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.
1,200 fundamentals
“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
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
K_c uses molar concentrations; K_p uses partial pressures — they're related by K_p = K_c(RT)^Δn for gases.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
A strong acid ionises completely (HCl); a weak acid ionises only partly (CH₃COOH) and has an equilibrium.
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
Heating an exothermic reaction shifts it backward (toward reactants); heating an endothermic one shifts it forward (toward products).
“Your blood holds its pH steady with buffers — Le Chatelier keeping you alive.”
It means equal forward/reverse RATES, not equal amounts of reactants and products.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
A frequent error is mixing up which process loses and which gains electrons. In reality, oxidation is loss of electrons and reduction is gain of electrons (remember OIL RIG).
Memory trick: OIL RIG — Oxidation Is Loss, Reduction Is Gain.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
A bookkeeping charge assigned by rules to track electron transfer.
Memory trick: free elements always have oxidation number 0.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
A frequent error is forgetting that oxidation number can be fractional or zero (e.g. in O₂, it's 0). In reality, a bookkeeping charge assigned by rules to track electron transfer.
Memory trick: free elements always have oxidation number 0.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
The oxidising agent gets reduced (gains electrons); the reducing agent gets oxidised (loses electrons).
Memory trick: the agent does the opposite to itself — an oxidising agent is itself reduced.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
A frequent error is calling the substance that is oxidised the 'oxidising agent'. In reality, the oxidising agent gets reduced (gains electrons); the reducing agent gets oxidised (loses electrons).
Memory trick: the agent does the opposite to itself — an oxidising agent is itself reduced.
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
Oxidation is loss of electrons (oxidation number rises); reduction is gain of electrons (oxidation number falls).
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
An oxidising agent accepts electrons (and is reduced); a reducing agent donates electrons (and is oxidised).
“A cut apple browning and iron rusting are the same story: oxidation in slow motion.”
The oxidising agent is itself reduced and the reducing agent is itself oxidised — name by what they DO to the other species.