5. Chemical energetics
A section of Chemistry, 9701
Listing 10 of 306 questions
Silver sulfate, Ag2SO4, is sparingly soluble in water. The concentration of its saturated solution is 2.5 × 10–2 mol dm–3 at 298 K. Write an expression for the solubility product, Ksp, of Ag2SO4, and state its units. Ksp = units: Calculate the value for Ksp(Ag2SO4) at 298 K. Ksp = Using Ag2SO4 as an example, complete the following Hess' Law energy cycle relating the ● lattice energy, , ● enthalpy change of solution, , and ● enthalpy change of hydration, . On your diagram: ● include the relevant species in the two empty boxes, ● label each enthalpy change with its appropriate symbol, ● complete the remaining two arrows showing the correct direction of enthalpy change. Ag2SO4 An electrochemical cell is set up as follows. V Ag2SO4Fe2(SO4)3+ FeSO4Ag2SO4Pt Ag Use the Data Booklet to calculate the value of under standard conditions, stating which electrode is the positive one. = positive electrode: How would the actual Ecell of the above cell compare to the under standard conditions? Explain your answer. How would the Ecell of the above cell change, if at all, if a few cm3 of concentrated Na2SO4were added to • the beaker containing Fe3++ Fe2+, • the beaker containing Ag2SO4? Explain any changes in Ecell you have stated in . Solutions of iron(sulfate are acidic due to the following equilibrium. [Fe(H2O)6]3+[Fe(H2O)5(OH)]2++ H+Ka = 8.9 × 10–4 mol dm–3 Calculate the pH of a 0.1 mol dm–3 solution of iron(sulfate, Fe2(SO4)3. pH =
9701_s15_qp_41
THEORY
2015
Paper 4, Variant 1
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Silver sulfate, Ag2SO4, is sparingly soluble in water. The concentration of its saturated solution is 2.5 × 10–2 mol dm–3 at 298 K. Write an expression for the solubility product, Ksp, of Ag2SO4, and state its units. Ksp = units: Calculate the value for Ksp(Ag2SO4) at 298 K. Ksp = Using Ag2SO4 as an example, complete the following Hess' Law energy cycle relating the ● lattice energy, , ● enthalpy change of solution, , and ● enthalpy change of hydration, . On your diagram: ● include the relevant species in the two empty boxes, ● label each enthalpy change with its appropriate symbol, ● complete the remaining two arrows showing the correct direction of enthalpy change. Ag2SO4 An electrochemical cell is set up as follows. V Ag2SO4Fe2(SO4)3+ FeSO4Ag2SO4Pt Ag Use the Data Booklet to calculate the value of under standard conditions, stating which electrode is the positive one. = positive electrode: How would the actual Ecell of the above cell compare to the under standard conditions? Explain your answer. How would the Ecell of the above cell change, if at all, if a few cm3 of concentrated Na2SO4were added to • the beaker containing Fe3++ Fe2+, • the beaker containing Ag2SO4? Explain any changes in Ecell you have stated in . Solutions of iron(sulfate are acidic due to the following equilibrium. [Fe(H2O)6]3+[Fe(H2O)5(OH)]2++ H+Ka = 8.9 × 10–4 mol dm–3 Calculate the pH of a 0.1 mol dm–3 solution of iron(sulfate, Fe2(SO4)3. pH =
9701_s15_qp_43
THEORY
2015
Paper 4, Variant 3
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Describe and explain the trend in the solubility of the hydroxides down Group 2. Calcium reacts vigorously with HCl producing H2. Ca+ 2HCl → CaCl 2+ H2How would you expect the enthalpy change for this reaction to compare with the enthalpy change for the reaction where HNO3is used in place of HCl but all other conditions are the same? Explain your answer. The ionic equation for this reaction is shown. Ca+ 2H+→ Ca2++ H2∆H o = x kJ mol–1 Construct a fully labelled Hess’ Law cycle to connect each side of this equation to the relevant gas phase ions. Use your cycle, the following data, and data from the Data Booklet, to calculate a value for x. standard enthalpy of atomisation of Ca, (Ca) +178 kJ mol–1 standard enthalpy of hydration of Ca2+, (Ca2+) –1576 kJ mol–1 standard enthalpy of hydration of H+, (H+) –1090 kJ mol–1 x = kJ mol–1 The standard enthalpy change for the reaction between Caand CH3CO2His less negative than x by 2 kJ mol–1. Suggest an explanation for this.
