10.1. Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds
A subsection of Chemistry, 9701, through 10. Group 2
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Disodium phosphate, (Na+)2(HPO4 2–), reacts with an acid to form monosodium phosphate, Na+(H2PO4 –). Identify the ions that are a conjugate acid–base pair in this reaction, using the formulae of the species involved. conjugate acid conjugate base Define buffer solution. Write two equations to show how a mixture of (Na+)2(HPO4 2–) and Na+(H2PO4 –) can act as a buffer solution. equation 1 equation 2 Identify one inorganic ion that acts as a buffer in blood. Compound E is the hydroxide of a Group 2 element. Compound E is a strong alkali. 2.63 g of E is dissolved in water to make 250 cm3 of solution F. Solution F has a pH of 13.09 at 298 K. Show that the concentration of hydroxide ions in solution F is 0.123 mol dm–3. Explain why the concentration of compound E in solution F is 0.0615 mol dm–3. Use the concentration given in to identify compound E. compound E Compound E is much more soluble than magnesium hydroxide. A saturated solution of magnesium hydroxide in water has a concentration of 1.40 × 10–4 mol dm–3 at 298 K. Calculate the solubility product, Ksp, of magnesium hydroxide. Include units. Ksp = units Explain why compound E is much more soluble than magnesium hydroxide.
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THEORY
2024
Paper 4, Variant 1
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Disodium phosphate, (Na+)2(HPO4 2–), reacts with an acid to form monosodium phosphate, Na+(H2PO4 –). Identify the ions that are a conjugate acid–base pair in this reaction, using the formulae of the species involved. conjugate acid conjugate base Define buffer solution. Write two equations to show how a mixture of (Na+)2(HPO4 2–) and Na+(H2PO4 –) can act as a buffer solution. equation 1 equation 2 Identify one inorganic ion that acts as a buffer in blood. Compound E is the hydroxide of a Group 2 element. Compound E is a strong alkali. 2.63 g of E is dissolved in water to make 250 cm3 of solution F. Solution F has a pH of 13.09 at 298 K. Show that the concentration of hydroxide ions in solution F is 0.123 mol dm–3. Explain why the concentration of compound E in solution F is 0.0615 mol dm–3. Use the concentration given in to identify compound E. compound E Compound E is much more soluble than magnesium hydroxide. A saturated solution of magnesium hydroxide in water has a concentration of 1.40 × 10–4 mol dm–3 at 298 K. Calculate the solubility product, Ksp, of magnesium hydroxide. Include units. Ksp = units Explain why compound E is much more soluble than magnesium hydroxide.
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THEORY
2024
Paper 4, Variant 3
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Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry. KI forms an ionic lattice that is soluble in water. Define enthalpy change of solution, ΔHsol. KIhas a high solubility in water although its enthalpy change of solution is endothermic. Explain how this high solubility is possible. Table 1.1 gives some data about the halide ions, Cl –, Br – and I–, and their potassium salts. Table 1.1 halide ion enthalpy change of hydration, ΔHhyd / kJ mol–1 lattice energy of potassium halide, ΔHlatt / kJ mol–1 Cl – –364 –701 Br – –335 –670 I– –293 –629 Explain the trend in the enthalpy change of hydration of the halide ions. The ΔHsol values of these potassium halides are almost constant. Use the ΔHhyd and ΔHlatt data in Table 1.1 to suggest why. The enthalpy change of solution of KIis +21.0 kJ mol–1. Use this information and the data in Table 1.1 to calculate the enthalpy change of hydration of the potassium ion, K+. ΔHhyd of K+= kJ mol–1 Solid PbI2 forms when KIis mixed with Pb2+ions. The solubility product, Ksp, of PbI2 is 7.1 × 10–9 mol3 dm–9 at 25 °C. Calculate the solubility, in mol dm–3, of PbI2. solubility of PbI2= mol dm–3 The ionic radius of Pb2+ is 0.120 nm compared to 0.133 nm for K+. Suggest how the ΔH o latt of PbI2differs from ΔH o latt of KI. Explain your answer. KI slowly oxidises in air, forming I2. reaction 1 4KI+ 2CO2+ O22K2CO3+ 2I2ΔH o = –203.4 kJ mol–1 Table 1.2 shows some data relevant to this question. Table 1.2 substance standard entropy, S o / J K–1 mol–1 CO2213.6 I2116.1 K2CO3155.5 KI106.3 O2205.2 Calculate the standard entropy change, ΔS o , of reaction 1. ΔS o = J K–1 mol–1 Use your answer to to show that reaction 1 is spontaneous at 298 K. The Group 1 carbonates are much more thermally stable than the Group 2 carbonates. State and explain the trend in the thermal stability of the Group 2 carbonates. A student electrolyses a solution of KIfor 8 minutes using a direct current. The half-equation for the reaction that occurs at the anode is given. 2I–I2+ 2e– Write a half-equation for the reaction that occurs at the cathode. Include state symbols. After the electrolysis, the I2produced requires 21.35 cm3 of 0.100 mol dm–3 Na2S2O3to react completely. I2+ 2Na2S2O32NaI+ Na2S4O6Calculate the average current used in 8 minutes during the electrolysis. current = A KI is used as a source of I– ions in organic synthesis. One example of this is shown in the synthetic route in . NO2 NH2 N2 + N2 I I step 1 A B C D E step 2 step 3 step 4 NaNO2 and HCl Identify the reagents required for steps 1 and 2. step 1 step 2 Step 3 occurs in two stages. stage I NaNO2 and HCl undergo an acid–base reaction to produce HNO2. stage II HNO2 reacts with C, C6H5NH2, to produce D, C6H5N2 +. Complete the equations for stage I and for stage II. stage I NaNO2 + HCl stage II The I– from KI reacts with D in step 4. The mechanism is shown in . Suggest the name for this mechanism.
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THEORY
2024
Paper 4, Variant 2
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Limestone is an important raw material, used in building, steel making and agriculture. The first stage in using limestone is often to heat it in a kiln. CaCO3→CaO+ CO2H = +178 kJ mol–1 reaction 1 Water is then added to the ‘quicklime’ produced in the kiln, to make ‘slaked lime’. CaO+ H2O→Ca(OH)2H = –82 kJ mol–1 reaction 2 Suggest two reasons why reaction 1 needs heating to a high temperature. Explain whether MgCO3 would require a higher or a lower temperature than CaCO3 for its decomposition. Before the widespread use of cement, bricks and stones used for buildings were bonded together with a mixture of slaked lime, sand and water, known as lime mortar. On exposure to the air, the lime mortar gradually set hard due to the following reaction. Ca(OH)2+ CO2→CaCO3+ H2OUse the data given above to calculate the enthalpy change for this reaction. Use One of the major ores of magnesium is the mixed carbonate called dolomite, CaMg(CO3)2. Calculate the percentage loss in mass that would be observed when a sample of dolomite is heated at a high temperature until the reaction had finished. Examiner's Use
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THEORY
2005
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