5. Chemical energetics
A section of Chemistry, 9701
Listing 10 of 306 questions
Chemical reactions are accompanied by enthalpy changes. Explain the meaning of the term standard enthalpy change of reaction. The enthalpy change of hydration of anhydrous magnesium sulfate, ∆Hhyd MgSO4, can be calculated by carrying out two separate experiments. In the fi rst experiment 45.00 g of water was weighed into a polystyrene cup and 3.01 g of MgSO4 was added and stirred until it was completely dissolved. The temperature of the water rose from 23.4 °C to 34.7 °C. Calculate the amount of heat energy transferred to the water during this dissolving process. You can assume that the specifi c heat capacity of the solution is the same as that of water, 4.18 J g–1 K–1. heat energy = J Calculate the amount, in moles, of MgSO4 dissolved. amount = mol Calculate the enthalpy change of solution, ∆Hsoln, of MgSO4. You must include a sign with your answer. ∆Hsoln, of MgSO4= kJ mol–1 In the second experiment, the enthalpy change of solution for the hydrated salt, MgSO4.7H2O, was calculated and found to be +9.60 kJ mol–1. Use the equation below for the hydration of anhydrous magnesium sulfate to construct a suitable, fully labelled energy cycle that will allow you to calculate the enthalpy change for this reaction, ∆Hhyd MgSO4. MgSO4+ 7H2O→ MgSO4.7H2O Calculate the enthalpy change for this reaction, ∆Hhyd MgSO4. Include a sign in your answer. ∆Hhyd MgSO4 = kJ mol–1
9701_w15_qp_22
THEORY
2015
Paper 2, Variant 2
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The dissolving of an ionic compound in water is accompanied by an energy change, the enthalpy change of solution, ∆H sol. MgCl 2+ aq → Mg2++ 2Cl –Describe, in terms of bond breaking and bond making, what happens to the solid ionic lattice when an ionic compound dissolves in water. What is meant by the term enthalpy change of solution, ∆H sol? Use the following data to calculate the standard enthalpy change of hydration, , of chloride ions, Cl –. You may find it helpful to construct an energy cycle. enthalpy change value (Mg2+) –1925 kJ mol–1 lattice energy of MgCl 2–2524 kJ mol–1 enthalpy change of solution for MgCl 2–155 kJ mol–1 (Cl –) = kJ mol–1 The enthalpy change of hydration for Na+, (Na+), is –410 kJ mol–1. Suggest an explanation for why the of the Na+ ion is less exothermic than the of the Mg2+ ion. Describe and explain how the solubility of the Group II sulfates varies down the group.
9701_w15_qp_43
THEORY
2015
Paper 4, Variant 3
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For many compounds the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy changes of combustion can be used. The enthalpy change of combustion can be found by a calorimetry experiment in which the heat energy given off during combustion is used to heat a known mass of water and the temperature change recorded. Explain the meaning of the term standard enthalpy change of combustion. Write the equation for the complete combustion of ethanol, C2H5OH. In an experiment to determine the enthalpy change of combustion of ethanol, 0.23 g of ethanol was burned and the heat given off raised the temperature of 100 g of water by 16.3 °C. Calculate the heat energy change, q, during the combustion of 0.23 g of ethanol. q = J Calculate the enthalpy change on burning 1 mole of ethanol. Include a sign in your answer. ∆H = kJ mol–1 Suggest two reasons why the value for the enthalpy change of combustion of ethanol determined by a simple laboratory calorimetry experiment is likely to be lower than the true value. The table gives some enthalpy change of combustion values. substance enthalpy change of combustion / kJ mol–1 C–393.5 H2–285.8 C3H7OH–2021.0 Construct a labelled energy cycle to show how these values could be used to calculate the enthalpy change of formation of C3H7OH, ∆Hf. 3C+ 4H2+ 1 2 O2∆Hf C3H7OH Calculate the enthalpy change of formation, ∆Hf, of C3H7OH. ∆Hf = kJ mol–1
9701_w16_qp_21
THEORY
2016
Paper 2, Variant 1
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For many compounds the enthalpy change of formation cannot be calculated directly. An indirect method based on enthalpy changes of combustion can be used. The enthalpy change of combustion can be found by a calorimetry experiment in which the heat energy given off during combustion is used to heat a known mass of water and the temperature change recorded. Explain the meaning of the term standard enthalpy change of combustion. Write the equation for the complete combustion of ethanol, C2H5OH. In an experiment to determine the enthalpy change of combustion of ethanol, 0.23 g of ethanol was burned and the heat given off raised the temperature of 100 g of water by 16.3 °C. Calculate the heat energy change, q, during the combustion of 0.23 g of ethanol. q = J Calculate the enthalpy change on burning 1 mole of ethanol. Include a sign in your answer. ∆H = kJ mol–1 Suggest two reasons why the value for the enthalpy change of combustion of ethanol determined by a simple laboratory calorimetry experiment is likely to be lower than the true value. The table gives some enthalpy change of combustion values. substance enthalpy change of combustion / kJ mol–1 C–393.5 H2–285.8 C3H7OH–2021.0 Construct a labelled energy cycle to show how these values could be used to calculate the enthalpy change of formation of C3H7OH, ∆Hf. 3C+ 4H2+ 1 2 O2∆Hf C3H7OH Calculate the enthalpy change of formation, ∆Hf, of C3H7OH. ∆Hf = kJ mol–1
9701_w16_qp_23
THEORY
2016
Paper 2, Variant 3
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