5.1. Enthalpy change, \(\Delta H\)
A subsection of Chemistry, 9701, through 5. Chemical energetics
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Hydrogen peroxide is a liquid at 298 K. It is moderately stable under room conditions but will decompose quickly if a catalyst is added. reaction 1 2H2O22H2O + O2 Define entropy. Predict the sign of the standard entropy change of reaction 1. Explain your answer. sign explanation Some bond energy data are shown in Table 3.1. Table 3.1 type of bond bond energy / kJ mol–1 O–O O–H O=O Use the data in Table 3.1 to show that the enthalpy change of the following reaction is –196 kJ mol–1. 2H2O22H2O+ O2 Some standard entropies, S o , are shown in Table 3.2. Table 3.2 substance S o / J K–1 mol–1 H2O2+102 H2O+70 The enthalpy change and Gibbs free energy change for the following reaction are shown. 2H2O22H2O+ O2ΔH o = –196 kJ mol–1 ΔG o = –238 kJ mol–1 Use the data given to calculate the standard entropy of oxygen, S o , O2. S o , O2= J K–1 mol–1 The decomposition of H2O2is catalysed by aqueous iron(chloride and by silver metal. Identify which of these two catalysts is acting as a homogeneous catalyst. Explain your answer. homogeneous catalyst explanation The E o values for two electrode reactions are given. H2O2 + 2H+ + 2e− 2H2O E o = +1.77 V Cr3+ + e− Cr2+ E o = −0.41 V An electrochemical cell is constructed with the following half-cells : • an acidified solution of H2O2, a platinum wire • Cr2+ mixed with Cr3+, a platinum wire. Identify the positive half-cell and calculate the standard cell potential, E o cell. positive half-cell E o cell = V Calculate the value of ΔG o for the cell reaction that occurs, per mole of H2O2. ΔG o = kJ mol–1 The E o values for two electrode reactions are given. H2O2 + 2H+ + 2e– 2H2O E o = +1.77 V Co3+ + e– Co2+ E o = +1.82 V An electrochemical cell is constructed with the following half-cells. half-cell 1 an acidified solution of H2O2 under standard conditions, a platinum wire half-cell 2 a solution containing 0.020 mol dm–3 Co3+ and 2.0 mol dm–3 Co2+, a platinum wire Use the Nernst equation to calculate the value of E, the electrode potential of half-cell 2 under these conditions. E = V Write an equation for the cell reaction that occurs in this cell under these conditions. Define enthalpy change of hydration, ΔHhyd. Aluminium fluoride, Al F3, is an ionic solid. Complete and label the energy cycle to show the relationship between: ● the enthalpy change of solution of Al F3, ΔH o sol ● the lattice energy of Al F3, ΔH o latt ● the enthalpy changes of hydration of Al 3+ and F−, ΔH o hyd. Include state symbols for all substances and ions. …………………………… …………………………… …………………………… …………… ΔH o …………… ΔH o …………… ΔH o Relevant data for this question are given. ΔH o sol Al F3 = –209 kJ mol−1 ΔH o hyd Al 3+ = – 4690 kJ mol−1 ΔH o hyd F– = –506 kJ mol−1 Use these data and your energy cycle in to calculate the ΔH o latt of Al F3. ΔH o latt of Al F3 = kJ mol−1
9701_w23_qp_41
THEORY
2023
Paper 4, Variant 1
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Hydrogen peroxide is a liquid at 298 K. It is moderately stable under room conditions but will decompose quickly if a catalyst is added. reaction 1 2H2O22H2O + O2 Define entropy. Predict the sign of the standard entropy change of reaction 1. Explain your answer. sign explanation Some bond energy data are shown in Table 3.1. Table 3.1 type of bond bond energy / kJ mol–1 O–O O–H O=O Use the data in Table 3.1 to show that the enthalpy change of the following reaction is –196 kJ mol–1. 2H2O22H2O+ O2 Some standard entropies, S o , are shown in Table 3.2. Table 3.2 substance S o / J K–1 mol–1 H2O2+102 H2O+70 The enthalpy change and Gibbs free energy change for the following reaction are shown. 2H2O22H2O+ O2ΔH o = –196 kJ mol–1 ΔG o = –238 kJ mol–1 Use the data given to calculate the standard entropy of oxygen, S o , O2. S o , O2= J K–1 mol–1 The decomposition of H2O2is catalysed by aqueous iron(chloride and by silver metal. Identify which of these two catalysts is acting as a homogeneous catalyst. Explain your answer. homogeneous catalyst explanation The E o values for two electrode reactions are given. H2O2 + 2H+ + 2e− 2H2O E o = +1.77 V Cr3+ + e− Cr2+ E o = −0.41 V An electrochemical cell is constructed with the following half-cells : • an acidified solution of H2O2, a platinum wire • Cr2+ mixed with Cr3+, a platinum wire. Identify the positive half-cell and calculate the standard cell potential, E o cell. positive half-cell E o cell = V Calculate the value of ΔG o for the cell reaction that occurs, per mole of H2O2. ΔG o = kJ mol–1 The E o values for two electrode reactions are given. H2O2 + 2H+ + 2e– 2H2O E o = +1.77 V Co3+ + e– Co2+ E o = +1.82 V An electrochemical cell is constructed with the following half-cells. half-cell 1 an acidified solution of H2O2 under standard conditions, a platinum wire half-cell 2 a solution containing 0.020 mol dm–3 Co3+ and 2.0 mol dm–3 Co2+, a platinum wire Use the Nernst equation to calculate the value of E, the electrode potential of half-cell 2 under these conditions. E = V Write an equation for the cell reaction that occurs in this cell under these conditions. Define enthalpy change of hydration, ΔHhyd. Aluminium fluoride, Al F3, is an ionic solid. Complete and label the energy cycle to show the relationship between: ● the enthalpy change of solution of Al F3, ΔH o sol ● the lattice energy of Al F3, ΔH o latt ● the enthalpy changes of hydration of Al 3+ and F−, ΔH o hyd. Include state symbols for all substances and ions. …………………………… …………………………… …………………………… …………… ΔH o …………… ΔH o …………… ΔH o Relevant data for this question are given. ΔH o sol Al F3 = –209 kJ mol−1 ΔH o hyd Al 3+ = – 4690 kJ mol−1 ΔH o hyd F– = –506 kJ mol−1 Use these data and your energy cycle in to calculate the ΔH o latt of Al F3. ΔH o latt of Al F3 = kJ mol−1
