Tamil Nadu HSE Second Year Chemistry common mistakes
Tamil Nadu HSE Second Year Chemistry · Understand the traps and how to avoid them.
When naming or drawing a coordination compound, students write the ligands in the order the formula appears (e.g., [Co(NH3)4Cl2]Cl as 'tetraamminedichloridocobalt(III) chloride' but then place chloro before ammine in the name, or reverse the metal-ligand order when converting a name back to a formula).
The formula and the IUPAC name follow different ordering rules: in the name, ligands are listed alphabetically (ammine before chlorido), while in the formula the metal is written first and ligand order is conventional. Students memorize names as a single string without learning the alphabetisation step, and Tamil-medium candidates often translate ligand prefixes loosely, losing the ordering cue.
What to do instead
For every complex, build the name in three fixed passes: (1) list ligands alphabetically ignoring prefixes di/tri/tetra, (2) add the metal with its oxidation state in Roman numerals, (3) add the counter-ion as 'ate' if the complex is anionic. Practise 10 conversions in both directions and check the alphabetical order aloud before writing.
Students calculate the oxidation state of the metal in a complex by adding all ligand charges as positive or by ignoring the overall charge of the complex ion (e.g., assigning Fe as +2 in [Fe(CN)6]3-).
The rule 'sum of oxidation states equals the charge on the species' is known verbally but not applied as an equation. Students also confuse the charge on the complex with the charge on the counter-ion, and CN- is often treated as neutral because it is written without a visible charge in the formula.
What to do instead
Always write an explicit algebraic equation: x + (number of ligands)(ligand charge) = overall charge. Memorise common ligand charges (CN-, Cl-, OH-, NO2- = -1; NH3, H2O, CO = 0; oxalate = -2). Do five complexes per session and verify by checking that the Roman numeral in the name matches the computed x.
Students predict the number of unpaired electrons and magnetic behaviour of a transition-metal ion from its free-ion d-electron count without applying crystal-field splitting and the spectrochemical series, so they call [Fe(CN)6]3- paramagnetic with five unpaired electrons.
The distinction between weak-field and strong-field ligands is taught as a list, but students do not connect it to whether Δo exceeds the pairing energy. They also forget that CN- is a strong-field ligand that forces low-spin configuration, and they do not draw the split d-orbital diagram before counting.
What to do instead
For each complex, draw the t2g/eg diagram first, place electrons according to Δo versus pairing energy, then count unpaired electrons and compute the spin-only moment √(n(n+2)) BM. Learn the series I- < Br- < Cl- < F- < H2O < NH3 < en < CN- < CO and mark strong-field ligands before filling.
In ionic equilibrium, students substitute concentrations directly into the Henderson–Hasselbalch or buffer equation without checking whether the acid or base is weak, or they use the initial concentration of a weak acid as [H+] without solving the equilibrium expression.
Textbook problems often use simplified formulas, and students apply them mechanically to strong acids, or to very dilute solutions where the approximation fails. They also forget that Ka and Kb are linked by Kw and mix up pKa with pH.
What to do instead
Before substituting, classify each species as strong or weak and write the equilibrium table. Use [H+] = √(Ka·C) only when C/Ka > 100 and state that condition. For buffers, verify the ratio of conjugate base to acid and use pH = pKa + log([salt]/[acid]). Cross-check the answer lies within a plausible pH range (0–14).
In chemical kinetics, students confuse order with molecularity, or they calculate the rate constant using concentrations in the wrong units or forget that the half-life of a first-order reaction is independent of initial concentration.
Order is experimental while molecularity is theoretical, and textbooks present both near each other. Students also memorise t1/2 = 0.693/k for first order but then apply it to second-order problems, and unit conversions (mol L-1 s-1 versus min-1) are frequently skipped.
What to do instead
Write the rate law from data, not from the balanced equation. State the order explicitly, then select the correct integrated form and half-life expression. Always carry units through the calculation and check that k has units consistent with the order (s-1 for first order, L mol-1 s-1 for second).
In electrochemistry, students balance redox equations in acidic medium but forget to add H2O and H+ to balance oxygen and hydrogen, or they reverse the sign of the cell potential when writing the anode half-reaction.
The half-reaction method has several bookkeeping steps that are easy to drop under exam pressure. Students also treat E° as an intensive property incorrectly, multiplying it when multiplying a half-reaction, or they confuse E°cell with the sign convention for spontaneity.
What to do instead
Use a fixed five-step checklist: balance atoms other than O and H, balance O with H2O, balance H with H+, balance charge with electrons, then equalise electrons and add. Never multiply E° values; only ΔG° scales with n. Confirm spontaneity by checking E°cell > 0 and ΔG° < 0.
In solid state, students calculate the packing efficiency or density of a unit cell using the wrong number of atoms per cell (e.g., using Z = 1 for FCC or Z = 4 for BCC) or they use the atomic radius instead of the edge length in the density formula.
The relation between Z, edge length a, and radius r differs for simple cubic, BCC, and FCC, and students memorise the density formula without the geometry. They also forget that for FCC, 4r = a√2, and for BCC, 4r = a√3.
What to do instead
Draw the unit cell and mark where atoms touch before choosing a relation. Memorise Z (SC = 1, BCC = 2, FCC = 4) and the radius–edge relations together as a set. Substitute into ρ = ZM/(NA·a3) with a in cm, and check that the computed density is of the order of a few g cm-3.
In thermodynamics, students mix up state functions with path functions, or they apply ΔH = ΔU + ΔnRT with the wrong sign or wrong Δn (counting only gases on one side).
The distinction between state and path functions is conceptual, and students memorise examples without understanding why. In ΔnRT, only gaseous species count, and students often include solids or liquids, or reverse the sign of Δn.
What to do instead
Classify each quantity (U, H, S, G = state; q, w = path) before solving. For ΔH = ΔU + ΔnRT, list gaseous moles of products minus gaseous moles of reactants explicitly. Check sign conventions: work done on the system is positive in the IUPAC convention, and verify that an exothermic reaction gives ΔH < 0.
In haloalkanes and haloarenes, students predict SN1 versus SN2 reactivity by the size of the halogen rather than by substrate class and steric hindrance, or they claim aryl halides undergo easy nucleophilic substitution.
The reactivity order for SN1 (3° > 2° > 1°) and SN2 (1° > 2° > 3°) is memorised as strings without the underlying carbocation stability and steric arguments. Aryl halides are drawn like alkyl halides, so students miss the partial double-bond character of the C–X bond due to resonance.
What to do instead
For each substrate, decide first whether it is alkyl, allylic/benzylic, or aryl/vinyl. Then apply: SN1 needs a stable carbocation (3° > 2° > 1°), SN2 needs an accessible backside (1° > 2° > 3°). For aryl halides, state explicitly that resonance with the ring strengthens the C–X bond and blocks substitution unless electron-withdrawing groups activate the ring.
In metallurgy, students write the wrong reducing agent or the wrong equation for a named process (e.g., using carbon to reduce alumina in the Hall–Héroult process, or writing the blast-furnace reactions for haematite with incorrect stoichiometry).
Metallurgy is taught as a set of named processes with many equations, and students memorise them as isolated facts without linking each step to the reactivity of the metal. They also confuse calcination with roasting and forget that highly reactive metals require electrolytic reduction.
What to do instead
Organise metallurgy by the reactivity series: electrolytic reduction for K, Na, Ca, Mg, Al; carbon reduction for Zn, Fe, Pb; self-reduction or displacement for less reactive metals. For each named process, write the balanced equation and label the reducing agent. Distinguish calcination (carbonate ore, limited air) from roasting (sulphide ore, excess air) with one example each.