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
“A chef’s oven timer ticks faster when the heat is turned up, not when more dough is placed inside.”
The rate constant k depends only on temperature and the presence of a catalyst; it is independent of reactant concentrations. Mistaking k for a concentration-dependent factor leads to wrong rate-law forms. Remember: k is temperature-only, concentration-free.
Memory trick: k is temperature-only, concentration-free.
“A mountain pass (Ea) decides how many cars can cross; a catalyst builds a shortcut, lowering the pass height but not the destination.”
Activation energy is the minimum energy barrier reactants must overcome to form products. A catalyst provides an alternative pathway with lower Ea, accelerating both forward and reverse reactions equally, without changing ΔH or the equilibrium position. The common error is to think a catalyst shifts equilibrium.
Memory trick: Catalyst = lower hill, same summit.
“A radioactive clock ticks the same 5-minute interval regardless of how many atoms remain, just like first-order decay.”
Half-life (t½) is the time for a reactant’s concentration to drop to half its initial value. For a first-order reaction t½ = 0.693/k, a constant independent of the starting concentration. The trap is applying a constant half-life to zero- or second-order reactions.
Memory trick: t½ const ⇒ first order.
“Assuming every marathon runner halves their speed every 10 km is like thinking all reactions share a constant half-life.”
Only first-order reactions have a concentration-independent half-life. For zero-order, t½ = [A]₀/2k; for second-order, t½ = 1/(k[A]₀). The mistake is treating t½ as universally constant. The exam pitfall is using t½ = 0.693/k for non-first-order cases.
Memory trick: Constant t½? → first order only.
“Just like a car’s engine runs faster when you press the accelerator, reaction rate constant k rises as temperature climbs.”
The Arrhenius equation k = A·e^(−Ea/RT) shows k grows with temperature because more molecules have energy ≥Ea. A is the frequency factor, assumed temperature-independent in basic form. Exam trap: keep Ea positive and don’t treat A as varying with T.
k = A·e^(−Ea/RT)
Memory trick: Higher T → smaller −Ea/RT → larger k.
“A radioactive isotope losing half its atoms every minute is like a first-order reaction’s half-life.”
For a first-order reaction, t½ = 0.693/k, derived from ln2 = k·t½. It holds only when rate = k·[A]. Trap: applying the same formula to zero- or second-order reactions gives wrong half-life.
t½ = 0.693/k
Memory trick: t½ = ln2/k ≈ 0.693/k.
“In a billiard game, the number of balls that collide in a single shot is the molecularity; the win-rate’s dependence on ball speed is the reaction order.”
Molecularity counts actual reactant molecules in an elementary step, always an integer. Reaction order is the sum of concentration exponents, found experimentally, and can be zero, fractional or negative. Trap: using stoichiometric coefficients as order for a complex mechanism.
Memory trick: Order = experiment, Molecularity = integer count.
“When you pour water, the flow speed changes with pipe width (rate), but the pump’s power rating (k) stays the same at a given temperature.”
Rate = k·[A]^m (or product of reactants), so it varies as concentrations change during the reaction. The rate constant k depends only on temperature (and catalyst) and is fixed for that condition. Trap: treating k as a function of concentration.
Memory trick: Rate ∝ k·[C]ⁿ, k fixed at T.
“A shortcut through a mountain (catalyst) lets hikers reach the summit faster without changing the hill’s height (equilibrium).”
A catalyst provides an alternate pathway with lower activation energy, raising k and speeding the reaction. It does not alter ΔG°; therefore the equilibrium constant K remains unchanged. Trap: assuming a catalyst shifts the position of equilibrium.
Memory trick: Catalyst ↑ rate, K unchanged.
“A recipe that calls for 2 cups flour and 1 cup sugar doesn’t guarantee the cake’s rise depends on 2 and 1; only an elementary step does.”
Reaction order is determined experimentally from rate data; it coincides with stoichiometric coefficients only for a single elementary step. For overall reactions, coefficients give no order information. Trap: blindly equating overall order to sum of coefficients.
Memory trick: Order = coeffs only if elementary.
“A household AA battery is a galvanic cell that powers a torch by spontaneous redox, while an electrolytic cell is like a charger that forces a reaction.”
In a galvanic (voltaic) cell the redox reaction is spontaneous (ΔG < 0) and produces a positive cell voltage; it delivers electrical work. In an electrolytic cell the reaction is non-spontaneous (ΔG > 0) and needs an external voltage, giving a negative cell voltage. The key: galvanic gives energy, electrolytic consumes it.
Memory trick: Galvanic = gives, Electrolytic = takes
“Students often think a rechargeable battery works like a charger; actually the charging mode is electrolytic, the discharging mode is galvanic.”
The frequent mistake is to label a rechargeable cell always as galvanic. During discharge it behaves as a galvanic cell (spontaneous, positive E°). When charging, the same cell operates as an electrolytic cell (non-spontaneous, external voltage required). Direction of electron flow decides the type.
