6.3. Reversible reactions and equilibrium
A subsection of Chemistry, 0620, through 6. Chemical reactions
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This question is about compounds of sulfur. Sulfuricacid, H2SO4, is manufactured using the Contact process. This manufacture involves four stages. stage 1 Molten sulfur burns in air to produce sulfurdioxide. stage 2 Sulfurdioxide reacts with oxygen to form sulfurtrioxide. stage 3 Sulfurtrioxide combines with concentrated sulfuricacid to form oleum, H2S2O7. stage 4 Oleum reacts to form concentrated sulfuricacid. Write a chemical equation for the reaction occurring in stage1. State the essential conditions that are necessary for stage2. Write an equation for the chemical reaction that occurs. Write a chemical equation for the reaction occurring in stage3. Name the substance that reacts with oleum in stage4. Hydrogensulfide has the formula H2S. Complete the dot-and-cross diagram to show the electron arrangement in a molecule of hydrogensulfide. Show outer shell electrons only. S H H  Balance the chemical equation for the reaction of hydrogensulfide with sulfur dioxide shown. H2S + SO2 → S + H2O 
0620_s22_qp_41
THEORY
2022
Paper 4, Variant 1
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This question is about sulfur and compounds of sulfur. Sulfur is converted into sulfuricacid, H2SO4, by the Contact process. The process involves four stages. stage 1 Molten sulfur is converted into sulfur dioxide. stage 2 Sulfur dioxide reacts with oxygen to form sulfur trioxide. stage 3 Sulfur trioxide combines with concentrated sulfuric acid to form oleum, H2S2O7. stage 4 Oleum reacts to form concentrated sulfuric acid. In stage 1, iron pyrites, FeS2, can be used instead of molten sulfur. The iron pyrites is heated strongly in air. Balance the equation for the reaction occurring when iron pyrites reacts with oxygen in the air. FeS2 + O2 → Fe2O3 + SO2 Name Fe2O3. Include the oxidation number of iron. The equation for stage 2 is shown. 2SO2+ O22SO3The forward reaction is exothermic. The reaction is carried out at a temperature of 450 °C and a pressure of 2 atm. Using explanations that do not involve cost: explain why a temperature greater than 450 °C is not used explain why a pressure lower than 2 atm is not used. When sulfuricacid reacts with ammonia the salt produced is ammoniumsulfate. Write the symbol equation for this reaction. Lead(sulfate is an insoluble salt. Lead(sulfate can be made from aqueous ammoniumsulfate using a precipitation reaction. Name a solution that can be added to aqueous ammoniumsulfate to produce a precipitate of lead(sulfate. Write an ionic equation for this precipitation reaction. Include state symbols. The precipitate of lead(sulfate forms in an aqueous solution. Describe how pure lead(sulfate can be obtained from the mixture. 
0620_s23_qp_41
THEORY
2023
Paper 4, Variant 1
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Over 200 million tonnes of sulfuricacid are manufactured every year. State the name of the process used to manufacture sulfuricacid. Part of the manufacture of sulfuricacid involves converting sulfurdioxide to sulfurtrioxide. Describe two methods by which sulfurdioxide is obtained.  The conversion of sulfurdioxide to sulfurtrioxide is a reversible reaction which can reach equilibrium. 2SO2+ O22SO3State two features of an equilibrium.  State the typical conditions and name the catalyst used in the conversion of sulfurdioxide to sulfurtrioxide. temperature °C pressure kPa catalyst  Complete Table3.1 to show the effect, if any, when the following changes are applied to the conversion of sulfurdioxide to sulfurtrioxide. 2SO2+ O22SO3The forward reaction is exothermic. Only use the words increases, decreases or no change. Table 3.1 change effect on the rate of the forward reaction effect on the concentration of SO3at equilibrium temperature decreases decreases pressure increases no catalyst decreases  Explain in terms of collision theory why reducing the temperature decreases the rate of the forward reaction. Sulfuricacid contains SO4 2– ions. The oxidation number of O atoms in SO4 2– ions is –2. Determine the oxidation number of S atoms in SO4 2– ions. Show your working.  oxidation number = 
0620_s23_qp_42
THEORY
2023
Paper 4, Variant 2
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