TG POLYCET Chemistry: Reactions, Moles and Periodic Trends
TG POLYCET Chemistry covers Telangana SSC topics: chemical reactions and equations, acids, bases and salts, atomic structure, periodic classification, chemical bonding, metallurgy and carbon compounds. Questions mix equation balancing, simple mole calculations and conceptual trends, so both numerical and descriptive recall are needed.
Core rules
- Balance equations by changing coefficients only, never subscripts. Recognise combination, decomposition, displacement, double displacement and redox reactions.
- pH scale: acids pH < 7, neutral pH = 7, bases pH > 7. Neutralisation: acid + base → salt + water.
- Mole concept: 1 mole = 6.022 × 10²³ particles; n = mass / molar mass. At STP, 1 mole of any gas occupies 22.4 L.
- Atomic structure: atomic number Z = protons; mass number A = protons + neutrons. Isotopes have the same Z but different A. Electronic configuration follows 2, 8, 8 for the first three shells.
- Periodic trends: atomic radius decreases across a period and increases down a group; metallic character decreases across a period; valency depends on valence electrons.
- Bonding: ionic bonds involve transfer of electrons, usually metal + non-metal; covalent bonds involve sharing, usually non-metal + non-metal. Carbon shows tetravalency and catenation.
- Metallurgy: roasting is heating a sulphide ore in air; calcination is heating a carbonate ore in limited air; reduction converts metal oxide to metal.
Worked example
Find the moles, molecules and STP volume of 11 g of CO₂. Molar mass = 12 + 2(16) = 44 g/mol. Moles = 11/44 = 0.25 mol. Molecules = 0.25 × 6.022 × 10²³ = 1.5055 × 10²³. Volume at STP = 0.25 × 22.4 = 5.6 L. The calculation works because CO₂ is a gas at STP and the molar mass uses both oxygen atoms.
Traps
Using 28 g/mol for CO₂ instead of 44 g/mol; applying 22.4 L/mol to solids or liquids; balancing by altering subscripts; reversing periodic trends; confusing roasting with calcination.