Questions
5 questions in standard papers
Difficulty
Medium
Importance
Core — never skip
Overview
Chemical Reactions and Equations forms the foundation of physical chemistry, focusing on the transformation of substances and the law of conservation of mass. Mastery of this topic is essential for scoring in both board exams and competitive engineering entrance tests, as it bridges the gap between atomic theory and macroscopic chemical change.
Balancing Chemical Equations
A balanced chemical equation follows the Law of Conservation of Mass, ensuring the total number of atoms for each element is equal on both sides. Mastering the Hit-and-Trial method is essential for efficiency in board examinations.
- Mass of reactants equals mass of products
- Never alter the subscripts of chemical formulas
- Start balancing with the compound containing the maximum number of atoms
- Ensure physical states (s, l, g, aq) are indicated for full marks
Types of Chemical Reactions
Chemical reactions are classified based on the nature of atomic rearrangement and energy exchange. Recognizing these patterns allows for the prediction of products in unseen chemical equations.
- Combination: A + B -> AB
- Decomposition: AB -> A + B (Thermal, Electrolytic, Photolytic)
- Displacement: A + BC -> AC + B (Based on reactivity series)
- Double Displacement: AB + CD -> AD + CB
- Exothermic vs Endothermic energy profiles
Oxidation and Reduction (Redox)
Redox reactions involve the simultaneous transfer of electrons, oxygen, or hydrogen between species. These processes are critical for understanding electrochemical cells and industrial metal protection.
- Oxidation: Gain of O2, loss of H2, or loss of electrons
- Reduction: Gain of H2, loss of O2, or gain of electrons
- Redox Agent: Substance oxidized is the reducing agent; substance reduced is the oxidizing agent
- Corrosion and Rancidity as practical examples of slow oxidation
Formula Sheet
Zn + H2SO4 -> ZnSO4 + H2
2H2 + O2 -> 2H2O
Fe + CuSO4 -> FeSO4 + Cu
Exam Tip
Always verify your equation balance by checking the net charge and total atom count for every element individually after balancing.
Common Mistakes
- Changing the subscripts in a chemical formula instead of modifying the stoichiometric coefficients.
- Forgetting to mention the physical states of reactants and products which results in mark deduction.
- Confusing oxidizing and reducing agents by identifying the process (oxidation) instead of the species causing it.
More Revision Notes
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