Chemistry revision goes fastest when organized the way exams actually test it: periodic trends for inorganic chemistry, reaction mechanisms for organic chemistry, and formula-based calculations for physical chemistry. This guide covers all three in exam-ready form.
Periodic Trends (Inorganic Chemistry)
Atomic radius: Decreases across a period (left to right), increases down a group. Across a period, added protons pull electrons closer despite added electrons; down a group, additional electron shells outweigh the increased nuclear charge.
Ionization energy: Increases across a period, decreases down a group — the opposite pattern to atomic radius, since smaller atoms hold their electrons more tightly.
Electronegativity: Increases across a period, decreases down a group. Fluorine has the highest electronegativity of all elements — a fact frequently used as a reference point in comparison questions.
Metallic character: Decreases across a period, increases down a group — directly opposite to electronegativity, which is why metals are typically found on the left and bottom of the periodic table.
Why trends matter more than memorized facts: A question like “which has a larger atomic radius, sodium or magnesium?” doesn’t require memorizing specific radius values — understanding the trend (both same period, sodium is further left, so sodium is larger) answers it directly and applies to elements you’ve never specifically studied.
Chemical Bonding
Ionic bonds form between metals and non-metals through electron transfer, creating oppositely charged ions held together by electrostatic attraction.
Covalent bonds form between non-metals through electron sharing — single, double, and triple bonds differ in how many electron pairs are shared, which directly affects bond strength and length (more shared pairs = stronger, shorter bond).
VSEPR theory predicts molecular shape based on electron pair repulsion: 2 electron groups → linear, 3 → trigonal planar, 4 → tetrahedral. This is tested both directly and as the basis for predicting polarity.
Organic Chemistry Reaction Types
Substitution reactions: One atom or group replaces another. SN1 reactions go through a carbocation intermediate (favored by tertiary substrates and polar protic solvents); SN2 reactions happen in one concerted step (favored by primary substrates and strong nucleophiles).
Addition reactions: Occur in unsaturated compounds (alkenes, alkynes) where atoms add across a double or triple bond, converting it to a single bond. Markovnikov’s rule predicts that in addition of HX to an alkene, hydrogen bonds to the carbon that already has more hydrogens.
Elimination reactions: The reverse of addition — atoms are removed to form a double bond, commonly seen in dehydration and dehydrohalogenation reactions.
Naming (IUPAC) essentials: Identify the longest carbon chain containing the principal functional group, number the chain to give the lowest possible locants to substituents, and name substituents alphabetically. This is pure practice — there’s no shortcut except doing many examples.
Physical Chemistry Formulas
Molarity: M = moles of solute ÷ liters of solution.
Ideal gas law: PV = nRT, connecting pressure, volume, moles, and temperature — commonly tested by asking how one variable changes when another is altered while holding the rest constant.
Chemical equilibrium (ICE tables): For a reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc = [C]^c[D]^d / [A]^a[B]^b. Setting up an ICE (Initial, Change, Equilibrium) table for each species is the fastest way to organize equilibrium calculations without losing track of the stoichiometry.
Le Chatelier’s principle: A system at equilibrium shifts to counteract any imposed change — increasing reactant concentration shifts equilibrium toward products, increasing pressure shifts equilibrium toward the side with fewer gas moles, and increasing temperature shifts equilibrium in the endothermic direction.
pH and pOH:
- pH = −log[H⁺]
- pOH = −log[OH⁻]
- pH + pOH = 14 (at 25°C)
- A pH below 7 is acidic, above 7 is basic, exactly 7 is neutral
Common Named Reactions Worth Knowing
- Esterification: Carboxylic acid + alcohol → ester + water (acid catalyst required)
- Saponification: Ester + base → alcohol + carboxylate salt (the reaction behind soap-making)
- Combustion: Hydrocarbon + O₂ → CO₂ + H₂O (complete combustion; incomplete combustion produces CO or soot)
- Neutralization: Acid + base → salt + water
Exam Technique Notes
- Always balance equations before submitting an answer — an otherwise correct answer loses marks for an unbalanced equation.
- State reaction conditions explicitly in organic chemistry answers (temperature, catalyst, solvent), since many reactions produce different products under different conditions.
- Use ICE tables for every equilibrium problem, even simple ones — the habit prevents sign and stoichiometry errors under exam time pressure.
- Double-check units in molarity and gas law problems — grams vs. moles and liters vs. milliliters are the most common sources of numerical errors.
Frequently Asked Questions
How do I predict the product of an unfamiliar organic reaction? Identify the reaction type (substitution, addition, or elimination) from the reagents given, then apply the general mechanism for that type rather than trying to recall the exact reaction from memory.
What’s the fastest way to memorize periodic trends? Don’t memorize the trends as separate facts — memorize just one underlying idea (effective nuclear charge and shielding), and derive atomic radius, ionization energy, and electronegativity trends from it whenever needed.
Why does my equilibrium calculation keep coming out wrong? Almost always a stoichiometry error in the ICE table — double-check that the “change” row uses the correct mole ratios from the balanced equation, not just a flat ±x for every species.
Is memorizing named reactions necessary? A handful of high-frequency ones (esterification, saponification, neutralization) are worth memorizing directly; less common named reactions are usually explained in the question itself.
How can I tell if a reaction is exothermic or endothermic just from its equation? If the equation doesn’t state an enthalpy value directly, look at bond energies: if more energy is released forming new bonds than was used breaking old ones, the reaction is exothermic; if breaking bonds requires more energy than is released forming new ones, it’s endothermic.
What’s the quickest way to check if an equation is balanced? Count the atoms of each element on both sides separately and compare — a quick tally catches most balancing errors faster than re-reading the whole equation as a block.