Memory trick: Discharge = galvanic; Charge = electrolytic
“In a zinc-carbon flashlight, the zinc outer case slowly disappears because it is the anode where oxidation occurs.”
Oxidation always occurs at the anode and reduction at the cathode. In a galvanic cell the anode is negative and the cathode positive; in an electrolytic cell the signs are reversed. Assuming the anode is always positive gives the wrong sign for Ecell.
Memory trick: Ox @ Anode, Red @ Cathode – O-A, R-C
“When a copper strip and a silver strip sit in salty water, electrons flow to silver because silver has the higher reduction potential.”
Electrode potential is the tendency of an electrode to gain electrons. Cell emf = Ecathode − Eanode; a positive emf means the overall reaction is spontaneous. A frequent trap is reversing the subtraction or adding the two potentials.
Memory trick: Ecell = Ecath – Eanod; + → spontaneous
“If you look up the standard reduction table, you must always write both half-reactions as reductions before subtracting.”
Both half-reactions must be written as reductions; then Ecell = Ecath – Eanod. If you reverse a half-reaction you must change the sign of its potential. Skipping this sign change yields an incorrect EMF.
Memory trick: Keep both as reductions; flip sign when you reverse a half-reaction.
“A pH-meter uses the Nernst equation to turn the H⁺ concentration of a solution into a voltage reading.”
The Nernst equation corrects the standard potential for actual concentrations: E = E° – (0.0592/n) log Q at 25 °C. Using E° alone when concentrations differ gives a wrong cell voltage.
Memory trick: E = E° – (0.059/n) log Q; 0.059 ≈ 0.0592 at 25 °C.
“Measuring a Daniell cell with a voltmeter gives the voltage equal to the copper electrode’s potential minus the zinc electrode’s.”
Ecell = Ecath – Eanod gives the standard EMF when all species are at 1 M, 1 atm and 25 °C. It is derived from tabulated reduction potentials. Remember to use reduction values for both electrodes before subtracting.
E_cell = E_cathode − E_anode
Memory trick: Ecell = Ecath – Eanod; both from reduction table.
“The ‘12 V, 2 Ah’ label on a battery tells you the free-energy change is ΔG = –nF·E, the energy you can draw out.”
ΔG = –nF E links the Gibbs free energy change to the cell potential; n is the number of electrons transferred, F = 96485 C mol⁻¹. A negative ΔG indicates a spontaneous cell; using the wrong sign flips the conclusion.
ΔG = −nFE
Memory trick: ΔG = –nF·E; negative means spontaneous.
“A flashlight’s battery delivering light is a galvanic cell, while plugging it into a charger makes it work as an electrolytic cell.”
In a galvanic cell the redox reaction is spontaneous (ΔG < 0) and the cell delivers an emf; electrons flow from the oxidation-site anode to the reduction-site cathode. In an electrolytic cell an external source forces a non-spontaneous reaction (ΔG > 0); the anode becomes positive. Trap: the anode always hosts oxidation, but its polarity reverses between the two cells.
Memory trick: Galvanic = gives; Electrolytic = takes
“In a Daniell cell zinc metal dissolves at the zinc electrode – that’s the anode, even though it’s the negative pole.”
Oxidation always occurs at the anode and reduction always at the cathode, no matter if the cell is galvanic or electrolytic. Only the electrode’s polarity changes between the two types. Trap: mixing up the sign of the electrode with the redox process.
Memory trick: A = Oxidation, C = Reduction (O→A, R→C).
“When you charge a rechargeable battery, the charger’s positive lead attaches to the cell’s anode, making it positive.”
Oxidation always occurs at the anode, but its charge sign depends on the cell type. In a galvanic cell the anode is negative because it supplies electrons; in an electrolytic cell the anode is positive because the external source removes electrons. Trap: never label anode as positive or negative before knowing whether the cell delivers or consumes electricity.
Memory trick: AnOx = Anode Oxidation, polarity flips with cell type.
“Like a hot metal rod warming the piece it touches, a more electronegative atom pulls electron density through σ bonds.”
The inductive effect is the permanent shift of electron density through σ bonds caused by differences in electronegativity. It withdraws or donates electrons but drops sharply with each additional bond, becoming negligible beyond three bonds. Do not assume it acts over long distances.
Memory trick: I-effect fades 1-bond > 2-bond > 3-bond
“In the telephone game, the message blurs after a few whispers, just as inductive influence fades down a carbon chain.”
A common mistake is treating the inductive effect as unchanged along a chain. In reality the effect diminishes rapidly; only atoms within two-three bonds feel a noticeable shift. Applying I-effect to remote groups in acidity or reactivity questions leads to wrong answers.
Memory trick: I-effect: strong near, weak far
“A resonance hybrid is like a photo made from several exposures, not a picture that flips between each shot.”
Resonance structures are not real intermediates; the actual molecule is a hybrid where π electrons are delocalised over several atoms, giving extra stability. Use a double-headed arrow (↔) between forms; never draw a single-arrow as if a reaction occurs.
Memory trick: Resonance = ↔, not →