Introduction
This chapter introduces you to chemical reactions and how they are represented using balanced chemical equations. You will learn the Law of Conservation of Mass, how to balance chemical equations, identify different types of reactions (combination, decomposition, displacement, double displacement, and oxidation-reduction), and understand everyday phenomena like corrosion and rancidity.
Writing and Balancing Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction. The reactants are written on the left and products on the right, separated by an arrow. According to the Law of Conservation of Mass (Lavoisier), matter can neither be created nor destroyed in a chemical reaction, so the number of atoms of each element must be equal on both sides. Balancing is done by adjusting coefficients (numbers before formulae), never by changing subscripts. The hit-and-trial method is commonly used: start with the element that appears in the fewest formulae, then adjust others accordingly. State symbols are added: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution.
Key Points
- •Chemical equation: reactants -> products
- •Law of Conservation of Mass: atoms are conserved
- •Balance by adjusting coefficients, never subscripts
- •Use hit-and-trial method starting with the least common element
- •Add state symbols: (s), (l), (g), (aq)
- •An unbalanced equation is called a skeletal equation
Worked Example
Balance: Fe + H2O -> Fe3O4 + H2 Step 1: Fe appears as Fe3O4, so put 3 before Fe: 3Fe + H2O -> Fe3O4 + H2 Step 2: O has 4 on right, put 4 before H2O: 3Fe + 4H2O -> Fe3O4 + H2 Step 3: H has 8 on left (4x2), put 4 before H2: 3Fe + 4H2O -> Fe3O4 + 4H2 Balanced: 3Fe(s) + 4H2O(g) -> Fe3O4(s) + 4H2(g)
Watch Out
In board exams, always write the balanced equation with state symbols. Marks are deducted for missing state symbols or unbalanced equations.
Types of Chemical Reactions
Chemical reactions are classified into five main types based on how reactants transform into products. (1) Combination reaction: two or more substances combine to form a single product (A + B -> AB). Example: CaO + H2O -> Ca(OH)2. (2) Decomposition reaction: a single reactant breaks down into two or more simpler products (AB -> A + B). It requires energy input — thermal decomposition (heat), electrolytic decomposition (electricity), or photolytic decomposition (light). (3) Displacement reaction: a more reactive element displaces a less reactive one from its compound (A + BC -> AC + B). Example: Zn + CuSO4 -> ZnSO4 + Cu. (4) Double displacement reaction: two compounds exchange their ions (AB + CD -> AD + CB). Often forms a precipitate. (5) Oxidation-reduction (Redox): oxidation is gain of oxygen or loss of hydrogen; reduction is the reverse. In a redox reaction, one substance is oxidised while another is reduced simultaneously.
Key Points
- •Combination: A + B -> AB (e.g., 2Mg + O2 -> 2MgO, exothermic)
- •Decomposition: AB -> A + B (requires energy: heat/light/electricity)
- •Displacement: more reactive metal replaces less reactive (use reactivity series)
- •Double displacement: exchange of ions, often forms precipitate
- •Redox: oxidation and reduction happen simultaneously
- •OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons
- •The substance that gets oxidised is the reducing agent, and vice versa
Worked Example
CuO + H2 -> Cu + H2O CuO loses oxygen -> Cu (CuO is reduced, it is the oxidising agent) H2 gains oxygen -> H2O (H2 is oxidised, it is the reducing agent) This is a redox reaction.
Watch Out
Remember: the substance that is oxidised is the REDUCING agent (it reduces the other). This is a common trick question in board exams.
Corrosion and Rancidity
Corrosion is the slow deterioration of metals when exposed to moisture, oxygen, acids, or other chemicals in the environment. The most common example is rusting of iron: iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust), which is reddish-brown and flaky. Unlike iron, copper develops a green coating (basic copper carbonate) and silver turns black (silver sulphide). Rancidity occurs when fats and oils in food get oxidised, producing foul-smelling substances. Both are oxidation processes.
Key Points
- •Rusting: 4Fe + 3O2 + 2xH2O -> 2Fe2O3.xH2O (needs both oxygen and moisture)
- •Copper corrosion: green coating of basic copper carbonate
- •Silver tarnishing: black silver sulphide layer
- •Corrosion prevention: painting, oiling, greasing, galvanising, electroplating, alloying
- •Galvanising: coating iron with zinc (even if scratched, zinc corrodes first)
- •Rancidity prevention: adding antioxidants (BHA), storing in airtight containers, nitrogen flushing, refrigeration
Watch Out
Galvanising is the most commonly asked prevention method. Remember that zinc is more reactive than iron, so it corrodes preferentially (sacrificial protection).
Exothermic and Endothermic Reactions
Reactions that release energy (usually as heat) are exothermic. The products have lower energy than the reactants. Examples include combustion, neutralisation, respiration, and most combination reactions. Reactions that absorb energy are endothermic. The products have higher energy than the reactants. Examples include decomposition of calcium carbonate, photosynthesis, and dissolving ammonium chloride in water. Quick lime (CaO) reacting with water is a classic example of a highly exothermic reaction — it releases so much heat that it can burn skin.
Key Points
- •Exothermic: releases heat (combustion, respiration, neutralisation)
- •Endothermic: absorbs heat (decomposition, photosynthesis)
- •CaO + H2O -> Ca(OH)2 + heat (highly exothermic)
- •CaCO3 --(heat)--> CaO + CO2 (endothermic decomposition)
- •Natural decomposition: sunlight decomposes silver compounds (used in photography)
Worked Example
Photosynthesis is endothermic: 6CO2 + 6H2O + sunlight -> C6H12O6 + 6O2 (energy absorbed from sunlight) Respiration is exothermic: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (energy released for body functions)
Quick Summary
- ✓Chemical equations must be balanced according to the Law of Conservation of Mass
- ✓Five types: combination, decomposition, displacement, double displacement, redox
- ✓Oxidation is gain of oxygen or loss of hydrogen; reduction is the reverse
- ✓The substance oxidised is the reducing agent; the substance reduced is the oxidising agent
- ✓Corrosion: metals react with environment (rusting needs both O2 and moisture)
- ✓Prevention of corrosion: painting, oiling, galvanising, electroplating, alloying
- ✓Rancidity: oxidation of fats; prevented by antioxidants, airtight storage, N2 flushing
- ✓Exothermic reactions release energy; endothermic reactions absorb energy
Key Formulas
Rusting: 4Fe + 3O2 + 2xH2O -> 2Fe2O3.xH2O
Quick lime: CaO + H2O -> Ca(OH)2 + heat
Thermite: 2Al + Fe2O3 -> Al2O3 + 2Fe + heat
Thermal decomposition: CaCO3 -> CaO + CO2
Electrolytic decomposition: 2H2O -> 2H2 + O2
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