reaction has taken place by observing one or more of these changes:

• Change in state, such as a solid forming from two liquids.
• Change in colour, such as a cut apple turning brown.
• Evolution of a gas, such as bubbles when zinc reacts with dilute acid.
• Change in temperature, either heating up or cooling down.
• Formation of a precipitate, an insoluble solid that separates from a solution.

A chemical change differs from a physical change, such as melting ice or dissolving sugar, where no new substance is formed and the change can usually be reversed easily.

2. Writing and Balancing Chemical Equations

A word equation like "magnesium + oxygen → magnesium oxide" becomes a chemical equation when we use symbols and formulas: Mg + O₂ → MgO. This is a skeletal equation, because the number of oxygen atoms is not equal on both sides. Since atoms cannot be created or destroyed, we must balance the equation by placing numbers (coefficients) in front of formulas.

Balanced equation: 2Mg + O₂ → 2MgO. Now there are 2 Mg atoms and 2 O atoms on each side.

Steps to Balance an Equation (Hit-and-Trial Method)

1. Write the correct formulas of reactants and products. Never change a formula to balance; only change coefficients.
2. Count the atoms of each element on both sides.
3. Start with the compound having the most atoms, and balance one element at a time.
4. Balance hydrogen and oxygen last, as they often appear in many compounds.
5. Check the final count of all atoms.

Example: Fe + H₂O → Fe₃O₄ + H₂. Balance Fe: 3Fe. Balance O: 4H₂O. Balance H: 4H₂. Final: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

We can also show physical states: (s) solid, (l) liquid, (g) gas and (aq) aqueous solution, and write conditions such as heat or a catalyst above the arrow.

3. Types of Chemical Reactions

Combination Reaction

Two or more reactants combine to form a single product. A + B → AB.

• Burning of coal: C + O₂ → CO₂
• Formation of water: 2H₂ + O₂ → 2H₂O
• Slaking of lime: CaO + H₂O → Ca(OH)₂ + heat. Quicklime (calcium oxide) reacts vigorously with water to form slaked lime (calcium hydroxide), which is used for whitewashing. Over a few days, it reacts with carbon dioxide in the air to form a shiny layer of calcium carbonate on walls.

Decomposition Reaction

A single reactant breaks down into two or more simpler products. AB → A + B. Decomposition usually needs energy in the form of heat, light or electricity.

• Thermal decomposition: CaCO₃ → CaO + CO₂ (limestone heated to make quicklime, used in cement).
• Electrolytic decomposition: 2H₂O → 2H₂ + O₂ (electrolysis of water).
• Photolytic decomposition: 2AgCl → 2Ag + Cl₂ in sunlight; silver chloride turns grey. This reaction was the basis of black-and-white photography.

Displacement Reaction

A more reactive element displaces a less reactive element from its compound. A + BC → AC + B.

Example: Fe + CuSO₄ → FeSO₄ + Cu. When an iron nail is placed in blue copper sulphate solution, a brown coating of copper forms on the nail and the solution turns pale green. This happens because iron is more reactive than copper. The reactivity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au) tells us which metals can displace others.

Double Displacement Reaction

Two compounds exchange their ions to form two new compounds. AB + CD → AD + CB.

• Precipitation reaction: Na₂SO₄ + BaCl₂ → BaSO₄ (white precipitate) + 2NaCl.
• Neutralisation reaction: NaOH + HCl → NaCl + H₂O. An acid and a base react to form salt and water.

Oxidation and Reduction (Redox) Reactions

Oxidation is the gain of oxygen, loss of hydrogen or loss of electrons. Reduction is the loss of oxygen, gain of hydrogen or gain of electrons. They always occur together, which is why such reactions are called redox reactions.

Example: CuO + H₂ → Cu + H₂O. Copper oxide loses oxygen (reduced), while hydrogen gains oxygen (oxidised). The substance that causes oxidation is the oxidising agent (here CuO), and the one that causes reduction is the reducing agent (here H₂).

Memory trick: OIL RIG – Oxidation Is Loss (of electrons), Reduction Is Gain.

4. Exothermic and Endothermic Reactions

Respiration is an exothermic process: glucose combines with oxygen in our cells to release energy. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

5. Effects of Oxidation in Daily Life

Corrosion

Corrosion is the gradual damage of metals due to reaction with air, moisture or chemicals. Rusting of iron needs both oxygen and water; rust is mainly hydrated iron(III) oxide (Fe₂O₃·xH₂O). Silver turns black because of silver sulphide, and copper develops a green coating (basic copper carbonate). Corrosion damages bridges, ships, cars and pipelines, causing huge economic losses. It can be prevented by painting, oiling, greasing, galvanising (coating with zinc), electroplating and making alloys such as stainless steel.

Rancidity

When fats and oils are oxidised, they develop a bad smell and taste; this is called rancidity. It can be slowed by adding antioxidants, storing food in airtight containers, refrigerating it, and flushing packets of chips with nitrogen gas, which keeps oxygen out.

6. Rate of Reaction and Catalysts

Some reactions, like an explosion, happen in a fraction of a second, while others, like rusting, take days or years. The speed of a reaction depends on temperature (higher temperature usually speeds it up), concentration of reactants, surface area (powdered substances react faster than lumps) and the presence of a catalyst. A catalyst speeds up a reaction without being used up itself. Examples include iron in the Haber process for making ammonia, nickel in the hydrogenation of vegetable oils to make vanaspati ghee, platinum and rhodium in catalytic converters of vehicles, and enzymes such as amylase and pepsin, which act as biological catalysts in digestion.

7. Practice Questions

1. Balance: H₂ + Cl₂ → HCl. Answer: H₂ + Cl₂ → 2HCl.
2. Which type of reaction is the electrolysis of water? Answer: Decomposition.
3. What colour is the precipitate of barium sulphate? Answer: White.
4. Which gas is used to flush chip packets? Answer: Nitrogen.
5. Which metal is used for galvanising iron? Answer: Zinc.

Related Notes

• Chemistry Basics: Acids, Bases, Salts and Metals
• Atomic Structure, Carbon Compounds and Fuels
• Vitamins, Nutrition and Diseases Notes
• All General Science Notes

Frequently Asked Questions

What is a chemical reaction?

A process in which reactants change into new substances called products with different properties.

Why must chemical equations be balanced?

Because of the law of conservation of mass: atoms are neither created nor destroyed in a reaction.

What is a combination reaction?

A reaction in which two or more substances combine to form a single product, such as CaO + H₂O → Ca(OH)₂.

What is a decomposition reaction?

A reaction in which one compound breaks down into simpler substances, such as CaCO₃ → CaO + CO₂.

What happens when an iron nail is put in copper sulphate solution?

Iron displaces copper; a brown copper coating forms on the nail and the blue solution turns pale green.

What is a precipitation reaction?

A double displacement reaction that forms an insoluble solid, such as barium sulphate.

What is oxidation?

Gain of oxygen, loss of hydrogen or loss of electrons.

What is reduction?

Loss of oxygen, gain of hydrogen or gain of electrons.

Is respiration exothermic or endothermic?

Exothermic, because it releases energy.

What is needed for iron to rust?

Both oxygen (air) and water (moisture).

What is rancidity and how is it prevented?

Rancidity is the oxidation of fats and oils that spoils food; it is prevented by antioxidants, airtight packing, refrigeration and nitrogen flushing.

What is galvanisation?

Coating iron or steel with zinc to prevent rusting.