Chemistry 0620/42 May/June 2025: question paper and mark scheme

Question

Using numbers only, state the: (a) percentage of nitrogen in clean, dry air …… [1] (b) typical operating temperature of the Contact process in °C …… [1] (c) number of metals in Period 3 of the Periodic Table …… [1] (d) number of halogens which are gases at r.t.p. …… [1] (e) typical temperature for the fermentation of aqueous glucose in °C …… [1] (f) number of unbranched structural isomers of C4H8 which decolourise aqueous bromine …… [1] (g) number of covalent bonds in one molecule of ethanol …… [1] (h) number of carbon atoms in one molecule of propyl butanoate. …… [1] [Total: 8]

Question

This question is about covalent compounds. (a) State what is meant by a covalent bond. …… [2] (b) Chlorine(I) oxide, Cl 2O, is a simple molecule with covalent bonds. (i) State what is meant by (I) in the name chlorine(I) oxide. …… [2] (ii) Complete the dot-and-cross diagram in Fig. 2.1 of a molecule of chlorine(I) oxide. Show outer electrons only. Cl O Cl Fig. 2.1 [3] (iii) Explain, in terms of structure and bonding, why Cl 2O boils at a low temperature and does not thermally decompose into its constituent elements, Cl 2 and O2. …… [3] (iv) Give two reasons why liquid Cl 2O is a poor conductor of electricity. 1 …… 2 …… [2] (c) Carbon and silicon(IV) oxide both exist as giant covalent structures. (i) Name a giant covalent structure of carbon which conducts electricity. …… [1] (ii) Identify the particles responsible for the conduction of electricity in this covalent structure of carbon. …… [1] (iii) Silicon(IV) oxide contains silicon atoms, Si, and oxygen atoms, O. Fig. 2.2 shows part of the giant covalent structure of silicon(IV) oxide. Complete the diagram in Fig. 2.2 by adding the symbol for each of the 9 atoms shown. Fig. 2.2 [2] [Total: 16]

Question

This question is about electrolysis. (a) Complete the definition of electrolysis by filling in the missing words. The decomposition of …… compounds when …… or …… by the passage of an electric current. [3] (b) Electrolysis of aqueous copper(II) sulfate is carried out using the apparatus shown in Fig. 3.1. The electrodes are made of platinum. power + supply – Fig. 3.1 (i) State whether the mass of the cathode increases, decreases or remains the same when platinum electrodes are used. …… [1] (ii) Describe the change in appearance, if any, of the electrolyte when platinum electrodes are used. …… [1] (iii) Describe what is seen at the anode when platinum electrodes are used. …… [1] (iv) Write the ionic half-equation for the reaction at the anode. …… [3] (c) The electrolysis is repeated using copper electrodes. (i) State whether the mass of the cathode increases, decreases or remains the same when copper electrodes are used. …… [1] (ii) Describe the change in appearance, if any, of the electrolyte when copper electrodes are used. ……[1] (d) Aluminium is extracted from its purified ore by electrolysis. (i) Name this ore of aluminium. …… [1] (ii) Name the substance added to this process to reduce the operating temperature. …… [1] (iii) Explain why the anodes need to be continually replaced during this process. …… [2] [Total: 15]

Question

Chromium is the element with atomic number 24 in the Periodic Table. (a) The main ore of chromium is chromite. Chromite contains FeCr2O4. FeCr2O4 reacts with carbon. (i) Complete the equation for this reaction. FeCr2O4 + 4C Fe + 2Cr + …… [1] (ii) Suggest one disadvantage of extracting chromium by reacting chromite with carbon. …… [1] (b) Chromium can be mixed with nickel and other elements to form stainless steel. Name the type of substance formed when a metal is mixed with other elements. …… [1] (c) (NH4)2Cr2O7 is a compound containing chromium. The negative ion in (NH4)2Cr2O7 is Cr2O72–. (i) State the sum of the oxidation numbers in the Cr2O72– ion. …… [1] (ii) The oxidation number of each O in Cr2O72– ions is –2. Determine the oxidation number of each Cr in Cr2O72– ions. Show your working. …… [2] (iii) When (NH4)2Cr2O7 is heated the following reaction occurs. (NH4)2Cr2O7 Cr2O3 + 4H2O + N2 Calculate the volume of nitrogen gas produced at r.t.p., in cm3, when 1.26 g of (NH4)2Cr2O7 is heated using the following steps. The Mr of (NH4)2Cr2O7 is 252. • Calculate the number of moles of (NH4)2Cr2O7 used. …… mol • Determine the number of moles of N2 formed. …… mol • Calculate the volume of N2 formed at r.t.p. in cm3. …… cm3 [3] [Total: 9]

