DHS Prelim P2_Ans

1 H2 Chemistry Prelims P2 Answers Scheme © DHS 2017 9729/02 Answer all questions in the spaces provided. 1 The chalcogens, or the oxygen family, are the elements in group 16 of the Periodic Table. These elements are common in both organic and inorganic compounds. (a) The graph below shows the trend in the first ionisation energies of oxygen, sulfur and selenium. (i) Explain the trend in the first ionisation energies of oxygen, sulfur and selenium. The first ionisation energy decreases from oxygen to selenium. This is because down the group while the nuclear charge increases, number of quantum shells increases and valence electrons are further away from the nucleus. Hence, down the group, the valence electrons experience weaker attraction to the nucleus and a smaller amount of energy is required to remove this electron from the atom. [2] (ii) On the same grid above, sketch the trend in the first ionisation energies of nitrogen, phosphorus and arsenic. [1] 1st Ionisation Energy / kJ mol-1 1st I.E. / kJ mol–1 Oxygen Sulfur Selenium Nitrogen Phosphorus Arsenic Oxygen Sulfur Selenium

2 H2 Chemistry Prelims P2 Answers Scheme © DHS 2017 9729/02 (b) A common chalcogen-containing reagent used in both organic and inorganic synthesis is hydrogen peroxide, H2O2. Hydrogen peroxide readily decomposes at room temperature. Iodide ions, I–, catalyse this decomposition, as shown below: Step I: H2O2 + I – → H2O + IO– (slow) Step II: H2O2 + IO– → H2O + O2 + I – The overall equation for the decomposition of hydrogen peroxide is shown below: 2H2O2 → 2H2O + O2 The enthalpy and entropy changes for the reaction above are shown in the table below: Enthalpy change / kJ mol–1 –98 Entropy change / J K–1 mol–1 +71 (i) Using the data above, complete the diagram below to show the energy profile diagram for the decomposition of hydrogen peroxide in the presence of iodide ions. [2] (ii) An unknown amount of hydrogen peroxide was allowed to decompose in a 5 dm 3 closed vessel at 120 ºC. When all the hydrogen peroxide was decomposed, a pressure of 177 kPa was measured in the vessel. Determine the amount of hydrogen peroxide that decomposed in the vessel. (Assume that H2O and O2 are ideal gases under the above reaction conditions) pV = nRT Total moles of gas in vessel, n = 177000 × 5 ×10−3 8.31 ×(273+120) = 0.27098 mol Hence, moles of hydrogen peroxide that decomposed = 0.27098 ÷ 3 × 2 = 0.181 mol [2] Energy Progress of reaction Ea2 2H2O2 + I – 2H2O + O2 + I – –98 kJ mol–1 Ea1 H2O + H2O2 + IO –

3 H2 Chemistry Prelims P2 Answers Scheme © DHS 2017 9729/02 (c) Chalcogens are also very commonly found in organic compounds. Table 1 below shows some common oxygen or sulfur containing organic functional groups. Oxygen- containing