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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Group 2 carbonates decompose when heated to form the metal oxide and carbon dioxide. Suggest a mechanism for the decomposition of the carbonate ion by adding two curly arrows in . O O– –O C CO2 O2– + Describe the variation in the thermal stability of Group 2 carbonates. Explain your answer. Define lattice energy. The lattice energy of the Group 2 carbonates, ΔH o latt(MCO3), becomes less exothermic down the group. The lattice energy of the Group 2 oxides, ΔH o latt(MO), also becomes less exothermic down the group. ΔH o latt(MCO3) and ΔH o latt(MO) change by different amounts going down the group. Suggest how the standard enthalpy change of the decomposition reaction for Group 2 carbonates changes down the group. Explain your reasoning in terms of the relative sizes of the anions and the relative changes in lattice energy down the group. Potassium sulfite, K2SO3, is used as a food additive. The concentration of sulfite ions, SO3 2–, can be determined by titration using aqueous acidified manganate(ions, MnO4 –. • A 250 cm3 solution contains 3.40 g of impure K2SO3. • 25.0 cm3 of this solution requires 22.40 cm3 of 0.0250 mol dm–3 acidified MnO4 – to reach the end-point. All the SO3 2– ions are oxidised. None of the other species in the impure K2SO3 are oxidised. The reaction occurs as shown by the two half-equations. H2O + SO3 2– SO4 2– + 2H+ + 2e– MnO4 – + 8H+ + 5e– Mn2+ + 4H2O Give the ionic equation for the reaction between SO3 2– and acidified MnO4 –. Calculate the percentage purity of the sample of K2SO3. Show your working. percentage purity of K2SO3 = Potassium disulfite, K2S2O5, is another food additive. The disulfite ion, S2O5 2–, has the displayed formula shown in . O O O O– O– S S α Deduce the geometry around the S(α) atom in S2O5 2–. geometry around S(α)
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THEORY
2023
Paper 4, Variant 2
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Describe the trend in the solubility of the hydroxides of magnesium, calcium and strontium. Explain your answer. > > most soluble least soluble Suggest the variation in pH of saturated solutions of the hydroxides of magnesium, calcium and strontium. Explain your answer. Barium hydroxide, Ba(OH)2, is a strong base. A 250.0 cm3 solution of Ba(OH)2 with a pH of 12.2 is made by dissolving Ba(OH)2 in distilled water. Calculate the mass of Ba(OH)2 required to make this solution. Show your working. [Mr: Ba(OH)2, 171.3] mass of Ba(OH)2 = g The solubility of iron(hydroxide, Fe(OH)2, is 5.85 × 10–6 mol dm–3 at 298 K. Write the expression for the solubility product, Ksp, of Fe(OH)2. Ksp = Calculate the value of Ksp of Fe(OH)2. Include its units. Ksp = units =
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THEORY
2024
Paper 4, Variant 1
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When a sample of hydrated lithium ethanedioate, Li2C2O4•H2O, is gently heated, two gaseous products are formed and a white solid residue remains. The residue is added to HNO3. A gas is produced that turns limewater milky. Complete the equation for the decomposition of Li2C2O4•H2O. Li2C2O4•H2O + + The trend in the decomposition temperatures of the Group 2 ethanedioates is similar to that of the Group 2 nitrates. Suggest which of CaC2O4 and BaC2O4 will decompose at the lower temperature. Explain your answer. Potassium iron(ethanedioate, K3[Fe(C2O4)3], dissolves in water to form a green solution. Explain why transition elements can form coloured complexes. The anhydrous iron(compound K3[Fe(C2O4)3] decomposes on heating to form a mixture of K2[Fe(C2O4)2], K2C2O4 and CO2. Complete the equation for the decomposition