Schleswig-Holstein Abitur Chemistry common mistakes
Schleswig-Holstein Abitur Chemistry · Understand the traps and how to avoid them.
When deducing the electron configuration of a transition metal cation, the candidate incorrectly removes electrons from 3d before emptying 4s, e.g., writing Fe²⁺ as [Ar]3d⁴4s² instead of [Ar]3d⁶.
Candidates memorize the aufbau filling order (4s before 3d) and apply it mechanically to ionization, overlooking that once 3d is occupied, the 4s electrons are higher in energy and are removed first. This is reinforced by textbooks that list the neutral atom configuration with 4s before 3d.
What to do instead
Always write the neutral atom's full configuration with the 4s subshell explicitly before 3d. For cations, remove electrons from the highest n first (4s before 3d) and then re-express the result with 3d before 4s. Practice with Fe, Co, Ni, Cu, and Zn ions until the pattern is automatic.
The candidate counts octets without checking formal charges or total charge, for example assigning avoidable charge separation when a reasonable lower-charge Lewis contributor is available.
Candidates focus on counting electrons and completing octets, treating the octet rule as the sole criterion. They often forget that formal charge minimization and electronegativity dictate the preferred resonance contributor.
What to do instead
After drawing any Lewis structure, calculate formal charge on every atom using FC = valence electrons − (nonbonding electrons + ½ bonding electrons). If nonzero formal charges appear, try multiple bonds or alternative arrangements to minimize them. The best structure has formal charges closest to zero and any negative formal charge on the more electronegative atom.
When predicting the effect of temperature on vapor pressure or the state of a substance, the candidate states that increasing temperature decreases vapor pressure because gas solubility decreases, or confuses vapor pressure with total pressure in a closed container.
The candidate conflates Henry's law (gas solubility decreases with temperature) with vapor pressure (which increases with temperature). They also may not distinguish between the partial pressure of vapor and the total pressure when a gas is present.
What to do instead
Memorize that vapor pressure of a liquid always increases with temperature because more molecules have sufficient kinetic energy to escape. Use the Clausius–Clapeyron equation to quantify the change. When a gas is present, the total pressure is the sum of the gas partial pressure and the vapor pressure of the liquid; the vapor pressure is independent of the gas.
The candidate predicts that the atomic radius increases across a period because atomic number increases, or that ionization energy decreases across a period because the nucleus has more protons but electrons are added to the same shell.
The candidate confuses the trend with the underlying cause. They may think that more protons attract electrons less, or they may mix up shielding and effective nuclear charge. The increase in atomic number is misinterpreted as adding shells.
What to do instead
Across a period, nuclear proton number rises while, in the main-group trend being compared, added valence electrons occupy the same principal shell, increasing effective nuclear charge (Zeff) and pulling electrons closer; thus atomic radius decreases and ionization energy increases. Down a group, new shells are added, increasing radius and decreasing ionization energy. Use Zeff = Z − S (shielding) to explain trends.
In a limiting-reactant problem, the candidate calculates moles of product from each reactant separately and then averages the two results, or selects the reactant with the smaller initial mass as the limiting reactant.
Averaging is a mathematical habit from other contexts; selecting by mass ignores molar mass and stoichiometry. The candidate may also skip the step of dividing moles by the stoichiometric coefficient.
What to do instead
For each reactant, convert mass to moles, then divide by its coefficient in the balanced equation to get the 'reaction extent'. The smallest extent identifies the limiting reactant. Calculate product moles from that reactant only. Always check that the other reactant is in excess.
When using Hess's law or calorimetry data, the candidate forgets to reverse the sign of ΔH when reversing a reaction, or fails to multiply ΔH by the stoichiometric factor when scaling a reaction.
The candidate treats ΔH as an ordinary number rather than a state function that depends on direction and amount. They may also confuse ΔH of formation with ΔH of reaction and use tabulated values without considering the coefficients.
What to do instead
Write each reaction with its ΔH value. When reversing, change the sign; when multiplying by a factor, multiply ΔH by the same factor. Then add the equations and their ΔH values. For formation enthalpies, use ΔH°rxn = Σ nΔH°f(products) − Σ mΔH°f(reactants), being careful with coefficients.
The candidate calculates an equilibrium constant using initial concentrations instead of equilibrium concentrations, or uses the reaction quotient Q and calls it K without evaluating at equilibrium.
The candidate confuses the initial state with the equilibrium state, especially in ICE table problems where they forget to subtract the change. They may also assume that K can be calculated directly from stoichiometry without experimental data.
What to do instead
Always set up an ICE table: write initial concentrations, define the change in terms of x, and express equilibrium concentrations. Substitute only the equilibrium expressions into the K expression. For Q, use current concentrations; compare Q to K to determine direction. Practice problems where the change is not negligible.
When calculating the pH of a weak acid solution, the candidate uses the initial concentration directly in the Ka expression without accounting for the degree of ionization, or assumes pH = pKa for any buffer regardless of concentration ratio.
The candidate overgeneralizes the approximation that weak acids ionize only slightly, forgetting that for moderately weak acids the x cannot be neglected. For buffers, they may misapply the Henderson–Hasselbalch equation without checking the ratio of concentrations or the validity of the approximation.
What to do instead
For weak acids, write Ka = x²/(C − x) and solve the quadratic if x > 5% of C. For buffers, use pH = pKa + log([A⁻]/[HA]) and verify that the ratio is between 0.1 and 10 and that concentrations are much larger than Ka. Always check assumptions.
When balancing a redox reaction in basic solution, the candidate balances oxygen by adding H₂O and hydrogen by adding H⁺, but then forgets to neutralize the H⁺ by adding OH⁻ to both sides, leaving an acidic solution.
The half-reaction method is often taught first in acidic solution, and candidates memorize the steps without adapting them to basic conditions. They may also forget to combine the half-reactions and cancel electrons.
What to do instead
After balancing in acidic solution, add OH⁻ to both sides equal to the number of H⁺ ions, combine H⁺ and OH⁻ to form H₂O, and cancel water molecules. Verify that the final equation has no H⁺ and is balanced in both atoms and charge.
When naming an organic compound, the candidate selects the longest carbon chain without considering the principal functional group, or numbers the chain from the end that gives the lowest locant to a substituent rather than to the principal group.
The candidate applies IUPAC rules mechanically but forgets the hierarchy: the principal functional group determines the suffix and the numbering direction. They may also ignore that multiple bonds and substituents have different priorities.
What to do instead
Identify the principal functional group first (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane). The parent chain must contain the principal group, and numbering starts from the end that gives the principal group the lowest locant. Then apply lowest locants to multiple bonds and substituents in alphabetical order.