NEB Class 12 Chemistry common mistakes
NEB Class 12 Chemistry · Understand the traps and how to avoid them.
Writing the full electron configuration of a transition-metal ion without removing 4s electrons before 3d electrons, e.g. giving Fe2+ as [Ar]3d4 4s2.
Students memorise the aufbau filling order (4s before 3d) and apply it unchanged to cations, forgetting that once the 3d shell is occupied the 4s orbital lies higher in energy and is emptied first.
What to do instead
Build the neutral atom, then remove electrons from the highest-n shell (4s) first. Practise with Fe2+, Fe3+, Cr3+, Cu+, Mn2+ and check the resulting d-count against the known magnetic behaviour.
Balancing redox equations in acidic or basic medium by inspection, leaving H and O unbalanced or adding H2O on the wrong side.
Candidates try to balance everything at once instead of using the half-reaction method, and they forget that in basic medium each H+ must be neutralised by adding OH- to both sides.
What to do instead
Always use the four-step half-reaction method: balance atoms other than H and O, balance O with H2O, balance H with H+, then add OH- to both sides in basic medium. Verify charge balance on each half before combining, then cancel electrons.
Treating reversible reactions as if they go to completion, e.g. calculating product yield from the limiting reagent without using the equilibrium constant.
Stoichiometry problems from earlier years train students to assume complete reaction, and the distinction between the extent of reaction and the equilibrium position is not consolidated.
What to do instead
Before calculating, ask whether the reaction is quantitative or reversible. For reversible reactions set up an ICE table in terms of extent of reaction, substitute into Kc or Kp, and solve; only use limiting-reagent logic when K is very large or the question states completion.
Applying Le Chatelier's principle to changes in pressure by counting moles of all species, including solids and pure liquids, or by predicting a shift when the number of gaseous moles is equal on both sides.
Students rely on a vague rule of thumb ('pressure favours the smaller volume side') without restricting it to gaseous species, and they expect a shift even when there is no change in gas moles.
What to do instead
Count only gaseous moles when evaluating pressure changes. If Δn(gas) = 0, state explicitly that pressure has no effect on position (only on concentrations). Practise with N2O4/NO2, PCl5/PCl3 + Cl2 and the Haber process.
Confusing the order of a reaction with its molecularity, or writing a rate law from the overall stoichiometric equation rather than from the rate-determining step.
The coefficients of the balanced equation look like exponents, and textbooks often show simple examples where order happens to equal stoichiometry, reinforcing the error.
What to do instead
State the rule explicitly: order is experimental and comes from the rate law; molecularity applies only to an elementary step. For a proposed mechanism, derive the rate law from the slow step, treating intermediates by steady-state or pre-equilibrium approximation.
Drawing organic structures with wrong valency or ignoring stereochemistry, e.g. drawing a carbon with five bonds, or showing an SN1 product without recognising racemisation.
Students focus on naming and forget that structure drawing is a chemical claim; they also treat stereochemistry as decoration rather than a consequence of mechanism.
What to do instead
After drawing any structure, count bonds at every carbon and heteroatom. For each substitution or addition, decide the mechanism first (SN1/SN2, syn/anti addition) and then state the stereochemical outcome explicitly, including inversion, retention or racemisation.
Predicting the major product of an electrophilic aromatic substitution without considering the directing effect of the substituent already present.
Candidates memorise isolated reactions of benzene and then apply them to substituted rings, forgetting that -OH, -NH2, -CH3 are ortho/para directing while -NO2, -COOH, -SO3H are meta directing.
What to do instead
Before predicting, classify each existing substituent as activating or deactivating and ortho/para or meta directing. Then apply the rule and, where two substituents compete, let the more strongly activating group decide the position.
Describing a qualitative test without stating the specific observation, e.g. saying 'a precipitate forms' for the brown ring test or the Lassaigne sodium fusion test.
Students learn the name of the test but not the diagnostic colour, state or gas, so answers cannot be credited even when the correct reagent is chosen.
What to do instead
For every named test, memorise a fixed three-part answer: reagent, condition, and exact observation (colour, gas, ring, smell). Build a one-page table for the brown ring test, Lassaigne's test for N/S/halogens, Tollens', Fehling's and Baeyer's tests.
Reporting a titration or gravimetric result to more significant figures than the apparatus allows, or omitting units and the correct indicator choice.
Calculators return long decimals and students copy them, and the link between burette readability, indicator selection and the equivalence point is not practised.
What to do instead
Record burette readings to 0.05 cm3 and report the final concentration to the precision justified by the data. Always state units and justify the indicator by comparing its pH range with the equivalence-point pH calculated from the titration curve.
Writing a numerical answer to a thermodynamics or kinetics question without checking the sign convention, e.g. reporting ΔG as negative for a non-spontaneous process or ΔH as negative for an endothermic reaction.
Formulas are memorised without the physical meaning of the sign, and the conventions for q (system vs surroundings) and work are easily reversed under exam pressure.
What to do instead
After every calculation, interpret the sign in words: negative ΔH means heat released, positive ΔG means non-spontaneous at that temperature. Cross-check with the relation ΔG = ΔH - TΔS and confirm the sign of ΔS from the change in moles of gas.