Chemistry: Structure, Reactions, and Quantitative Work
Core concepts
Chemistry at Class XII spans atomic structure, chemical bonding, thermodynamics, equilibrium, electrochemistry, kinetics, organic functional groups, and coordination compounds. You must draw electron configurations, predict shapes using VSEPR, balance redox equations, and apply Le Chatelier's principle. Organic chemistry requires recognising functional groups, reaction mechanisms (substitution, addition, elimination), and isomerism.
Traps in questions
- Mole calculations: mixing mass, moles, and molarity; always convert to moles first.
- Limiting reagent: the reactant in excess is not the one that determines product yield.
- Equilibrium shifts: a catalyst changes rate, not position; only temperature changes $K$.
- Oxidation numbers: hydrogen is +1 except in metal hydrides; oxygen is −2 except in peroxides.
- Organic naming: longest chain, lowest locants, and correct suffix priority.
Worked example
How many moles of $\text{CO}_2$ form when 4 g of $\text{CH}_4$ burns completely? ($M_r$ of $\text{CH}_4 = 16$) Equation: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$. Moles of methane: $4/16 = 0.25$ mol. Ratio 1:1, so 0.25 mol of $\text{CO}_2$, i.e. $0.25 \times 44 = 11$ g.
Rule of thumb
For every calculation, write the balanced equation and convert everything to moles. For organic answers, count carbons, then check functional-group priority before naming.