St. Patrick's School 2026 Mastery Review (Mid-Year) Chemistry Mark Scheme

1 ST. PATRICK'S SCHOOL MASTERY REVIEW 2026 MARK SCHEME LEVEL : SECONDARY 4 EXPRESS SUBJECT : CHEMISTRY SECTION A 1 2 3 4 5 6 7 8 9 10 B A C B C D A D A C Section B 1 a FeSO4/ Ag2CO3 1 5 b Ag2CO3 1 c Al2O3 1 d CaCl2 1 e SiO2 1 2 a Line through the points is smooth and has the correct profile Ignore 0–5 cm3 section of the graph. (1) Lose this mark if graph is kinked or not a single line. (1) Line ignores the point at 12 cm3 Lose this mark if point clearly not treated as an anomaly. (1) 3 9 bi 24.4 cm3 ± 0.2 If no answer in (i) allow answer written on the graph. Allow this answer only. Do not penalise precision. 1 bii 12.2 cm3 ± 0.1 If no answer in (ii), allow answer written on the graph. Allow answer to (i) divided by 2. Do not penalise precision. 1 biii 3.9 ± 0.2 If no answer in (iii), allow answer written on the graph or can be detected from the graph). Consequential marking from (ii) Lose this mark if answer not given to 1 dp.( whole no without 1 dp accepted) 1 c The pH is about 3.8-4.0, which is higher than the pH expected for a strong acid/strong acid has pH 0-2. (Accept has pKa> 0/pH which is of a weak acid) This shows that acid Y only partially ionises in water and is therefore a weak acid. 1 1 d pKa = 3.8-4.0 gives methanoic acid. Accept based on their value of pH, the corresponding named acid from pKa No lucky guesses – candidates must apply answer from (c). 1 3 a Cl2 + 2OH– → ClO– + Cl – + H2O Chlorine oxidation state decreased from 0 in Cl2 to -1 in Cl – (1) 3 3

2 and increased from 0 in Cl2 to +1 in ClO–. (1) Both an oxidising and reducing agent.(1) Accept : Cl2is an oxidising agent, oxidation states decreases from decreased from 0 in Cl2 to -1 in Cl – (1m only) or Cl2is a reducing agent , oxidation sate of chlorine increased from 0 in Cl2 to +1 in ClO–. ( 1m only) 4 a any one from: • carbon monoxide toxic / poisonous as cause starvation of oxygen in body cells/ combined with haemoglobin in red blood cells which carries oxygen. do not accept carbon monoxide is harmful / dangerous / deadly • greater public concern / awareness about pollution do not accept comments about the effects of other pollutants ignore unspecified comments about carbon monoxide pollution • more cars so otherwise there would be more carbon monoxide entering atmosphere • improved engine technology • catalytic converters have been introduced 1 6 b any one from: • (to reduce) health problems allow (to reduce) specified health problems e.g. accept caused breathing difficulties, asthma, lung cancer ( accept lungs or eyes irritation ) • (to reduce) smog / (to reduce) the formation of particulates ignore global warming do not accept to reduce soot 1 c nitrogen (from atmosphere) reacts with oxygen (from atmosphere) at high temperature (in engine) and provide enough energy (1) to break N≡N bond or to provide Ea for N2/O2 reaction (1) 1 1 di 2NO2(g) → N2(g) + 2O2(g) or 2CO (g) + 2NO(g)→2CO2(g) + N2(g) 1 d Catalyst arranged in honeycombed structures that maximise /increase the surface area for gases to interact with 1 5 a O2-, one oxygen atom does not have complete/full valence shell/ 7 valence electrons instead of 8 after bonding accept one oxygen atom missing one electron/ one ato has electronic configurtion (2.7) 1 6 bi Na2O2 1 bii Mg(O2)2

