Secondary 5 Chemistry Practice Paper: Atomic Structure And The Periodic Table, Chemical Bonding And Structure, Energetics, Reaction Rates And Equilibrium, Electrolysis And Redox, Organic Chemistry (100 Marks)

Secondary 5 Chemistry Practice Paper: atomic structure and the periodic table, chemical bonding and structure, energetics, reaction rates and equilibrium, electrolysis and redox, organic chemistry (100 marks)

2026-11-14

Chemistry – Secondary 5

Original S5 Chemistry practice paper with 24 questions on atomic structure, bonding, energetics, rates, equilibrium, electrolysis and organic chemistry, with answers.

Paper details　S5 Chemistry practice paper with 24 structured questions and full worked answers on atomic structure, bonding, energetics, rates, equilibrium, electrolysis and organic chemistry.

Section A – Atomic structure and the periodic table (17 marks)

Study the information given and answer all questions.

#	Question	Answer and marking notes
1	Element X has atomic number 12 and its most abundant isotope has mass number 24. (a) State the number of protons, neutrons and electrons in one atom of X; (b) write the electronic arrangement; (c) state the period and group of X.	Answer: (a) 12 protons, 12 neutrons and 12 electrons; (b) the arrangement is 2, 8, 2; (c) period 3, group IIA, with 2 electron in the outermost shell.
2	An element has three isotopes with mass numbers 24, 25 and 26, present at 79, 10 and 11 respectively. Calculate the relative atomic mass of the element, correct to two decimal places.	Answer: relative atomic mass = (24 x 79 + 25 x 10 + 26 x 11) / 100 = 24.32.
3	A magnesium atom (atomic number 12) loses two electrons and a chlorine atom (atomic number 17) gains one electron. State the number of electrons in each ion and name the noble gas with the same electronic arrangement.	Answer: the magnesium ion has 10 electrons and the chloride ion has 18. Both have the same arrangement as neon, which has 10 electrons.
4	The formula of magnesium oxide is MgO, with molar mass 40 g/mol. A sample of 8.0 g of magnesium oxide is taken. (a) Calculate the number of moles; (b) estimate the number of formula units, taking 6.02 x 10^23 as Avogadro constant.	Answer: (a) moles = 8.0 / 40 = 0.2 mol; (b) about 1.2 x 10^23 formula units.

Section B – Chemical bonding and structure (17 marks)

Compare the structures and properties of the substances below.

#	Question	Answer and marking notes
5	Compare sodium chloride and water: (a) state the type of chemical bond in each; (b) explain why solid sodium chloride does not conduct electricity while the molten substance does.	Answer: (a) sodium chloride is held by ionic bonds and water by covalent bonds; (b) in the solid the ions are fixed in the lattice and cannot move, whereas on melting the ions become mobile and carry current.
6	The molecular formula of ethane is C2H6. Calculate, for one ethane molecule, (a) the total number of covalent bonds; (b) the total number of valence electrons.	Answer: (a) 7 covalent bonds in total, namely 6 C-H bonds and 1 C-C bond; (b) 14 valence electrons.
7	Both diamond and graphite are made of carbon atoms. (a) State the main difference in their structures; (b) explain why graphite conducts electricity but diamond does not.	Answer: (a) in diamond each carbon atom bonds to four others in a three-dimensional network, while in graphite each carbon atom bonds to three others forming layers; (b) each carbon atom in graphite has one delocalised electron that can move within a layer, so graphite conducts.
8	Carbon dioxide and iodine are both simple molecular substances. (a) Explain why both have very low melting points; (b) explain why carbon dioxide is a gas at room temperature while iodine is a solid.	Answer: (a) only weak forces between molecules must be overcome on melting, so the melting points are low; (b) iodine molecules are larger with more electrons and stronger intermolecular forces, so iodine is solid at room temperature.

Section C – Energetics (16 marks)

The following data come from a calorimetry experiment on neutralisation.

#	Question	Answer and marking notes
9	Dilute acid and dilute alkali are mixed and 200.0 g of water rises in temperature from 18.0 °C to 21.0 °C. Given the specific heat capacity of water is 4.2 J g-1 °C-1, calculate the energy released in kJ.	Answer: energy = 200 x 4 x 3 = 2520 J = 2.52 kJ, released as heat.
10	In the same experiment 0.05 mol of acid is neutralised. Find the molar enthalpy of neutralisation in kJ/mol.	Answer: molar enthalpy = 2.52 / 0.05 = 50.4 kJ/mol, with a negative sign for delta H.
11	Classify each reaction as exothermic or endothermic and give the sign of delta H: (a) burning methane; (b) thermal decomposition of calcium carbonate.	Answer: (a) burning methane is exothermic, delta H is negative; (b) the decomposition of calcium carbonate is endothermic, delta H is positive.
12	Explain how a catalyst affects the activation energy and delta H of a reaction, and state whether it changes the overall energy change.	Answer: a catalyst provides an alternative route of lower activation energy, so the activation energy falls; the energies of reactants and products are unchanged, so delta H and the overall energy change stay the same.

