SPM Chemistry common mistakes
SPM Chemistry · Understand the traps and how to avoid them.
When an equation will not balance, the candidate edits the chemical formula itself instead of the coefficient - writing H2 + O2 → H2O2, or Fe + O2 → FeO for the rusting of iron, or 'correcting' Al2O3 to AlO. They also skip any written check that both sides match.
Practice is framed as 'make the atoms equal on both sides', which sounds like permission to change anything. Subscripts and coefficients are both just small numbers on the page, and no tally is written, so the error survives to the final answer.
What to do instead
Force a written tally (element | left | right) under every equation and state the rule out loud: only the big number in front may change, because a formula is the identity of the substance. After balancing, re-tally, then divide through to the lowest whole-number ratio. Drill specifically on equations containing O2, H2 and Fe2O3, where the temptation is strongest.
Uses 22.4 dm3 mol-1 for solids, liquids and solutions as well as gases, substitutes a volume in cm3 straight into n = V/22.4, ignores whether the gas is at STP or room conditions, or treats 6.02 × 1023 as though it were a molar mass.
The three mole formulae look interchangeable on the formula sheet, and molar volume is memorised as a number rather than as a quantity that only applies to a gas under stated conditions. Unit conversion between cm3 and dm3 is treated as trivial and dropped.
What to do instead
Write the unit beside every quantity before substituting and cancel units explicitly: n = m ÷ Mr gives g ÷ g mol-1; n = V ÷ 22.4 applies only to a gas at STP with V in dm3; n = N ÷ 6.02 × 1023 counts particles. Convert cm3 to dm3 by dividing by 1000 first. Habit-check: if the substance is a solid or a solution, the 22.4 route is automatically wrong.
Treats proton number and nucleon number as interchangeable, describes an isotope as having a different number of protons, and writes 'an atom is neutral so neutrons equal protons'. Electron arrangements are filled without first counting the electrons, e.g. giving chlorine 2.8.7.
Notation such as 35Cl with 17 subscript is read left-to-right without mapping the superscript to nucleon number, and 'isotope' is memorised as a sentence rather than as same protons, different neutrons.
What to do instead
Apply A = Z + N every single time: write protons = Z, electrons = Z for a neutral atom, neutrons = A − Z. Count the electrons before filling shells 2, 8, 8. Self-test with 35Cl and 37Cl: both have 17 protons, with 18 and 20 neutrons respectively - the chemistry is identical because the electron arrangement is identical.
Explains periodic trends in the wrong direction - 'sodium is more reactive than potassium because its nucleus attracts the outer electron more strongly', or 'fluorine is less reactive than iodine because it is a smaller atom'. The trend word is recalled but the reasoning is inverted.
The trend direction and the phrase 'attraction of the nucleus' are memorised as two separate facts and then joined by guesswork, with no chain linking shells, distance and shielding to the actual behaviour of the valence electron.
What to do instead
Reason in a fixed four-step chain every time: number of shells → distance of the valence electron from the nucleus → shielding by inner shells → strength of attraction → ease of losing or gaining electrons. Down Group 1 the outer electron is further out and better shielded, so it is lost more easily and reactivity rises. Down Group 17 the incoming electron is attracted less, so reactivity falls.
Draws an ionic compound as a molecule with a shared pair, shows every electron in a dot-and-cross diagram instead of only the valence electrons, omits brackets and charges on ions, and describes sodium chloride as paired Na+Cl- molecules rather than a lattice.
Dot-and-cross is learned as a drawing routine rather than a decision about transfer versus sharing, and 'formula unit' and 'molecule' are used as if they were synonyms.
What to do instead
Decide first: metal plus non-metal means transfer, drawn as [Na]+[Cl]- with brackets and charges; non-metal plus non-metal means sharing, drawn with only the outer shells overlapping and the shared pair in the overlap. Check that the total valence electrons equal the sum of the group numbers. For ionic solids, always describe a giant ionic lattice with strong electrostatic forces in all directions, which is why the melting point is high.
Equates strong with concentrated: 'dilute hydrochloric acid is a weak acid', 'a weak acid is one with a low concentration', or explains pH by how much acid is present rather than by how far it ionises.
Everyday English uses 'strong' for 'a lot', and the two topics are taught in adjacent lessons, so the two ideas merge into one untested assumption.
What to do instead
Keep two separate definitions taped to the method: strong/weak = extent of ionisation in water (HCl fully ionised, CH3COOH partially); concentrated/dilute = amount of acid per unit volume. Prove it with a paired example: 0.1 mol dm-3 HCl has pH 1 while 0.1 mol dm-3 CH3COOH has pH about 3 - same concentration, different pH, so the difference must be ionisation. Likewise a very dilute HCl is still a strong acid.
Answers 'rate increases because more product is formed' or 'because the particles move faster' and stops there, and confuses rate with yield: claiming a catalyst increases the amount of product, or that using fewer, larger marble chips makes the reaction faster.
Rate is measured as volume of gas against time, so a curve that simply rises higher looks 'faster'; collision theory is recited as a label without the idea of frequency of effective collisions.
What to do instead
Write the full causal chain: factor → change in frequency of effective collisions between reacting particles per unit time → change in rate. Read the graph properly: steeper gradient means faster rate, same final volume means the same amount of product. State that a catalyst lowers activation energy and changes the gradient but never the final volume, and that smaller chips increase surface area and therefore collision frequency, not the yield.
Defines oxidation only as gain of oxygen, cannot assign oxidation numbers in species such as Fe3+, KMnO4 or H2O2, and names the oxidising agent as the substance that is itself oxidised.
The oxygen-based definition is taught first and is never formally replaced by the electron-transfer definition, while oxidation-number rules are learned as a list without being applied to unfamiliar formulae.
What to do instead
Use OIL RIG and always calculate oxidation numbers before answering: element = 0, simple ion = its charge, O = −2 (except peroxides, −1), H = +1 (except hydrides, −1), and the sum equals the overall charge. Then write the conclusion in one sentence: the oxidising agent contains the element whose oxidation number falls, because the oxidising agent is itself reduced.
Gets the sign convention backwards - 'heat is released, so ΔH is positive' - and draws energy level diagrams with unlabelled axes, no arrow, and products placed above reactants for an exothermic reaction.
The system/surroundings convention is abstract and is memorised as a sign rather than derived from a diagram, so under exam pressure the sign flips and nothing in the drawing contradicts it.
What to do instead
Anchor everything to one relation: ΔH = energy of products − energy of reactants. Exothermic: products lower, arrow points down, ΔH negative. Endothermic: products higher, arrow points up, ΔH positive. Every diagram must carry 'energy' on the y-axis, labelled reactant and product levels, and a signed ΔH on the arrow. Check the sign against the direction of the arrow before moving on.
Describes heating a solid as 'the particles expand' or 'the particles melt', and explains the flat part of a heating curve as 'the temperature stops because the heating was switched off'. Also claims particles in a liquid are not moving.
Kinetic theory is learned as a list of properties of the three states, so the particles are imagined as the material itself swelling, and the plateau is read as a pause in energy input rather than a change of use of that energy.
What to do instead
Use a fixed particle script: heat → particles gain kinetic energy and vibrate faster → at the melting point the energy is used to overcome the forces between particles, not to raise kinetic energy, so the temperature stays constant. Particles never expand - the spacing between them does. In a liquid the particles are still moving, sliding past one another; only in a solid are they locked in fixed positions.