Slovakia Maturita Chemistry common mistakes
Slovakia Maturita Chemistry · Understand the traps and how to avoid them.
When writing electron configurations for transition metals, the candidate fills the 4s orbital after the 3d orbital (e.g., writes 3d^10 4s^2 for Zn instead of the expected 3d^10 4s^2 but often misorders or misplaces electrons, or writes 3d^4 4s^2 for Cr instead of 3d^5 4s^1).
The candidate memorizes the n+l rule but forgets that half-filled and fully-filled d subshells have extra stability, and confuses the order of filling with the order of writing.
What to do instead
Practice writing configurations for Cr, Cu, and their ions; always check that the 4s orbital is filled before 3d during filling but write the configuration in increasing n (or as the exam requires). Use the mnemonic for exceptions: Cr and Cu promote one 4s electron to 3d.
In stoichiometry problems, the candidate calculates the amount of product from the limiting reactant by using the mass of the excess reactant or by ignoring the molar ratio from the balanced equation.
The candidate focuses on the given masses and forgets that the coefficients give the mole ratio; they also may not identify the limiting reactant when two reactant amounts are given.
What to do instead
Always convert all given quantities to moles, divide by the stoichiometric coefficient to find the limiting reactant, then use that reactant's mole ratio to calculate the product. Practice with problems where the limiting reactant is not obvious.
When naming organic compounds, the candidate chooses the longest chain but does not give the lowest locants to the principal functional group, or numbers from the wrong end when multiple substituents are present.
The candidate applies 'longest chain' but forgets that the principal functional group (e.g., -COOH, -OH) must get the lowest possible number, and that alphabetical order of substituents affects numbering only as a tiebreaker.
What to do instead
Follow IUPAC priority rules: identify the principal functional group, number the chain to give it the lowest locant, then apply lowest locants to substituents. Practice with molecules containing multiple functional groups and branches.
In acid-base calculations, the candidate uses the concentration of a weak acid directly as [H+] without considering the acid dissociation constant (Ka), or forgets to take the negative logarithm when calculating pH.
The candidate confuses strong and weak acids, assuming complete dissociation for all acids, and may mix up the definitions of pH and pOH.
What to do instead
For weak acids, set up an ICE table or use the approximation [H+] = sqrt(Ka * c) when dissociation is small. Always check if the acid is strong (HCl, HNO3, H2SO4 first proton) or weak. Remember pH = -log[H+] and pOH = -log[OH-], and pH + pOH = 14 at 25°C.
When predicting the spontaneity of a reaction from Gibbs free energy, the candidate states that a reaction with negative ΔH is always spontaneous, ignoring the entropy term and temperature dependence.
The candidate oversimplifies by associating exothermic reactions with spontaneity, forgetting that ΔG = ΔH - TΔS and that entropy can oppose enthalpy.
What to do instead
Analyze the signs of ΔH and ΔS separately, then use ΔG = ΔH - TΔS to determine spontaneity at a given temperature. Memorize the four cases (ΔH negative/positive, ΔS negative/positive) and the temperature at which ΔG changes sign (T = ΔH/ΔS).
In redox or electrochemistry problems, the candidate balances atoms but not charge, or incorrectly identifies the oxidizing and reducing agents.
The candidate focuses on mass balance and forgets that electrons must also balance; they may also confuse oxidation (loss of electrons) with reduction (gain of electrons).
What to do instead
Use the half-reaction method: balance atoms other than O and H, balance O with H2O, H with H+, then balance charge with electrons. Check that electrons lost equal electrons gained. Remember OIL RIG (Oxidation Is Loss, Reduction Is Gain).
When describing laboratory procedures, the candidate states that a pipette should be blown out to deliver the last drop, or that a burette should be filled without rinsing with the titrant.
The candidate generalizes from everyday experience (e.g., blowing out a pipette to get all liquid) and does not recall that pipettes are calibrated to deliver (TD) and that residual liquid should remain, and that rinsing with titrant prevents dilution.
What to do instead
Learn the standard techniques: pipettes are not blown out unless marked 'blow out'; burettes and pipettes should be rinsed with the solution they will contain. Practice with a checklist of proper techniques for each piece of glassware.
When explaining the periodic trends, the candidate states that atomic radius increases across a period and decreases down a group, or that ionization energy decreases across a period.
The candidate confuses the trends because they mix up the effects of increasing nuclear charge (across a period) and increasing principal quantum number (down a group).
What to do instead
Use the effective nuclear charge concept: across a period, Z_eff increases, pulling electrons closer (radius decreases, ionization energy increases). Down a group, n increases, so radius increases and ionization energy decreases. Create a summary table and practice with specific elements.
In biochemical pathways, the candidate claims that glycolysis produces 36 or 38 ATP directly, or that fermentation produces ATP by oxidative phosphorylation.
The candidate confuses the net ATP yield of glycolysis (2 ATP) with the total ATP from complete aerobic respiration, and does not distinguish between substrate-level phosphorylation and oxidative phosphorylation.
What to do instead
Memorize the net yields: glycolysis (2 ATP, 2 NADH), pyruvate oxidation (2 NADH), Krebs cycle (2 ATP, 6 NADH, 2 FADH2), and oxidative phosphorylation (~26-28 ATP). For fermentation, emphasize that it regenerates NAD+ and produces no additional ATP beyond glycolysis.
When discussing environmental chemistry, the candidate states that the greenhouse effect is solely caused by human activity or that the ozone hole is the main cause of global warming.
The candidate conflates two separate environmental issues (ozone depletion and climate change) and lacks understanding of natural versus anthropogenic greenhouse effects.
What to do instead
Clarify the distinction: the greenhouse effect is a natural process; human activities enhance it by increasing greenhouse gas concentrations. Ozone depletion is caused by CFCs and affects UV radiation, not global temperature directly. Study the specific causes and effects of each phenomenon.