Chemical Reactions and Equations

1. Chapter at a Glance

• A chemical reaction has taken place when we observe change in state, change in colour, evolution of a gas, or change in temperature.
• A chemical equation represents a chemical reaction; reactants are written on the LHS and products on the RHS, with an arrow showing the direction of the reaction.
• Chemical equations must be balanced so that the number of atoms of each element is the same on both sides (law of conservation of mass).
• Combination reaction: two or more substances combine to form a single product; decomposition is its opposite (one substance breaks into two or more).
• Exothermic reactions release heat with the products; endothermic reactions absorb energy (heat, light or electricity).
• Displacement: one element displaces another from its compound; double displacement: exchange of ions between reactants (often forming a precipitate).
• Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen; reactions involving both are redox reactions.
• Corrosion (e.g. rusting of iron) and rancidity (oxidation of fats/oils) are everyday effects of oxidation.

2. Definitions and Laws

Law of conservation of mass (as recalled from Class IX and used in the chapter):
Mass can neither be created nor destroyed in a chemical reaction. The total mass of the elements present in the products has to be equal to the total mass of the elements present in the reactants. The number of atoms of each element remains the same before and after a chemical reaction.

Word-equation: Shows change of reactants to products through an arrow. Reactants on LHS with ‘+’ between them; products on RHS with ‘+’ between them. Arrowhead points towards the products.

Skeletal chemical equation: An unbalanced chemical equation written using formulae (mass not the same on both sides).

Balanced chemical equation: Number of atoms of each element is the same on both sides of the arrow. Balancing is done by the hit-and-trial method using the smallest whole-number coefficients. Formulae inside boxes must not be changed.

Physical state symbols:
- (s) solid
- (l) liquid
- (g) gaseous
- (aq) aqueous (solution in water)

Reaction conditions (temperature, pressure, catalyst, etc.) may be written above/below the arrow.

Combination reaction: A reaction in which a single product is formed from two or more reactants.
(Also: when two or more substances (elements or compounds) combine to form a single product.)

Exothermic chemical reactions: Reactions in which heat is released along with the formation of products.

Decomposition reaction: A single reactant breaks down to give simpler products.
- Thermal decomposition: carried out by heating.
- Also possible by light or electricity.

Endothermic reactions: Reactions in which energy is absorbed.

Displacement reaction: A reaction in which an element displaces or removes another element from its compound.

Precipitate: An insoluble substance formed in a reaction. Any reaction that produces a precipitate can be called a precipitation reaction.

Double displacement reactions: Reactions in which there is an exchange of ions between the reactants. Two different atoms or groups of atoms (ions) are exchanged.

Oxidation: If a substance gains oxygen during a reaction, it is said to be oxidised. (Also: gain of oxygen or loss of hydrogen.)

Reduction: If a substance loses oxygen during a reaction, it is said to be reduced. (Also: loss of oxygen or gain of hydrogen.)

Oxidation-reduction reactions / redox reactions: Reactions in which one reactant gets oxidised while the other gets reduced.

Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. (Rusting of iron is a common example; black coating on silver and green coating on copper are other examples.)

Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change. Antioxidants are added; airtight containers and flushing with nitrogen slow oxidation.

3. Important Diagrams and Activities

4. Common Misconceptions and Exam Pitfalls

• Balancing errors: Changing subscripts inside formulae (e.g. writing H₂O₄) instead of using coefficients; forgetting to balance polyatomic ions as units when possible; leaving fractional coefficients instead of clearing to whole numbers.
• Physical states: Omitting (s), (l), (g), (aq) when the question specifically asks for state symbols; writing (g) for water when steam is intended vs (l).
• Identifying reaction type: Calling a reaction only “combination” when it is also exothermic/redox (e.g. burning of Mg); confusing displacement with double displacement; missing that precipitation is a type of double displacement.
• Oxidation/reduction: Thinking only “gain of oxygen” counts and forgetting “loss of hydrogen” (and vice versa for reduction); mixing up which species is oxidised and which is reduced in a redox pair.
• Electrolysis of water: Not recalling that volume of H₂ is double that of O₂; wrong identification of gases at electrodes when tested with burning candle.
• Exothermic vs endothermic: Assuming all combination reactions are exothermic or all decompositions endothermic without checking energy change; forgetting respiration and composting as exothermic examples.
• Rancidity vs corrosion: Confusing the two everyday oxidation effects; not linking prevention methods (paint, oil/grease, galvanisation for iron; antioxidants, airtight packing, N₂ flushing for fats/oils) to the underlying oxidation.
• Whitewashing: Forgetting the slow follow-up reaction of Ca(OH)₂ with CO₂ forming CaCO₃ (shiny finish) after the initial slaked-lime formation.
• Skeletal vs balanced: Presenting an unbalanced equation when a balanced one is required; not checking atom count on both sides after balancing.
• Word-equation vs chemical equation: Writing only words when formulae are expected, or vice versa.

5. Formula Sheet

Note: Physical states and conditions are included where the chapter emphasises them. Always balance and add states when the question requires an informative equation.