Bulgaria DZI Chemistry common mistakes
Bulgaria DZI Chemistry · Understand the traps and how to avoid them.
When drawing electron configurations for transition-metal cations, the candidate removes electrons from the 4s subshell only after emptying 3d, or writes 3d^n 4s^2 for a cation such as Fe2+.
The candidate memorised the neutral-atom filling order (4s fills before 3d) and applies it unchanged to ions, forgetting that once 3d is occupied the 4s electrons are higher in energy and leave first.
What to do instead
Drill the rule: build the neutral atom, then remove electrons from the highest-n subshell (4s) before 3d for d-block cations. Practise Fe, Fe2+, Fe3+, Cu, Cu+, Cr, Cr3+ and check each against the periodic-table position.
The candidate predicts periodic trends by a single sentence ('increases across, decreases down') and then mislabels a property such as atomic radius, ionisation energy, or oxidising strength of halogens when the trend reverses at a group boundary.
The candidate treats trends as slogans without linking them to effective nuclear charge, shielding, and atomic size, so anomalies (e.g. smaller radius of Ga vs Al, higher first ionisation energy of N vs O) are missed.
What to do instead
For each property, write the causal chain (Z_eff up across -> radius down -> ionisation energy up) and then work 5-6 anomaly examples with reasons: half-filled 2p in N, d-block contraction, lanthanide contraction. Never state a trend without its cause.
The candidate draws a Lewis structure with an incorrect octet or assigns a molecular shape directly from the number of electron pairs without subtracting lone pairs, e.g. calling H2O linear or NH3 trigonal planar.
The candidate counts bonds but not lone pairs, and does not separate electron-domain geometry from molecular geometry, so VSEPR is applied mechanically.
What to do instead
Use a fixed four-step routine: total valence electrons, skeleton, lone pairs to complete octets, then count domains (bonding + lone pairs). Build a table of AXnEm cases (linear, trigonal planar, tetrahedral, trigonal pyramidal, bent) and label both geometries for each molecule practised.
The candidate balances an equation by inspection and then does not check charge or atom balance, or balances a redox reaction in acidic medium but forgets to add H2O and H+ in the correct half-reaction order.
Under time pressure the candidate skips the verification step and treats redox balancing as a memorised trick rather than a systematic half-reaction method.
What to do instead
Always finish with a two-line audit: atoms of each element, then total charge on each side. For redox, practise the six-step half-reaction method (balance atoms other than O/H, balance O with H2O, balance H with H+, balance charge with electrons, equalise electrons, add and cancel) on at least 10 reactions.
The candidate calculates amount of substance using the mass given for a solution or a gas without converting to the correct quantity, e.g. using the mass of solution instead of mass of solute in a mass-fraction problem, or applying n = m/M to a gas at non-standard conditions.
The candidate conflates mass fraction, molar concentration, and molar volume, and does not identify which quantity the problem actually supplies.
What to do instead
Before calculating, write a data inventory: what is given (mass of solution, mass fraction, volume of solution, concentration, gas volume and conditions) and which formula links it to n. Practise a mixed set of 10 problems where the first line must be the conversion to n, with units written explicitly.
The candidate writes ionic equations that keep spectator ions, or writes a precipitate/weak-electrolyte reaction as a full molecular equation and then cannot explain conductivity changes during titration.
The candidate has not internalised which species are strong electrolytes and dissociate completely, so the distinction between molecular, complete ionic, and net ionic equations is blurred.
What to do instead
Memorise the strong-electrolyte list (strong acids, strong bases, soluble salts) and the weak/insoluble exceptions. For every reaction practised, write all three forms and cross out spectators explicitly. Add conductivity reasoning: which ions are removed or replaced.
The candidate predicts a reaction between a metal and an acid or a displacement reaction using reactivity intuition rather than the electrochemical series, and states that copper displaces hydrogen from dilute hydrochloric acid.
The candidate remembers 'active metals react with acids' as a blanket rule and never checks the standard electrode potential of the metal against H+/H2.
What to do instead
Keep a ranked activity series with E° values and require a written justification ('E°(Cu2+/Cu) = +0.34 V > 0 V, so Cu cannot reduce H+'). Apply this check to every displacement and metal-acid question practised.
The candidate answers qualitative-analysis questions by colour alone, e.g. identifying a precipitate as AgCl without confirming its solubility in ammonia, or confusing the flame colours of sodium and potassium.
The candidate learns a single observable per ion instead of the confirmatory test sequence, and the exam often tests the discriminating step.
What to do instead
Build a two-column table: preliminary observation and confirmatory test (solubility in excess reagent, flame colour, gas evolution). Rehearse the chloride/bromide/iodide sequence with AgNO3 and ammonia, and the cation flame tests, writing the confirmatory step for each ion.
The candidate names or draws an organic structure without applying the lowest-locant and principal-functional-group priority rules, producing 3-methylbutane instead of 2-methylbutane, or numbering an alcohol from the wrong end.
The candidate applies IUPAC rules in the order remembered rather than the prescribed hierarchy (principal group first, then lowest locants, then alphabetical order).
What to do instead
Follow a fixed naming checklist: find the longest chain containing the principal group, number to give that group the lowest locant, then apply lowest locants to substituents, then alphabetical order. Practise 10 structures with mixed functional groups and check each against the checklist.
The candidate predicts the major product of an addition or substitution reaction without tracking the intermediate, e.g. adding HBr to propene and giving 1-bromopropane, or oxidising a primary alcohol all the way to a carboxylic acid when a milder reagent is specified.
The candidate memorises 'addition follows Markovnikov' as a phrase but does not apply the carbocation-stability reasoning, and does not link reagent choice to the oxidation stage.
What to do instead
For every addition, draw the two possible carbocations and select the more stable before placing the substituent. For oxidations, pair each reagent with its product stage (primary alcohol to aldehyde with distillation and limited oxidant, to acid with excess). Practise 8 reaction-scheme questions writing the intermediate explicitly.
The candidate answers equilibrium questions by assuming the reaction goes to completion, or applies Le Chatelier qualitatively without checking whether the change actually alters the equilibrium constant or only the position.
The candidate mixes kinetics (rate changes) with thermodynamics (position of equilibrium) and does not distinguish K from Q.
What to do instead
For each disturbance, write two lines: what happens to Q relative to K, and which direction shifts. State explicitly that only temperature changes K. Practise 6 scenarios (pressure change with equal and unequal moles of gas, addition of inert gas at constant volume, catalyst addition).
The candidate calculates reaction rate or equilibrium quantities without converting to consistent units or without using the stoichiometric coefficients as exponents, e.g. writing rate = k[A][B] for a reaction with coefficients 2 and 3 without justification.
The candidate confuses the stoichiometric equation with the experimentally determined rate law and does not track units of k.
What to do instead
State that the rate law must be given or determined experimentally, never read from coefficients. For every calculation, write the units of each quantity and derive the units of k from the order. Practise 5 rate-law and 5 equilibrium-constant problems with unit checks.
The candidate calculates enthalpy change with the wrong sign convention, e.g. reporting a combustion as endothermic because the calculation produced a negative number, or adding bond-formation energies instead of subtracting them.
The candidate does not fix a convention (system vs surroundings, bond breaking endothermic, bond forming exothermic) and applies signs inconsistently across bond-energy and formation-enthalpy methods.
What to do instead
Adopt one written convention and apply it to every problem: ΔH = Σ(bonds broken) - Σ(bonds formed), and ΔH = ΣΔHf(products) - ΣΔHf(reactants). Label each term endothermic or exothermic before summing, and check the sign against the physical situation.