Cannizzaro reaction

“In a lab, adding concentrated NaOH to formaldehyde (no α-H) instantly yields methanol and formate, just like the base that saponifies fats.”

Aldehydes that lack α-hydrogen atoms undergo the Cannizzaro reaction in strong base. OH⁻ adds to the carbonyl, forming a tetrahedral alkoxide; a hydride is transferred to a second aldehyde, producing an alcohol and a carboxylate ion. The trap: if any α-hydrogen is present the substrate follows aldol condensation, not Cannizzaro.

Memory trick: No α-H → Cannizzaro; α-H present → aldol.

Basicity of amines

“Adding HCl to an amine is like a lock catching a key, forming an ammonium salt that stays in solution.”

Amines act as bases because the nitrogen lone pair can accept a proton, forming R₃NH⁺. Electron-donating alkyl groups raise basicity (+I effect), while aromatic resonance withdraws electron density, lowering it. The exam trap: compare pKa of the conjugate acid, not pKb, and note tertiary aliphatic amines are strongest, aromatic amines are weakest.

Memory trick: More alkyl = stronger base (↑+I)

Diazonium salts

“Aryl diazonium salts are like chilled fireworks; kept at 0-5 °C they sit still, but a gentle warm-up makes them burst, losing N₂ and forming a new aromatic product.”

Diazonium salts are prepared at 0–5 °C in strong acid (NaNO₂/HCl) and are stable only in this cold, acidic medium. On warming they lose N₂; the resulting product depends on the added reagent (CuCl → chlorobenzene, CuBr → bromobenzene, H₂O → phenol). Assuming they always give phenol or bromide on heating is wrong—Sandmeyer reagents are required.

Memory trick: Diazo = Cold, N₂ flies; add Cu-X for X-substituted benzene.

Amino acids

“Proline is like a hinge in a paperclip; its nitrogen is part of a ring, not a free –NH₂.”

The 20 standard amino acids have a central α-carbon with –COOH and –NH₂, but proline’s –NH is secondary, forming a pyrrolidine ring, so it lacks a free –NH₂. Remember this when drawing structures or counting –NH₂ groups. Exam trap: assuming every amino acid has a primary –NH₂ gives a wrong structure for proline.

Memory trick: Pro-line = ‘pro’ in a ring, no free NH₂.

Nucleic acids

“DNA is the library, and each nucleotide is a book containing a code for life.”

Nucleic acids are polymers of nucleotides, each consisting of a phosphate, a pentose sugar and a nitrogenous base. DNA’s deoxyribose lacks a 2′-OH, making it stable; RNA’s ribose has a 2′-OH, rendering it more reactive. Exams test the sugar difference and its impact on stability.

Memory trick: DNA = Deoxy, no 2′ OH; RNA = Ribose, has 2′ OH.

Essential vs non-essential amino acids

“When you bake a cake, the flour you add is like essential amino acids – you must supply them, the body can’t make them.”

Essential amino acids must be obtained from the diet; the body synthesises all other amino acids. For adults the nine are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine and lysine. A common trap is counting arginine as essential or forgetting histidine.

Memory trick: PVT TIM H L K

Soaps vs detergents

“When you wash a greasy spoon in hard water, the soap forms scum while detergent keeps it sparkling.”

Soap is the sodium or potassium salt of a fatty acid. In hard water Ca²⁺/Mg²⁺ precipitate with soap forming insoluble scum, reducing its cleaning power. Detergents contain sulfonate or sulfate groups that stay soluble, so they work even with Ca²⁺/Mg²⁺. Exam trap: assuming any surfactant works in hard water; remember only detergents avoid scum.

Memory trick: Soap + Ca = scum; Detergent ≠ scum

How soap cleans

“A soap molecule is like a tiny magnet, one pole pulls water, the other pulls oil, lifting dirt into the rinse.”

A soap molecule has a polar head that hydrogen-bonds with water and a non-polar tail that dissolves grease. Above the critical micelle concentration the tails cluster, trapping oil droplets inside micelles that are washed away. Trap: forgetting the need for micelle formation; cleaning fails below CMC.

Memory trick: Head H2O, tail oil → micelle

Antacids

“After a spicy curry, you sip an antacid tablet that fizzles, neutralizing the burning acid in your stomach.”

Antacids are basic salts such as Mg(OH)₂, CaCO₃ or Al(OH)₃ that react with excess H⁺ in gastric juice: H⁺ + OH⁻ → H₂O, raising pH and relieving acidity. They act locally in the stomach and do not alter systemic pH. Trap: confusing antacids with acid-suppressing drugs that act by inhibiting secretion.

Memory trick: Base + H⁺ → H₂O, less burn

Antibiotics

“A penicillin capsule stops a bacterial infection but does nothing against the flu virus you caught.”

Antibiotics are agents that either kill bacteria (bactericidal) or stop their growth (bacteriostatic) by targeting cell-wall synthesis, protein synthesis, etc. Viruses lack these structures, so antibiotics have no effect on viral infections. Trap: prescribing antibiotics for colds; resistance develops.

Memory trick: Bacteria → kill; Virus → no effect

Catalysts in industry

“In the Haber plant, iron filings act like a busy highway toll, letting nitrogen and hydrogen zip through to form ammonia faster.”

Iron (often promoted with K₂O, Al₂O₃) provides a surface where N₂ and H₂ adsorb, lowering the activation energy for N≡N bond breakage, thus increasing the rate without being consumed. Catalysts speed up reaching equilibrium but do not shift the equilibrium position. Trap: assuming a catalyst changes the equilibrium composition is wrong.