9701_w23_qp_43
THEORY
2023
Paper 4, Variant 3
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The unsaturated hydrocarbon ethyne , C2H2, is widely used in ‘oxy-acetylene torches’ for cutting and welding metals. In the torch, ethyne is burned in oxygen to produce a flame with a temperature of 3400 K. Ethyne is a linear molecule with a triple bond, CC, between the two carbon atoms. Draw a ‘dot-and-cross’ diagram of an ethyne molecule. When used for cutting or welding, ethyne is transported in cylinders which contain the gas under pressure. A typical cylinder has a volume of 76 dm3 and contains ethyne gas at 1515 kPa pressure at a temperature of 25 °C. Use the general gas equation, pV = nRT, to calculate the amount, in moles, of ethyne in this cylinder. In some countries, ethyne is manufactured from calcium carbide, CaC2, which is produced by heating quicklime and coke together at 2300 K. CaO + 3C CaC2 + CO When water is added to the CaC2, calcium hydroxide, Ca(OH)2, and ethyne, C2H2, are produced. Construct a balanced equation for the formation of ethyne from calcium carbide. Use this equation and your answer to part to calculate the mass of CaC2 which will react with an excess of water to produce enough ethyne to fill 100 cylinders of the gas. The equation for the complete combustion of ethyne is given below. Use appropriate bond energy data from the Data Booklet to calculate a value for the enthalpy change of combustion of ethyne. C2H2+ ⁵⁄₂O22CO2+ H2O The value for the standard enthalpy change of combustion of ethyne is –1300 kJ mol–1. Define the term standard enthalpy change of combustion. Explain why your answer to does not have the same value as the standard enthalpy change of combustion.
9701_s06_qp_2
THEORY
2006
Paper 2, Variant 0
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Alcohols such as methanol, CH3OH, are considered to be possible replacements for fossil fuels because they can be used in car engines. Defi ne, with the aid of an equation which includes state symbols, the standard enthalpy change of combustion, , for methanol at 298 K. equation defi nition Methanol may be synthesised from carbon monoxide and hydrogen. Relevant values for this reaction are given in the table below. compound CO–283 H2–286 CH3OH–726 Use these values to calculate for the synthesis of methanol, using the following equation. Include a sign in your answer. CO+ 2H2→ CH3OH= kJ mol–1 The operating conditions for this reaction are as follows. pressure 200 atmospheres (2 × 107 Pa) temperature 600 K catalyst oxides of Cr, Cu, and Zn In the spaces below, explain how each of these conditions affects the rate of formation of methanol. pressure temperature catalyst
9701_s12_qp_22
THEORY
2012
Paper 2, Variant 2
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Chromyl chloride, CrO2Cl 2, can be prepared by heating a mixture of potassium dichromate(and potassium chloride with concentrated sulfuric acid. K2Cr2O7 + 4KCl + 3H2SO4 → 2CrO2Cl 2 + 3K2SO4 + 3H2O Use the following data to complete the Hess’ Law cycle and calculate the enthalpy change of the reaction, ∆Hr. compound enthalpy change of formation, / kJ mol–1 K2Cr2O7 –2061 KCl –437 H2SO4 –814 CrO2Cl 2 –580 K2SO4 –1438 H2O –286 K2Cr2O7 + 4KCl + 3H2SO4 ∆Hr 2CrO2Cl 2 + 3K2SO4 + 3H2O elements ∆Hr = kJ mol–1 There are two isomeric complex ions with the formula [Cr(NH3)4Cl 2]+. One is green and the other is violet. Suggest the type of isomerism shown by these complex ions. Explain why these two complex ions • are coloured, • have different colours.
9701_s16_qp_41
THEORY
2016
Paper 4, Variant 1
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Chromyl chloride, CrO2Cl 2, can be prepared by heating a mixture of potassium dichromate(and potassium chloride with concentrated sulfuric acid. K2Cr2O7 + 4KCl + 3H2SO4 → 2CrO2Cl 2 + 3K2SO4 + 3H2O Use the following data to complete the Hess’ Law cycle and calculate the enthalpy change of the reaction, ∆Hr. compound enthalpy change of formation, / kJ mol–1 K2Cr2O7 –2061 KCl –437 H2SO4 –814 CrO2Cl 2 –580 K2SO4 –1438 H2O –286 K2Cr2O7 + 4KCl + 3H2SO4 ∆Hr 2CrO2Cl 2 + 3K2SO4 + 3H2O elements ∆Hr = kJ mol–1 There are two isomeric complex ions with the formula [Cr(NH3)4Cl 2]+. One is green and the other is violet. Suggest the type of isomerism shown by these complex ions. Explain why these two complex ions • are coloured, • have different colours.
9701_s16_qp_43
THEORY
2016
Paper 4, Variant 3
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