CEnT-S chemistry
Chemistry is the fourth section: ten questions in twenty minutes, worth ten of the fifty-five marks. It carries the widest syllabus of the five sections and the least room to work, which is why candidates consistently name it the hardest.
What the section covers
Quantitative chemistry
Stoichiometry, the limiting reagent, percentage yield, units of concentration and dilution. This is the largest single group and the one most likely to appear more than once.
Acids and bases
pH and pOH, strong and weak acids and bases, buffer solutions, and the hydrolysis of salts. A salt of a weak acid and a strong base gives an alkaline solution; the reverse gives an acidic one. That single rule accounts for a family of questions.
Redox
Assigning oxidation numbers and balancing equations in acidic solution. The usual route is to count the oxygen atoms in the oxidising agent, turn them into water molecules and derive the number of protons from there.
Structure and bonding
VSEPR and molecular shape, hybridisation, and the distinction between electron geometry and molecular geometry. Ammonia has four regions of electron density and therefore a tetrahedral arrangement, but the shape is named from the atoms alone, which makes it trigonal pyramidal.
Periodic trends
Atomic radius, ionisation energy and electronegativity, across a period and down a group. Moving down a group generally outweighs moving across one.
Gases and thermodynamics
The ideal gas equation, partial pressure and mole fraction, enthalpy, entropy and Gibbs free energy. An endothermic reaction can still be spontaneous if the entropy change is positive and the temperature is high enough.
Organic nomenclature
Naming and recognising the common functional groups.
Applied chemistry
An easily overlooked strand: reading product labels, acid rain, the greenhouse effect, photochemical smog and laboratory safety. It appears in the official syllabus and almost nowhere in preparation material.
What twenty minutes and no calculator actually mean
Two minutes a question, no calculator and no periodic table constrain the chemistry far more than the topic list suggests. A three-stage stoichiometric calculation cannot be done by hand in that time, so the questions do not ask for one.
In practice this means molar masses come out round, mole quantities fall on clean fractions, concentrations in pH questions are exact powers of ten, and any square root is of a perfect square. When a calculation starts producing awkward decimals, that is usually a sign that the method has gone wrong rather than that the arithmetic is hard.
Relative atomic masses are printed in the question whenever a calculation needs them, because no periodic table is supplied. What is not supplied is the relationship between quantities — n = m/M, c = n/V, pH = −log₁₀[H⁺] and the rest have to be brought with you. Full formula list →
Where marks are lost
Unit conversion. A volume given in cubic centimetres has to be divided by 1000 before it becomes a concentration in mol dm⁻³. Enthalpy in kilojoules and entropy in joules cannot be combined until one of them is converted. Both mistakes produce answers that are wrong by exactly a factor of a thousand, and both appear among the options.
The limiting reagent. Working out which reactant runs out first is a separate step, and skipping it gives a yield based on the reagent in excess.
Naming the wrong species. In a balanced redox equation the coefficient of water and the coefficient of the proton are related but not equal. In a gas mixture the two partial pressures add to the total, so quoting the wrong one gives a number that looks entirely reasonable.
With a quarter-mark penalty for a wrong answer, distinctions like these are expensive. How the penalty changes what is worth guessing →
Practise the section
Ten original chemistry questions
Under the real twenty-minute limit, with every constant stated in the question and every answer explained.
Start practice →