12.1. Nitrogen and sulfur
A subsection of Chemistry, 9701, through 12. Nitrogen and sulfur
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Sulfides are compounds that contain sulfur but not oxygen. Carbondisulfide, CS2, is a volatile liquid at room temperature and pressure. State the meaning of volatile. Draw a ‘dot-and-cross’ diagram of the CS2 molecule.  Suggest the bond angle in a molecule of CS2. CS2 is a liquid under room conditions, while CO2 is a gas. Explain what causes the difference in the physical properties between CS2 and CO2. The enthalpy change of combustion of CS2is represented by the following equation. ∆Hc CS2+ 3O2CO2+ 2SO2Define enthalpy change of combustion. The table shows the enthalpy changes of formation of CS2, CO2and SO2. compound enthalpy change of formation, ∆Hf / kJ mol–1 CS2+89.7 CO2–394 SO2–297 Use the data in the table to calculate the enthalpy change of combustion, ∆Hc, of CS2, in kJ mol–1. Show your working. ∆Hc of CS2= kJ mol–1  Hydrogensulfide gas, H2S, is slightly soluble in water. It acts as a weak acid in aqueous solution. State the meaning of weak acid. Give the formula of the conjugate base of H2S. H2Sreacts slowly with oxygen dissolved in water. The reaction is represented by the following equation. H2S+ 1 2O2→ H2O+ SExplain, with reference to oxidation numbers, why this reaction is a redox reaction. The compound As2S3 is a common mineral. When As2S3 is heated strongly in air, it forms a mixture of products, as shown. 2As2S3+ 9O2→ As4O6+ 6SO2A sample containing 0.198 g As2S3 is placed in 0.100 dm3 of pure oxygen, an excess, in a reaction chamber connected to a gas syringe at room temperature. The reactants are heated until no further change is observed. The products are then allowed to cool to room temperature. Calculate the volume, in dm3, of gas present at the end of the experiment. The molar volume of gas is 24.0 dm3 mol–1 under these conditions. Assume that the pressure is constant throughout the experiment. Show your working.  volume of gas remaining = dm3  State the environmental consequences of releasing SO2into the atmosphere. SO2can be removed from the air by reacting it with NaOH. Construct an equation for the reaction of SO2with NaOH. Include state symbols. 
9701_w21_qp_21
THEORY
2021
Paper 2, Variant 1
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Sulfides are compounds that contain sulfur but not oxygen. Carbondisulfide, CS2, is a volatile liquid at room temperature and pressure. State the meaning of volatile. Draw a ‘dot-and-cross’ diagram of the CS2 molecule.  Suggest the bond angle in a molecule of CS2. CS2 is a liquid under room conditions, while CO2 is a gas. Explain what causes the difference in the physical properties between CS2 and CO2. The enthalpy change of combustion of CS2is represented by the following equation. ∆Hc CS2+ 3O2CO2+ 2SO2Define enthalpy change of combustion. The table shows the enthalpy changes of formation of CS2, CO2and SO2. compound enthalpy change of formation, ∆Hf / kJ mol–1 CS2+89.7 CO2–394 SO2–297 Use the data in the table to calculate the enthalpy change of combustion, ∆Hc, of CS2, in kJ mol–1. Show your working. ∆Hc of CS2= kJ mol–1  Hydrogensulfide gas, H2S, is slightly soluble in water. It acts as a weak acid in aqueous solution. State the meaning of weak acid. Give the formula of the conjugate base of H2S. H2Sreacts slowly with oxygen dissolved in water. The reaction is represented by the following equation. H2S+ 1 2O2→ H2O+ SExplain, with reference to oxidation numbers, why this reaction is a redox reaction. The compound As2S3 is a common mineral. When As2S3 is heated strongly in air, it forms a mixture of products, as shown. 2As2S3+ 9O2→ As4O6+ 6SO2A sample containing 0.198 g As2S3 is placed in 0.100 dm3 of pure oxygen, an excess, in a reaction chamber connected to a gas syringe at room temperature. The reactants are heated until no further change is observed. The products are then allowed to cool to room temperature. Calculate the volume, in dm3, of gas present at the end of the experiment. The molar volume of gas is 24.0 dm3 mol–1 under these conditions. Assume that the pressure is constant throughout the experiment. Show your working.  volume of gas remaining = dm3  State the environmental consequences of releasing SO2into the atmosphere. SO2can be removed from the air by reacting it with NaOH. Construct an equation for the reaction of SO2with NaOH. Include state symbols. 
9701_w21_qp_23
THEORY
2021
Paper 2, Variant 3
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Separate samples of 0.01 mol of magnesiumnitrate and 0.01 mol of strontiumnitrate are heated until completely decomposed to the metal oxide, nitrogendioxide and oxygen. State which of these two Group2 nitrates requires the higher temperature before it begins to decompose. Explain your answer. After decomposition is complete the 0.01 mol sample of magnesium oxide is taken and increasing amounts of water are added to it, with stirring, until no solid remains. This procedure is repeated with the 0.01 mol sample of strontiumoxide. Identify the sample to which most water must be added to cause all the solid to dissolve. Explain your answer by reference to the solubilities of the products formed when water is added to the oxides. You should refer to relevant energy terms in your answer. The nitrogen dioxide given off by the decomposition of 0.0100 mol of strontium nitrate is dissolved in water. The oxidising agent H2O2is then added to give 150.0 cm3 of a solution in which nitricacid, HNO3, is the only nitrogen-containing product. Calculate the concentration, in mol dm–3, of HNO3 in the 150.0 cm3 of solution.  concentration = mol dm–3 The HNO3 present in 25.0 cm3 of this solution is neutralised using 0.125 mol dm–3 NaOH. Calculate the minimum volume, in cm3, of NaOHneeded. Give your answer to three significant figures.  volume = cm3
9701_w21_qp_42
THEORY
2021
Paper 4, Variant 2
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An excess of aqueous ammonia is added to a solution containing the complex ion [Co(H2O)6]2+. [Co(H2O)6]2+ + 6NH3 [Co(NH3)6]2+ + 6H2O Complete the sentence to describe the colour change that will be seen during this reaction. The colour changes from to ���������������������������������������������� . Write an expression for the stability constant, Kstab, of [Co(NH3)6]2+. Kstab =  The numerical value of Kstab of [Co(NH3)6]2+ is 7.7×104. What deduction about the properties of [Co(NH3)6]2+ and [Co(H2O)6]2+ can be made from this Kstab value? Oxygen can oxidise [Co(NH3)6]2+ to [Co(NH3)6]3+ under standard conditions in alkaline solutions. [Co(NH3)6]3+ + e– [Co(NH3)6]2+ E o = +0.10 V Use this information and the Data Booklet to calculate the value for this oxidation of [Co(NH3)6]2+.  = V  Write an ionic equation for this oxidation of [Co(NH3)6]2+. Predict, by selecting suitable data from the Data Booklet, whether oxygen can oxidise Co2+in acidic solution, in the absence of ammonia. Explain your answer. 
9701_w21_qp_42
THEORY
2021
Paper 4, Variant 2
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