Disproportionation

“Chlorine in water both gains and loses electrons at once.”

A single species is simultaneously oxidised and reduced.

Memory trick: Self oxidation + reduction.

Standard enthalpy of formation

“It's the reference book from which reaction enthalpies are computed.”

The enthalpy change when one mole of a compound forms from its elements in standard states.

Memory trick: Elements → 1 mole compound.

Spontaneity and temperature

“Ice melts spontaneously above 0°C but not below — temperature tips ΔG.”

Whether a reaction is spontaneous can flip with temperature, since ΔG = ΔH − TΔS.

Memory trick: T can flip spontaneity.

Electrolytic refining

“Copper wires are refined to 99.99% purity this way.”

Impure metal is purified by electrolysis, depositing pure metal on the cathode.

Memory trick: Pure metal plates the cathode.

Vulcanisation of rubber

“Goodyear's accidental discovery gave us durable car tyres.”

Heating rubber with sulphur cross-links the chains, making it tough and elastic.

Memory trick: Sulphur cross-links rubber.

Biodegradable polymers

“Green plastics that rot away instead of choking landfills.”

Polymers like PHBV break down naturally, easing plastic pollution.

Memory trick: Break down naturally.

Antiseptics vs disinfectants

“Dettol on a cut is antiseptic; floor cleaner is disinfectant.”

Antiseptics are safe on living tissue; disinfectants are for inanimate surfaces.

Memory trick: Skin-safe vs surface-only.

Artificial sweeteners

“They trick the tongue without feeding the waistline.”

Compounds like aspartame sweeten food with negligible calories.

Memory trick: Sweet taste, few calories.

Food preservatives

“They quietly extend the shelf life of your pantry.”

Substances like sodium benzoate slow microbial spoilage of food.

Memory trick: Slow spoilage.

Analgesics

“Aspirin also thins blood, protecting hearts.”

Painkillers such as aspirin and paracetamol relieve pain.

Memory trick: Relieve pain.

Molar volume at STP

“Count any gas by volume — 22.4 L is one mole of it.”

One mole of any ideal gas occupies 22.4 L at STP (0°C, 1 atm).

22.4 L/mol

Memory trick: 22.4 L per mole at STP.

Faraday constant

“It's the exchange rate between moles of electrons and coulombs.”

The charge carried by one mole of electrons, about 96500 C.

F ≈ 96500 C

Memory trick: ~96500 C per mole e⁻.

Effective nuclear charge

“Inner electrons act as a screen softening the nucleus's pull.”

The net positive pull felt by an outer electron after inner electrons shield it.

Memory trick: Actual pull after shielding.

Electron affinity

“Chlorine is greedy for that extra electron, releasing lots of energy.”

The energy change when a gaseous atom gains an electron; halogens release the most.

Memory trick: Halogens love electrons.

Elevation of boiling point

“Salted water boils a touch hotter than pure water.”

Dissolving a solute raises a solvent's boiling point (a colligative property).

Memory trick: Solute raises boiling point.

Depression of freezing point

“Salt melts icy roads by dropping water's freezing point.”

A dissolved solute lowers a solvent's freezing point.

Memory trick: Solute lowers freezing point.

Henderson–Hasselbalch equation

“It's the recipe for mixing a buffer at a target pH.”

Relates a buffer's pH to the acid's pKa and the salt-to-acid ratio.

Memory trick: pH = pKa + log(salt/acid).

Degree of dissociation

“Weak acids dissociate only a little; strong acids fully.”

The fraction of a substance that has ionised in solution.

Memory trick: Fraction ionised.

The mole

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

A counting unit for particles — one mole is 6.022×10²³ (Avogadro's number) of anything.

Memory trick: think of a mole like a 'dozen' — just a fixed count of particles.

The mole — common mistake

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

A frequent error is confusing number of moles with number of grams or molecules. In reality, a counting unit for particles — one mole is 6.022×10²³ (Avogadro's number) of anything.

Memory trick: think of a mole like a 'dozen' — just a fixed count of particles.

Molar mass — common mistake

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

A frequent error is using atomic number instead of atomic mass to find molar mass. In reality, the mass of one mole of a substance in grams, numerically equal to its atomic/molecular mass.

Memory trick: moles = given mass ÷ molar mass.

Limiting reagent — common mistake

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

A frequent error is assuming the reactant with the smaller mass is limiting. In reality, the reactant that runs out first and therefore caps how much product forms.

Memory trick: convert each reactant to moles, divide by its coefficient — the smallest wins.

Concentration terms

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

Molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent.

Memory trick: molarity changes with temperature (volume expands); molality doesn't.

Concentration terms — common mistake

“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”

A frequent error is thinking molarity and molality are interchangeable. In reality, molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent.

Memory trick: molarity changes with temperature (volume expands); molality doesn't.