Memory trick: Catalyst = speed-up, not equilibrium shift.

Acid-base: Arrhenius, Bronsted, Lewis

“Squeezing lemon into water releases H⁺ – an Arrhenius acid that also donates a proton, satisfying the Brønsted definition.”

Arrhenius acids increase [H⁺] in water; Brønsted acids are proton donors; Lewis acids accept electron pairs. Every Arrhenius acid is a Brønsted acid, but many Brønsted acids work outside aqueous media. Lewis is the broadest definition, yet not all Lewis acids are Brønsted.

Memory trick: Arrhenius = H⁺ in water; Brønsted = proton donor; Lewis = electron-pair acceptor.

Oxidation number rules

“In water (H₂O), hydrogen is +1 and oxygen –2, together giving a neutral molecule.”

Free elements have oxidation number 0; oxygen is usually –2 (except peroxides and OF₂); hydrogen is +1 (except metal hydrides). The sum of oxidation numbers equals the ion’s charge. Forgetting the exceptions (e.g., O in OF₂) is a frequent error.

Memory trick: O –2, H +1, free 0; sum = charge

Percentage yield

“If a lab synthesis should give 80 g of aspirin but you collect 60 g, the percentage yield is 75 % – the efficiency of the experiment.”

% yield = (actual yield ÷ theoretical yield) ×100. Reflects losses and side reactions. Trap: using mass for one side and moles for the other without proper conversion.

Memory trick: Yield % = (real ÷ theoretical)×100

Concentration: ppm

“Dissolving 1 g of salt in 1 m³ (1000 L) of water gives a concentration of 1 ppm.”

ppm means one part solute per 10⁶ parts of solution by mass. For water, 1 ppm ≈ 1 mg of solute per litre because 1 L ≈ 1 kg. The common mistake is treating ppm as a volume fraction or using it for non-aqueous media without converting mass to volume.

Memory trick: ppm = mg / L (water)

Law of conservation of mass

“In a sealed pressure cooker, the total mass of ingredients before cooking equals the mass of steam and food after.”

In any closed chemical reaction the total mass of reactants equals the total mass of products because atoms are neither created nor destroyed. This is the law of conservation of mass. In open systems mass appears to change if gases escape, so always consider a closed vessel.

Memory trick: Closed system → mass in = mass out

Law of definite proportions

“Table salt always contains sodium and chlorine in the same 23:35 mass ratio, no matter where it’s mined.”

A pure compound always contains its constituent elements in a fixed mass ratio, independent of the source or amount. This is the law of definite proportions. If the observed ratio varies, the sample is a mixture, not a single compound.

Memory trick: Pure compound = fixed % by mass

Accuracy vs precision

“On a dartboard, hitting the bullseye once is accurate; hitting the same spot many times shows precision.”

Accuracy measures how close a result is to the true value; precision measures the repeatability of results. High accuracy with low precision gives scattered values around the true value, while high precision with low accuracy clusters away from it. Exams often swap the terms.

Memory trick: Bullseye once = accurate; cluster = precise

Stoichiometry

“When you make lemonade, 2 lemons plus 1 cup sugar always yields 3 cups drink – the ratio is fixed by the balanced recipe.”

Stoichiometry uses a balanced chemical equation to relate the mole amounts of reactants and products via their coefficients. After balancing, the ratio of coefficients gives the quantitative relationship. A common trap is to use the equation before it is balanced.

Memory trick: Balance → Ratio → Amount

Osmosis

“Plant roots pull water from dry soil through membranes toward the salty cell interior.”

Osmosis: solvent moves from lower to higher solute concentration across a semipermeable membrane, creating osmotic pressure π = i·M·R·T. The flow stops only when an external pressure equal to π is applied.

Memory trick: Water → higher solute side (π = iMRT)

Osmotic pressure

“Raisins left in water swell as water rushes in, and the pressure that stops further inflow is the osmotic pressure.”

Osmotic pressure is the hydrostatic pressure needed to halt net solvent flow through a semipermeable membrane. For an ideal dilute solution π = i C R T, with i the van’t Hoff factor (i=1 for non-electrolytes). Forgetting i for salts or using the formula for concentrated solutions gives wrong results.

π = CRT

Memory trick: π = i·C·R·T – i counts particles

Van't Hoff factor

“When you stir NaCl into water, it instantly splits into Na⁺ and Cl⁻, doubling the number of particles.”

The van’t Hoff factor i equals the actual number of particles produced per formula unit in solution. For strong electrolytes i ≈ number of ions (NaCl →2). For weak electrolytes i < theoretical because of incomplete dissociation; for nonelectrolytes i = 1.

Memory trick: i = particles ÷ formula units

Molar conductivity

“When you add a pinch of salt to water, the solution conducts better per mole of salt as it gets more diluted, like spreading traffic over wider lanes.”

Molar conductivity Λm = κ·1000/c (κ = specific conductivity, c = mol L⁻¹). As concentration decreases, ion mobility rises, so Λm increases, even though κ itself falls. The trap: confusing rise of Λm with rise of κ on dilution.

Memory trick: Λm up, κ down on dilution

Standard hydrogen electrode

“When you zero a kitchen scale before weighing, you’re setting the same reference point the SHE uses for all electrode potentials.”

The Standard Hydrogen Electrode is the half-cell H₂(g, 1 atm)│Pt│H⁺(aq, 1 M). By definition its potential is 0.00 V at all temperatures, so every other electrode potential is measured relative to it. Remember: the 0 V is assigned, not experimentally obtained.

Memory trick: SHE = H2|H+ at 1 atm/1 M → 0 V