3 c 1m for shared electrons and 1 m for unshared electrons 2 d O2 has double bond between O atoms while peroxide has a single bond between oxygen atoms  the peroxide ion has gained two electrons, so fewer electrons need to be shared between the oxygen atoms. OR  Each oxygen already has more electrons in its outer shell , so only one pair of electrons is shared. 1 1 6 a The atomic radii increase due to the increased in the number of electrons of the halogens down the group. As the number of electrons increases, more electron shells are required( to ‘house’) the electrons, thus creating a bigger atom. 1 1 7 b Since electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons, this implies that the ability of attraction of another electron from other atom decreases on going down the group as electronegativity decreases on going down the group. Hence, the decrease in electronegativity(1) leads to the decrease of reactivity (1) down the group. -1 m if failed to note from Graph 2 that electronegativity decreases down the group or vice-versa and explain instead the electrostatic forces of attraction between nucleus and valence electrons decreases 1 2 c Fluorine is more reactive than bromine, which is brown in colour. The fluorine gas displaces bromide ions from potassium bromide, causing it to be oxidised to bromine, resulting in brown colour. 2 Br- (aq) + F2 (g)  2 F- (aq) + Br2 (l) F2 more reactive + displacement - 1m Bromine liquid is brown - 1m Correct balance equation and sate symbols -1m 1 1 1 7 Mol of acid = 24/1000 x 0.050 = 0.0012 mol Mol of OH- = 20/1000 x 0.060 =0.0012 mol Mol ratio of acid : Base is 1:1 With Hydrochloric Acid M(OH)2 + 2HCl → MCl2 + 2H2O Ratio is 1 hydroxide : 2 acid. 3 3

4 With Sulfuric Acid M(OH)2 + H2SO4 → MSO4 + 2H2O M1 Calculation of Moles Calculating the number of moles for both the metal hydroxide and the unknown acid M2 Stoichiometric Ratio Identifying the reacting mole ratio of acid to base as 1:1 based on the calculated values. M3 Deduction & Equation Identifying sulfuric acid by explaining that it is dibasic and provides the 2H+ ions needed to neutralize the 2OH- ions in 1 mole of M(OH)2. Accept in embedded chemical equations of metal hydroxide with hydrochloric acid is 1:2 and with sulfuric acid is 1:1 – 1m 8 a Zn + 2H+  Zn2+ + H2 1 6 b Catalyst 1 c CuSO4 reacts with Zn in a displacement reaction. Reduces the amount of Zn available to react with sulfuric acid 1 1 d No Different metals have Ar => same mass = different no. of moles of metal => different no. of moles of H2 = different vol of H2 Unreactive metals like silver will not produce any H2 1 1 9 ai Volume increases till 3rd dotted line 1 4 aii Steeper graph, same ending point 1 b Energy profile diagram for ENDOTHERMIC reaction - Shape – reactants < products - Labels – axis, delta H and Ea Ecf max 1m if exothermic reaction with correct labels 1 1 10 a C6H5NH2 : 2nd order Br2 : 1st order 1 1 10 b 0.028 mol/dm3s-1 1 c Concentration increase  Particles are closer to one another OR More reacting particles per unit volume  Frequency of collisions increase  Rate of reaction increase 1 1

5 d No Step 1 is not redox – no change in oxidation state, o.n. of S remains at +6 Step 2 is redox: - O.N. of S decrease from +6 in sulfuric acid to +4 in sulfur dioxide => S is reduced = sulfuric acid = O.A. Need to state either oxidation number of either S or Br 1 1 e Production of sulfur dioxide – reacts with oxygen and water in the air to produce sulfuric acid. 2SO2 + O2 + 2H2O  2H2SO4 Environmental problem: - Acidify waterbodies – kills marine life - Acidify soil – kills crops and plants - Corrode buildings and structures of limestone and metal R: HSO4- + H+  H2SO4 1 1 1 Section C 11 a Cathode: O2 + 4H+ + 4e  2H2O Anode: C2H4O2 + 2H2O  2CO2 + 8H+ + 8e 1 1 10 b Allowing only H+ ions to pass through from the anode to the cathode 1 c Fuel produces as much CO 2 as the CO 2 captured during the production – no nett CO2 is added to the environment through combustion 1 d Accept any reasonable answer: - MFC can act as biological filter and produce electrical energy at the same time, but HFC only produces electrical energy