Question

Alkali metals are reactive elements. (a) State the group number of the alkali metals. …… [1] (b) Identify the alkali metal which: (i) has the highest melting point …… [1] (ii) has the highest density …… [1] (iii) has the lowest reactivity …… [1] (iv) burns with a lilac flame …… [1] (v) is found in fertilisers to improve plant growth. …… [1] (c) Sodium sulfide, Na2S, is an ionic compound. Complete Fig. 5.1 to show the electronic configurations of the ions in sodium sulfide. Show the charges on the ions. Na S Na Fig. 5.1 [3] (d) Rubidium has only two naturally occurring atoms, 85Rb and 87Rb. (i) State the term given to these naturally occurring atoms of rubidium. …… [1] (ii) Complete Table 5.1 to show the number of protons, neutrons and electrons in the atom and ion of rubidium shown. Table 5.1 85Rb 87Rb+ protons neutrons electrons [3] (iii) The relative atomic mass of rubidium to one decimal place is 85.5. Determine the relative abundance of 85Rb in rubidium. Express your answer as a percentage. ……% [1] [Total: 14]

Question

Ethene, C2H4, is the first member of a family of similar compounds which contains the alkene functional group. (a) State the term for a family of similar compounds which contain the same functional group. …… [1] (b) Determine the difference in relative molecular mass between C2H4 and the next member in this family of similar compounds. …… [1] (c) Write the symbol equation for the complete combustion of C2H4. …… [2] (d) C2H4 reacts with steam to form ethanol. C2H4(g) + H2O(g) C2H5OH(g) ΔH = – 45 kJ / mol The process happens in a closed system and the reaction reaches an equilibrium. The conditions for this process are 300 °C and 60 atm pressure. H3PO4 is used as a catalyst. (i) Complete Table 6.1 to show the effect, if any, on the concentration of C2H4(g) when the following changes to the conditions are applied. Only use the words increases, decreases or no change. Table 6.1 effect on the concentration of C2H4(g) change to condition at equilibrium temperature is decreased some C2H5OH(g) is removed pressure is increased a more effective catalyst is used [4] (ii) Explain, in terms of collision theory, why the rate of the forward reaction increases if the temperature increases. …… [3] (e) Compound B has the displayed formula shown in Fig. 6.1. H O H H O C C H C C H C O O H Fig. 6.1 (i) Deduce the molecular formula of compound B. …… [1] (ii) State why compound B is unsaturated. …… [1] (iii) Draw the structure of one repeat unit of the polymer formed when compound B undergoes addition polymerisation. [2] (iv) Explain why 1 mol of compound B reacts with 2 mol of sodium hydroxide, NaOH. …… [1] (v) Calculate the volume, in cm3, of 0.250 mol / dm3 NaOH that reacts with 0.100 mol of compound B. volume = …… cm3 [2] [Total: 18] The Periodic Table of Elements Group I II III IV V VI VII VIII 1 2 H He hydrogen helium Key 1 4 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 7 9 relative atomic mass 11 12 14 16 19 20 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23 24 27 28 31 32 35.5 40 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39 40 45 48 51 52 55 56 59 59 64 65 70 73 75 79 80 84 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 16 85 88 89 91 93 96 – 101 103 106 108 112 115 119 122 128 127 131 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 133 137 178 181 184 186 190 192 195 197 201 204 207 209 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 139 140 141 144 – 150 152 157 159 163 165 167 169 173 175 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232 231 238 – – – – – – – – – – – The volume of one mole of any gas is 24 dm3 at room temperature and pressure (r.t.p.).