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
“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 →
“A nucleophile is the key that fits into the electrophile’s lock, donating its electron pair.”
Electrophiles are electron-deficient species that accept a pair of electrons; nucleophiles are electron-rich species that donate a pair. The classification depends on electron flow, not just charge—large, sterically hindered anions may be poor nucleophiles despite being negative.
Memory trick: Nu = Nucleus-loving = donor; Ele = electron-eager = acceptor
“Adding OH⁻ to a sugar’s carbonyl is like a magnet (the carbon) pulling the electron-rich OH⁻ toward it.”
Electrophiles are electron-poor species that accept a lone-pair (Lewis acids); nucleophiles are electron-rich species that donate a lone-pair (Lewis bases). The definition is based on the direction of electron flow in bond formation, not on redox behavior. A common trap is labeling any oxidizing ion as an electrophile—only species seeking electrons to form a covalent bond qualify.
Memory trick: Nuc-donates, Elec-accepts
“A benzyl carbocation spreads its positive charge over the aromatic ring, like a ripple making even a primary carbon feel safe.”
For simple alkyl carbocations, stability follows 3° > 2° > 1° because more alkyl groups give hyper-conjugation and inductive donation. Yet resonance delocalisation (allylic, benzylic) can override this order; a primary resonance-stabilised carbocation is often more stable than a tertiary alkyl one. Trap: ranking only by substitution and ignoring resonance effects.
Memory trick: Resonance beats alkyl: R > 3°
“A wire (σ bond) transmits a short electric pulse that quickly fades, like inductive effect, while a stretched rope (π system) passes a wave far along, like resonance.”
Inductive effect travels through σ bonds and diminishes rapidly with each bond; resonance delocalizes charge through a conjugated π system over several atoms. Trap: applying resonance reasoning to saturated chains where only inductive effects operate.
Memory trick: I-effect = short-range σ; R-effect = long-range π.
“A two-step dance where the lead leaves the floor first (carbocation) before the partner joins – SN1; versus a single quick spin where the partner slides in from behind – SN2.”
SN1 proceeds via a rate-determining carbocation, favored by 3° substrates, polar protic solvents, and gives racemization. SN2 is a concerted backside attack, favored by primary substrates, strong nucleophiles, polar aprotic solvents, and inverts configuration. Trap: forgetting SN2 rate depends on both substrate and nucleophile concentration.
Memory trick: SN1 = 2 steps, 1st order; SN2 = 1 step, 2nd order.
“A kaleidoscope shows many patterns, but the glass never flips between them; the molecule exists as one hybrid pattern, not jumping between drawings.”
Resonance structures are not real isomers that interconvert; they are contributors to a single resonance hybrid that represents the true electron distribution. The hybrid is more stable than any individual form. Trap: drawing only one resonance form as the actual structure.
Memory trick: Resonance = hybrid, not flipping pictures.
“Like a fully packed train carriage, an alkane has all seats (bonds) occupied with single links.”
Alkanes contain only C–C single bonds, so they are called saturated. Alkenes (C=C) and alkynes (C≡C) have double or triple bonds and are unsaturated, making them more reactive in addition reactions. The exam trap: don’t equate ‘saturated’ with ‘non-reactive’; it only describes bond type.
Memory trick: Saturated = Single bonds Only; Unsaturated = Double/Triple.
“Just as a cracked egg shell (double bond) breaks easily, unsaturated hydrocarbons react faster than the solid alkanes.”
Unsaturated hydrocarbons possess π bonds, which are weaker and lie above the σ framework, so they react more readily than saturated alkanes. A common mistake is to think ‘unsaturated’ means less reactive. Remember: hydrogenation of alkenes/alkynes occurs, alkanes stay unchanged.
Memory trick: π bonds = reaction hot spots.
“Adding HBr to propene is like a thirsty sponge: the carbon with more H (the less-substituted end) grabs the H first.”
In electrophilic addition of HX to an unsymmetrical alkene, H adds to the carbon that already has more hydrogens (the less-substituted carbon). The halide then bonds to the more-substituted carbon, giving the more stable carbocation. Trap: with peroxides HBr follows anti-Markovnikov, reversing the rule.
Memory trick: H-rich carbon gets H; X-rich carbon gets X.
“Benzene’s ring is like a perfectly balanced carousel, each horse (p orbital) sharing its seat, giving extra stability.”
Aromatic compounds are cyclic, planar, fully conjugated rings with (4n+2) π electrons (Hückel rule). This delocalisation gives extra stability, as seen in benzene. Exam nuance: 4n+2, not 4n, and the ring must be completely conjugated.
Memory trick: Hückel: 4n+2 π electrons = aromatic.
“Trying to add H₂ to benzene is like trying to add a seat to a full carousel—it breaks the perfect ride.”
Benzene does not undergo addition like alkenes because addition would break its aromatic π cloud and lose stability. Instead it reacts by electrophilic substitution, preserving the aromatic system. Mistake: treating benzene as a simple alkene; always check aromaticity first.
Memory trick: Add? No – substitute to keep aromatic.
“An alkane is a straight road with only single lanes; an alkene inserts a double lane, changing traffic rules.”
Alkanes have only single C–C bonds (CₙH₂ₙ₊₂) and react mainly by substitution. Alkenes contain a C=C double bond (CₙH₂ₙ) and undergo addition reactions. Trap: use the molecular formula to identify the functional group before choosing the reaction type.
Memory trick: CₙH₂ₙ₊₂ → single lane; CₙH₂ₙ → double lane.