of K3[Fe(C2O4)3]. K3[Fe(C2O4)3] K2[Fe(C2O4)2] + K2C2O4 + CO2 The [Fe(C2O4)3]3– complex ion shows stereoisomerism. Complete the three-dimensional diagrams in to show the two stereoisomers of [Fe(C2O4)3]3–. The C2O4 2– ligand can be represented using O O. isomer 1 Fe isomer 2 Fe Buffer solutions are used to regulate pH. Write two equations to describe how a solution containing HC2O4 – ions acts as a buffer solution when small amounts of acid or alkali are added. A fuel cell is an electrochemical cell that can be used to generate electrical energy by using oxygen to oxidise a fuel. Ethanedioic acid, (COOH)2, dissolved in an alkaline electrolyte is being investigated as a fuel. The relevant standard electrode potentials, Eo, for the cell are shown. O2+ 2H2O+ 4e– 4OH–Eo = +0.40 V 2CO2+ 2e– C2O4 2–Eo = –0.59 V Use these equations to deduce the overall cell reaction. Calculate the value of E o cell. overall cell reaction E o cell = V
9701_s24_qp_41
THEORY
2024
Paper 4, Variant 1
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Lithium nitrate, LiNO3, decomposes on heating in a similar way to Group 2 nitrates to give the metal oxide, a brown gas and oxygen. Write an equation for the decomposition of LiNO3. The other Group 1 nitrates, MNO3, decompose on heating to form the metal nitrite, MNO2, and oxygen. The thermal stability of these nitrates increases down the group. Suggest why the thermal stability of MNO3 increases down the group. Acidified manganate(ions, MnO4 –, can be used to analyse solutions containing nitrite ions, NO2 –, by titration. X is a solution of NaNO2. 250.0 cm3 of X is added to 50.0 cm3 of 0.125 mol dm–3 acidified MnO4 –. The MnO4 –ions are in excess; all the NO2 – ions are oxidised in the reaction. The unreacted MnO4 –required 22.50 cm3 of 0.0400 mol dm–3 Fe2+to reach the end‑point. The relevant half‑equations are shown. NO2 – + H2O NO3 – + 2H+ + 2e– MnO4 – + 8H+ + 5e– Mn2+ + 4H2O Fe2+ Fe3+ + e– Calculate the concentration, in mol dm–3, of NaNO2 in X. Table 2.1 shows electrode potentials for some electrode reactions involving manganese compounds. Table 2.1 electrode reaction E o / V Mn2+ + 2e– Mn –1.18 MnO2 + 4H+ + 2e– Mn2+ + 2H2O +1.23 MnO4 – + e– MnO4 2– +0.56 MnO4 – + 4H+ + 3e– MnO2 + 2H2O +1.67 MnO4 – + 8H+ + 5e– Mn2+ + 4H2O +1.52 MnO4 – + 2H2O + 3e– MnO2 + 4OH– +0.59 MnO4 2– + 2H2O + 2e– MnO2 + 4OH– +0.60 MnO4 2– + 4H+ + 2e– MnO2 + 2H2O +1.70 Aqueous manganate(ions, MnO4 2–, are unstable in acidic conditions and undergo a disproportionation reaction. The E o cell for this reaction is +1.14 V. Construct an overall ionic equation for this disproportionation reaction. Suggest and explain how the Ecell value of the disproportionation reaction changes with an increase in pH. ,  ,
9701_s24_qp_42
THEORY
2024
Paper 4, Variant 2
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Describe the trend in the solubility of the hydroxides of magnesium, calcium and strontium. Explain your answer. > > most soluble least soluble Suggest the variation in pH of saturated solutions of the hydroxides of magnesium, calcium and strontium. Explain your answer. Barium hydroxide, Ba(OH)2, is a strong base. A 250.0 cm3 solution of Ba(OH)2 with a pH of 12.2 is made by dissolving Ba(OH)2 in distilled water. Calculate the mass of Ba(OH)2 required to make this solution. Show your working. [Mr: Ba(OH)2, 171.3] mass of Ba(OH)2 = g The solubility of iron(hydroxide, Fe(OH)2, is 5.85 × 10–6 mol dm–3 at 298 K. Write the expression for the solubility product, Ksp, of Fe(OH)2. Ksp = Calculate the value of Ksp of Fe(OH)2. Include its units. Ksp = units =
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THEORY
2024
Paper 4, Variant 3
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