EJC Prelim H2 Chemistry Paper 2 Solution

1 © EJC 9729/02/J2Prelim/25 1 Mount Ijen in East Java is famous for its rare blue flames, visible at night. The phenomenon occurs when sulfur vapour , from the volcano’s cracks burns, producing bright blue flames and sulfur dioxide. In the cool high -altitude air, some of the vapour condenses into solid sulfur. (a) A team of environmental chemists were authorised to collect solid sulfur deposits near Mount Ijen’s crater to investigate volcanic activity. The chemists burnt the sulfur sample and measured the temperature change for a fixed amount of water placed in a calorimeter. The data from their experiment is shown in Table 1.1. Table 1.1 mass of solid sulfur powder burnt /g 0.76 mass of water in beaker /g 150 initial temperature of water /oC 29.8 final temperature of water / oC 39.7 (i) Construct an equation to represent the standard enthalpy change of combustion of solid sulfur, S. ................................ ................................ ................................ ...................... [1] (ii) Calculate the enthalpy change of combustion of solid sulfur , S, based on their experiment. [2] EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2025 General Certificate of Education Advanced Level Higher 2 Chemistry Paper 2 Suggested Solutions with Marker’s comments ( )150 4.18 39.7 29.8 6207.3 J q mc T=−  =−   − =− 1 1 6207.3 0.76 32.1 262176 J mol 262 kJ mol qH n − − − = = =− =− S (s) + O2 (g) → SO2 (g)

2 © EJC 9729/02/J2Prelim/25 (iii) During volcanic activity, many sulfur-containing gases such as hydrogen sulfide, H2S, and sulfur dioxide, SO2, are released. In the atmosphere, H2S can oxidise to form SO2, and subsequently sulfur trioxide, SO3, may be formed. H2S(g) + 3/2 O2(g) → SO2(g) + H2O(l) ∆Hr = –562 kJ mol–1 Use the data given in Table 1.2 to calculate the enthalpy change of combustion of solid sulfur, S. Table 1.2 compound ∆Hf / kJ mol–1 H2S(g) –20.6 H2O(l) –285.8 [2] (iv) Comment on the difference in values for the enthapy change of combustion of solid sulfur determined in (a)(ii) and (a)(iii). ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ...................... [1] (v) With reference to a relevant chemical equation, explain how the release of sulfur-containing gases during volcanic activity can have a negative impact on the environment. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ...................... [2] For (a)(ii), the heat lost to the surroundings was not a ccounted for / the experiment was not 100% efficient / the sulfur sample from the volcano was not pure. SO3 + H2O → H2SO4 SO3 dissolves in water to result in the formation of acid rain that can lead to soil degradation/destruction of crops and farmlands / corrision of buildings or structures / water acidification that harms marine life. S(s) + O2(g) → SO2(g) represents ∆Hf(SO2) and also represents ∆Hc(S) ∆Hr = [∆Hf(SO2) + (–285.8)] – (–20.6)] –562 = [∆Hf(SO2) + (–285.8)] – (–20.6)] ∆Hf(SO2) = –296.8 kJ mol–1 ∆Hf(SO2) = ∆Hc(SO2) = –297 kJ mol–1

3 © EJC 9729/J2Prelim/25 [Turn Over (b) Sulfur in volcanic emissions primarily exists as a mixture of two stable isotopes: 32S and 34S. The chemists analysed volcanic gas samples to determine the isotopes’ relative abundance, which reveals the sulfur’s origin. • If the sample was enriched in 32S, it originated from deep mantle degassing. • If the sample was enriched in 34S, it originated from hydrothermally recycled sources. (i) 1.994 g of SO 2 evolved at Mount Ijen allowed the chemists to extract 1.00 g of elemental sulfur containing mixture of isotopes, 32S and 34S, for further analysis. Calculate the percentage by mass of 32S in the elemental sulfur extracted. You may use the chemical formula S to represent elemental sulfur in your calculations. [3] (ii) Hence, suggest the likely origin of the sulfur sample. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ...................... [1] Deep mantle degassing, since % by mass of 32S is signficantly high er/ the sample is enriched in 32S. Let the mass of 32S to be x g and 34S to be (1.00 – x) g. = −= 32 34 amount of S mol32 1.00amount of S mol 34 x x Molar mass of 32SO2 = 32 + (16 × 2) = 64 g/mol Molar mass of 34SO2 = 34 + (16 × 2) = 66 g/mol S ≡ SO2 −    +  =       + − = = = 10064 66 1.994 32 34 2 1.941 1.941 1.994 0.059 0.053 0.898 xx xx x x alternatively 1.994 1.00amount of O 16 0.062125 mol −= = S ≡ 2O −+= = 1.00 0.062125 32 34 2 0.898 xx x = = 32 0.898% by mass of S 100%1.00 89.8%

4 © EJC 9729/02/J2Prelim/25 While volcanic emissions affect the isotopic fractions of sulfur, the natural isotopic abundance of sulfur in Earth's environment is generally found to be present as follows. Table 1.3 isotope relative isotopic mass percentage abundance / % 32S 31.972 95.02 33S 32.971 0.75 34S 33.968 4.21 36S 35.967 0.02 (iii) Use the data in Table 1.3 to calculate the relative atomic mass of sulfur, giving your answer to two decimal places. [1] [Total: 13]  +  +  + = = r 95.02 31.975 0.75 32.971 4.21 33.968 0.02 35.96 7 of sulfur 100 32.06 (2 d.p.) A

5 © EJC 9729/J2Prelim/25 [Turn Over 2 Group 17 elements form a range of oxoacids with different oxidation states , such as HClO4. (a) (i) HClO4 has two central atoms ; one chlorine atom and one oxygen atom. In addition, the H atom is bonded to a O atom. Draw the dot-and-cross diagram of HClO4. [1] (ii) Use VSEPR to describe and explain the shape and bond angle about central Cl and O atom in HClO4. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [3] (b) With reference to your answer in (a)(i), explain why HFO4 does not exist? ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ .............................. [1] (c) The Latimer diagram of some chlorine species in acidic solution is given in Fig. 2.1