Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Water is bent, not straight — and that single fact is why life exists.”
A frequent error is treating hydrogen bonds as ordinary covalent bonds. In reality, a strong dipole attraction when H is bonded to N, O or F.
Memory trick: hydrogen bonding explains water's unusually high boiling point.
“Water is bent, not straight — and that single fact is why life exists.”
An ionic bond transfers electrons (metal + non-metal, forms ions); a covalent bond shares electrons (between non-metals).
“Water is bent, not straight — and that single fact is why life exists.”
A sigma bond forms by head-on overlap and is strong; a pi bond forms by sideways overlap of p-orbitals and is weaker.
“Water is bent, not straight — and that single fact is why life exists.”
A polar molecule has a net dipole (water); a non-polar one has none because its dipoles cancel (CO₂).
“Water is bent, not straight — and that single fact is why life exists.”
Its C=O bonds are polar, but the linear shape makes the dipoles cancel, so CO₂ is non-polar.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
The heat content at constant pressure; ΔH is negative for exothermic and positive for endothermic reactions.
Memory trick: heat released → ΔH negative; heat absorbed → ΔH positive.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
A frequent error is getting the sign of ΔH the wrong way round. In reality, the heat content at constant pressure; ΔH is negative for exothermic and positive for endothermic reactions.
Memory trick: heat released → ΔH negative; heat absorbed → ΔH positive.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
A measure of disorder or the number of ways energy can be arranged.
Memory trick: it's the total entropy (system + surroundings) that rises in a spontaneous change.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
A frequent error is thinking entropy always increases for the system alone. In reality, a measure of disorder or the number of ways energy can be arranged.
Memory trick: it's the total entropy (system + surroundings) that rises in a spontaneous change.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
A frequent error is judging spontaneity from ΔH alone. In reality, ΔG = ΔH − TΔS decides spontaneity — negative ΔG means the process is spontaneous.
Memory trick: an endothermic reaction can still be spontaneous if TΔS is large enough.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
A frequent error is thinking ΔH depends on the reaction path. In reality, the total enthalpy change is the same no matter the route taken.
Memory trick: enthalpy is a state function — add steps freely.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
Gibbs free energy change deciding spontaneity. Use it when constant temperature and pressure.
ΔG = ΔH − TΔS
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
Relating enthalpy and internal energy changes. Use it when ideal gases, Δn_g = change in moles of gas.
ΔH = ΔU + Δn_gRT
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
Enthalpy is about heat content/energy; entropy is about disorder — spontaneity balances the two through ΔG.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
Exothermic releases heat (ΔH<0, surroundings warm); endothermic absorbs heat (ΔH>0, surroundings cool).
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
The system is the reaction you study; the surroundings are everything else that can exchange heat/work with it.
“An instant cold-pack turns freezing because it steals heat — an endothermic reaction in your hand.”
Spontaneity is decided by ΔG, not ΔH — entropy and temperature matter too.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
A frequent error is thinking the reaction has stopped at equilibrium. In reality, the state where forward and reverse reactions occur at equal rates, so concentrations stay constant.
Memory trick: equilibrium is busy, not still — both directions keep going at equal speed.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
A frequent error is predicting the wrong direction of shift. In reality, if you disturb a system at equilibrium, it shifts to oppose the change.
Memory trick: add a reactant → shifts right; increase pressure → shifts to fewer gas moles.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
K compares products to reactants at equilibrium; a large K means products are favoured.
Memory trick: leave out pure solids and liquids — only gases and solutions count.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
A frequent error is including pure solids/liquids in the K expression. In reality, K compares products to reactants at equilibrium; a large K means products are favoured.
Memory trick: leave out pure solids and liquids — only gases and solutions count.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
A frequent error is believing pH can't go below 0 or above 14. In reality, pH = −log[H⁺]; below 7 is acidic, above 7 is basic, 7 is neutral at 25°C.
Memory trick: concentrated strong acids/bases really can give pH outside 0–14.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
Acidity of a solution. Use it when aqueous solution.
pH = −log[H⁺]
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
The ion-product of water at 25°C. Use it when aqueous solution at 25°C.
K_w = [H⁺][OH⁻] = 10⁻¹⁴