9701_s16_qp_42
THEORY
2016
Paper 4, Variant 2
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Sucrose is a white crystalline solid, C12H22O11. In reaction Z, sucrose reacts with water in the presence of a catalyst, aqueous hydrochloricacid, to form glucose and fructose. C12H22O11 + H2O + glucose fructose reaction Z C CH2OH OH H C OH H C C OH H C H H O HO C CH2OH OH H C OH H C C H CH2OH O HO Suggest a name for the reaction that occurs when sucrose reacts with water to form glucose and fructose. If no catalyst is added in reaction Z, the reaction is very slow. Label the Boltzmann distribution to show the effect of adding a catalyst to the sample of sucrose and water molecules at constant temperature. number of sucrose molecules energy Explain your labelled diagram.  Both fructose and glucose contain chiral centres. Explain what is meant by the term chiral centre. On the diagram of the fructose molecule, label all the chiral centres with an asterisk (*). C H O C C H O H C H H H H C O O O H H C H O H H  Determine the empirical formula of fructose. Explain what is meant by the term enthalpy change of combustion. Write the equation for the complete combustion of sucrose. The enthalpy change of reaction Z, ∆Hr, can be calculated using the enthalpy change of combustion data given in the table. substance enthalpy change of combustion, ∆Hc / kJ mol–1 sucrose –5643 glucose –2805 fructose –2810 Use the data in the table to calculate the enthalpy change for the reaction occurring when sucrose reacts with water, ∆Hr. You should draw a labelled Hess’ cycle to show your working.  ∆Hr = kJ mol–1 
9701_s20_qp_23
THEORY
2020
Paper 2, Variant 3
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Complete Table 3.1 by placing one tick (3) in each row to indicate the sign of each type of energy change under standard conditions. Table 3.1 energy change always positive always negative can be either negative or positive bond energy enthalpy change of atomisation enthalpy change of formation Define standard enthalpy change of atomisation. Table 3.2 shows some energy changes. Table 3.2 energy change value / kJ mol–1 standard enthalpy change of atomisation of silver +285 first ionisation energy of silver +731 second ionisation energy of silver +2074 bond energy of O=O +496 bond energy of O–O +150 first electron affinity of oxygen –141 second electron affinity of oxygen +798 first ionisation energy of oxygen +1314 standard enthalpy change of formation of silver oxide, Ag2O–31 Calculate the lattice energy, ΔH o latt, of Ag2Ousing relevant data from Table 3.2. It may be helpful to draw a labelled energy cycle. Show your working. ΔH o latt of Ag2O= kJ mol–1 Suggest the trend in the magnitude of the lattice energies of the silver compounds Ag2S, Ag2O and Ag2Se. Explain your answer. least exothermic most exothermic Silver sulfite, Ag2SO3, is sparingly soluble in water. Give an expression for the solubility product, Ksp, of Ag2SO3. Ksp = Calculate the equilibrium concentration of Ag+ in a saturated solution of Ag2SO3 at 298 K. [Ksp: Ag2SO3, 1.50 × 10–14 mol3 dm–9 at 298 K] [Ag+] = mol dm–3 The standard enthalpy change of solution, ΔH o sol, of AgNO3in water is +22.6 kJ mol–1. Suggest how the feasibility of dissolving AgNO3in water changes with temperature. Explain your answer.
9701_s23_qp_42
THEORY
2023
Paper 4, Variant 2
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