Section D – Reaction rates and equilibrium (17 marks)

The following data come from rate measurements in the laboratory.

#	Question	Answer and marking notes
13	A metal reacts with dilute acid and 60 cm3 of hydrogen is collected in the first 20 seconds. (a) Calculate the average rate over this period; (b) explain why the rate falls later in the reaction.	Answer: (a) average rate = 60 / 20 = 3.0 cm3/s; (b) the concentration of the reactants falls, collisions become less frequent, so the rate decreases.
14	Consider N2(g) + 3H2(g) ⇌ 2NH3(g) with a negative delta H. (a) What is the effect of raising the pressure on the yield of ammonia? (b) What is the effect of raising the temperature? Explain both answers.	Answer: (a) raising the pressure shifts the equilibrium to the right and increases the yield, because the forward reaction produces fewer moles of gas; (b) raising the temperature shifts the equilibrium to the left and lowers the yield, because the forward reaction is exothermic.
15	At 700 K the reaction H2(g) + I2(g) ⇌ 2HI(g) reaches equilibrium with [H2] = 0.10, [I2] = 0.10 and [HI] = 0.80 mol/dm3. Calculate Kc.	Answer: Kc = [HI]^2 / ([H2] x [I2]) = 0.80^2 / (0.10 x 0.10) = 64.
16	State and explain the effect of adding a catalyst on (a) the time taken to reach equilibrium and (b) the value of Kc.	Answer: (a) a catalyst speeds up both the forward and the reverse reactions, so equilibrium is reached sooner; (b) Kc is unchanged because a catalyst does not alter the position of equilibrium.

Section E – Electrolysis and redox (17 marks)

Answer the following questions on oxidation numbers and electrolysis.

#	Question	Answer and marking notes
17	Find the oxidation number of (a) manganese in KMnO4 and (b) sulfur in H2SO4, then state which element has the highest oxidation number in these compounds.	Answer: (a) manganese is +7; (b) sulfur is +6; manganese is the highest.
18	A copper(II) sulfate solution is electrolysed and 0.020 mol of electrons passes. The cathode reaction is Cu2+ + 2e- -> Cu and the relative atomic mass of copper is 63.5. Calculate the mass of copper deposited in g.	Answer: moles of copper = 0.020 / 2 = 0.010 mol; mass = 0.010 x 63.5 = 0.635 g.
19	Compare the anode products when dilute and concentrated sodium chloride solutions are electrolysed, and write the electrode reactions.	Answer: with the dilute solution oxygen forms at the anode by 4OH- -> O2 + 2H2O + 4e-; with the concentrated solution chlorine forms by 2Cl- -> Cl2 + 2e-.
20	A zinc strip is placed in copper(II) sulfate solution and a red deposit appears. (a) Identify the oxidising agent and the reducing agent; (b) write the ionic equation.	Answer: (a) the oxidising agent is the copper(II) ion and the reducing agent is zinc; (b) Zn + Cu2+ -> Zn2+ + Cu.

Section F – Organic chemistry (16 marks)

Answer the following questions on alkanes, alkenes and carboxylic acids.

#	Question	Answer and marking notes
21	(a) Write the general formula of the alkane homologous series; (b) an alkane contains 5 carbon atoms, give its molecular formula and molar mass.	Answer: (a) the general formula is CnH2n+2; (b) the formula is C5H12 and the molar mass is 72 g/mol.
22	Propane burns completely: C3H8 + 5O2 -> 3CO2 + 4H2O. At the same temperature and pressure, 2.0 dm3 of propane burns completely. Find (a) the volume of oxygen needed; (b) the volume of carbon dioxide formed.	Answer: (a) 10.0 dm3 of oxygen is needed; (b) 6.0 dm3 of carbon dioxide forms.
23	Name the functional groups of alkenes and carboxylic acids, and give one chemical reaction used to identify each.	Answer: the alkene functional group is the carbon-carbon double bond, which decolourises bromine water by addition; the carboxylic acid group reacts with sodium hydrogencarbonate solution to release carbon dioxide.
24	Two colourless liquids, hexane and hexene, are given. Describe one experiment to distinguish them and state the observation.	Answer: add bromine water and shake. Hexene decolourises the orange-brown solution while hexane shows no reaction, the bromine merely dissolving to give an orange solution.

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