LC Physics and Chemistry common mistakes
LC Physics and Chemistry · Understand the traps and how to avoid them.
When using the equations of motion, candidates substitute a displacement or a time with a negative sign without checking consistency with the chosen positive direction, or they mix up initial and final velocities (e.g., using v = u + at with v and u swapped).
The sign convention is often set implicitly and not written down; candidates memorise formulas but do not anchor them to a diagram. Also, they treat symbols as interchangeable labels rather than quantities with direction.
What to do instead
Before any calculation, draw a quick sketch and write 'positive direction = ...' at the top. Label u, v, a, s, t on the sketch with arrows. Substitute only after checking that each symbol matches the sketch. For vertical motion, always state whether up or down is positive and keep g's sign consistent.
In calorimetry and latent heat problems, candidates add or subtract temperatures directly (e.g., averaging 20°C and 80°C to get 50°C) or use Celsius temperatures in gas-law ratios instead of converting to kelvin.
Everyday experience treats Celsius as a ratio scale; the ideal-gas law and many thermodynamic relations require absolute temperature, and mixing °C with K is a common oversight. Also, candidates confuse heat capacity with specific heat capacity.
What to do instead
Adopt a habit: in any equation containing T (gas laws, Carnot efficiency, thermal energy), immediately convert all temperatures to kelvin and write 'K' next to each. For heat exchange, set up ΣQ = 0 with Q = mcΔT and Q = mL, and never average temperatures; solve the energy balance explicitly.
For waves and sound, candidates use the wave equation v = fλ with frequency in kHz without converting to Hz, or they apply the Doppler formula with the wrong sign for source versus observer motion.
Units are often given in kHz or cm in the stem; candidates plug numbers without unit conversion. The Doppler formula has several sign conventions, and candidates memorise one form without understanding the relative motion.
What to do instead
Always convert to SI base units before substitution (kHz → ×10³ Hz, cm → ×10⁻² m). For Doppler, derive the sign from first principles: if source and observer approach, observed frequency increases; if they recede, it decreases. Write the formula as f' = f(v ± v_o)/(v ∓ v_s) and check that the sign choice makes f' larger when approaching.
In electric circuits, candidates treat a parallel combination as series (or vice versa) when computing equivalent resistance, or they assume current is used up as it passes through resistors.
Circuit diagrams are often drawn in non-standard layouts; candidates pattern-match on visual appearance rather than tracing nodes. The 'current gets consumed' misconception comes from a water-flow analogy taken too literally.
What to do instead
Redraw the circuit by colouring nodes: all points connected by ideal wire are the same node. Then identify series (same current, no junction between) and parallel (same pair of nodes). Apply Kirchhoff's current law explicitly at each junction to confirm that current splits and recombines, not disappears.
When writing electron configurations and orbital diagrams, candidates fill 3d before 4s, or they violate Hund's rule by pairing electrons in degenerate orbitals before singly occupying all of them.
The Aufbau order is taught as a list, but the (n+l) rule and the actual energy ordering at the 3d/4s crossover are subtle. Hund's rule is often memorised as a statement without practice in drawing orbital boxes.
What to do instead
Use the (n+l) rule: lower n+l fills first; for equal n+l, lower n fills first. Practise by drawing orbital boxes for the first 30 elements and checking with the periodic table block structure. Remember that 4s fills before 3d but empties first when ionising transition metals.
In stoichiometry, candidates use the mass of a reactant directly as moles, or they ignore the limiting reactant and calculate product yield from the reactant with the larger mass.
The mole concept is abstract; candidates shortcut by treating grams as particles. Limiting reactant problems require comparing mole ratios, which is an extra step often skipped under time pressure.
What to do instead
Always convert every given mass to moles first (n = m/M) and write the balanced equation above the calculation. Then divide each reactant's moles by its stoichiometric coefficient; the smallest result identifies the limiting reactant. Use that reactant to calculate product moles, then convert back to mass if required.
For acid-base equilibria, candidates calculate pH from the initial concentration of a weak acid without accounting for partial dissociation, or they use the Henderson–Hasselbalch equation for a strong acid–strong base titration region where it does not apply.
Strong-acid habits carry over; the square-root approximation for weak acids is forgotten, and the distinction between initial and equilibrium concentrations is blurred. The buffer equation is over-applied.
What to do instead
For weak acids, set up an ICE table and solve Ka = x²/(C − x); check whether C/Ka > 100 to justify the approximation x ≈ √(KaC). For titrations, identify the region (initial, buffer, equivalence, excess titrant) before choosing a method. Use Henderson–Hasselbalch only in the buffer region, and calculate pH at equivalence from the hydrolysis of the conjugate base or acid.
In organic reaction mechanisms, candidates draw curly arrows starting from the wrong atom (e.g., from a positive charge rather than the nucleophile) or they omit the formal charges and lone pairs that drive the mechanism.
Mechanisms are often memorised as pictures rather than electron bookkeeping. Candidates do not consistently track valence electrons, so they miss the nucleophile/electrophile logic.
What to do instead
Before drawing any arrow, label all formal charges and lone pairs. Arrows must start from an electron-rich site (lone pair or π bond) and point to an electron-poor atom. After each step, recount valence electrons and formal charges to confirm conservation. Practise with a checklist: nucleophile → electrophile, then proton transfer if needed.
In thermochemistry, candidates use bond enthalpies with the wrong sign convention (e.g., treating bond breaking as exothermic) or they confuse ΔH of reaction with activation energy in energy profile diagrams.
Sign conventions for bond breaking (endothermic) and bond forming (exothermic) are easily reversed. Energy profile diagrams show both activation energy and ΔH, and candidates conflate the two.
What to do instead
Write the rule explicitly: breaking bonds requires energy (+), forming bonds releases energy (−). For ΔH from bond enthalpies, use ΔH = Σ(bonds broken) − Σ(bonds formed). On energy profiles, label the reactants and products, then mark ΔH as the difference in their levels and Ea as the peak minus reactant level; they are not the same.
In kinetics and electrochemistry, candidates read the rate from the slope of a concentration–time graph without drawing a tangent, or they determine the cell potential by adding half-cell potentials without reversing the sign of the oxidation half-reaction.
Rate is the derivative, not the average gradient; candidates use Δ[ ]/Δt over a long interval. For electrochemistry, standard reduction potentials are tabulated as reductions, and candidates forget to flip the sign for the oxidation half-cell.
What to do instead
For rate, draw a tangent at the specific time and calculate its gradient, or use the initial-rate method. For E°cell, write both half-reactions as reductions, then reverse the oxidation one and change its sign; E°cell = E°cathode − E°anode. Alternatively, use E°cell = E°reduction(cathode) + E°oxidation(anode) with the oxidation potential sign already flipped.