12. Nitrogen and sulfur
A section of Chemistry, 9701
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Sulfuric acid is manufactured by the Contact process. One stage in this process is the conversion of sulfur dioxide into sulfur trioxide in the presence of a heterogeneous catalyst of vanadium(oxide, V2O5. 2 O=S=O+ O=O2 O=S=OΔH = –196 kJ mol–1 = O State the effect of a catalyst on a reaction. Explain how a catalyst causes this effect. State the meaning of the term heterogeneous as applied to catalysts. Some bond energies are given. bond bond energy / kJ mol–1 S=O (in SO2) O=O Use the data, and the enthalpy change for the conversion of sulfurdioxide into sulfurtrioxide, to calculate a value for the S=O bond energy in SO3.  S=O bond energy in SO3 = kJ mol–1 The Contact process is usually carried out at a temperature of about 400 °C and a pressure just above atmospheric pressure. Using a higher or lower temperature and pressure would affect both the rate of production of sulfurtrioxide and the yield of sulfurtrioxide. A reaction pathway diagram for both the catalysed and uncatalysed reactions between SO2 and O2 is shown. A B C D E energy progress of reaction 2SO2 + O2 2SO3 The letters A–E represent energy changes. Complete the table by stating which letter, A–E, represents the energy change described. energy change letter the energy change for the production of SO3 the activation energy for the production of SO3 in the absence of a catalyst the activation energy for the first step in the decomposition of SO3 in the presence of a catalyst  The equation for this stage of the Contact Process is shown. 2SO2+ O22SO3ΔH = –196 kJ mol–1 State and explain the effect of increasing temperature on the rate of production of SO3. State and explain the effect of increasing temperature on the yield of SO3. The SO3 produced is converted to sulfuricacid in two stages. In the first stage the SO3 is reacted with concentrated sulfuricacid to produce oleum, H2S2O7. The oleum is then reacted with water to form sulfuric acid. Suggest an equation for the reaction of oleum, H2S2O7, with water to form sulfuricacid. SO2 reacts with water to form sulfurous acid. Sulfurous acid is a weak Brønsted‑Lowry acid, while sulfuric acid is a strong Brønsted‑Lowry acid. Complete the ‘dot-and-cross’ diagram to show the bonding in a molecule of SO2. Show outer electrons only.  State the meaning of the term strong Brønsted‑Lowry acid. Write an equation to show the acid-base behaviour of sulfuricacid with water. Include state symbols. 
9701_s18_qp_21
THEORY
2018
Paper 2, Variant 1
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Ammonium iron(sulfate, (NH4)2Fe(SO4)2, has a relative formula mass, Mr, of 284. Define the term relative formula mass. One of the cations in ammonium iron(sulfate is the ammonium ion, NH4 +. Draw a ‘dot-and-cross’ diagram of an ammonium ion. Show outer shell electrons only. Use × to show electrons from nitrogen. Use ● to show electrons from hydrogen.  Suggest the shape of an ammonium ion and predict the bond angle. shape bond angle  In aqueous solution the ammonium ion acts as a weak Brønsted-Lowry acid. Explain the meaning of the term weak Brønsted-Lowry acid. Write an equation to show this behaviour of the ammonium ion in water. Include state symbols. Mohr's salt, (NH4)2Fe(SO4)2•xH2O, is the hydrated form of ammonium iron(sulfate. x represents the number of moles of water in 1mole of the salt. A student wanted to determine the value of x. 0.784 g of the hydrated salt was dissolved in water and this solution was acidified. All of the solution was titrated with 0.0200 mol dm–3 potassiummanganate(. 20.0 cm3 of this potassiummanganate(solution was required for complete reaction with the Fe2+ ions. Use changes in oxidation numbers to balance the equation for the reaction taking place. MnO4 –+ Fe2++ H+Mn2++ Fe3++ H2O State the role of the Fe2+ ions in this reaction. Explain your answer. Calculate the amount, in moles, of manganate(ions that reacted.  amount = mol Calculate the amount, in moles, of Fe2+ ions in the sample of the salt.  amount = mol Calculate the relative formula mass of (NH4)2Fe(SO4)2•xH2O.  relative formula mass = Calculate the value of x.  x = 
9701_s18_qp_22
THEORY
2018
Paper 2, Variant 2
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