5. Chemical energetics
A section of Chemistry, 9701
Listing 10 of 306 questions
Magnesium has a melting point of 650 °C and high electrical conductivity. Explain these properties of magnesium by referring to its structure and bonding. When magnesium is heated in air, magnesium oxide, MgO, is the major product. Smaller amounts of magnesiumnitride, Mg3N2, are also made. Calculate the oxidation number for magnesium and for the nitrogen species in Mg3N2 to complete Table 1.1. Table 1.1 species magnesium in Mg3N2 nitrogen in Mg3N2 oxidation number  Identify the type of reaction which takes place between magnesium and nitrogen. Explain your answer. Define enthalpy change of formation. When 3.645 g of Mgburns in excess N2to form Mg3N2, 23.05 kJ of energy is released. Calculate the enthalpy change of formation, ∆Hf, of Mg3N2. Show your working.  ∆Hf (Mg3N2) = 
9701_s22_qp_22
THEORY
2022
Paper 2, Variant 2
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0.025 mol of HIis added to a closed vessel and left to reach dynamic equilibrium. The total pressure of the vessel is 100 kPa. equation 1 2HIH2+ I2Explain what is meant by dynamic equilibrium. Describe one difference in the initial appearance of the reaction mixture compared to the mixture at equilibrium. Write an expression for Kp for the reaction described in equation1. Kp =  At equilibrium the partial pressure of HIis 86.4 kPa. Calculate the amount of HIpresent in the mixture at equilibrium. Show your working.  amount of HI= mol Use equation1 and the bond energy values in Table3.1 to calculate the change in enthalpy, ∆H, for the thermal decomposition of 1mole of HI. Show your working. Table 3.1 bond bond energy / kJ mol–1 H–H I–I H–I  ∆H = kJ mol–1 Describe the effect of increasing pressure on the value of Kp for the decomposition of HI. HCl is prepared by adding NaCl to concentrated H2SO4. HIis not prepared by adding NaIto concentrated H2SO4 because the HIproduced also reacts with concentrated H2SO4. Identify the type of reaction that occurs when NaIreacts with concentrated H2SO4 to form HI. Write an equation for the reaction of HIand concentrated H2SO4. Explain why HIreacts with concentrated H2SO4 whereas HCl does not. 
9701_s22_qp_22
THEORY
2022
Paper 2, Variant 2
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Calciumcarbonate decomposes on heating. CaCO3→ CaO+ CO2Table 4.1 shows the values of the Gibbs free energy change, ∆G o, for this reaction at various temperatures. Table 4.1 T / K ∆G o / kJ mol–1 9.9 4.3 –1.3 –5.8 –10.3 Assume the standard enthalpy change, ∆H o, and the standard entropy change, ∆S o, for this reaction remain constant over this temperature range. Use the data in Table4.1 to plot a graph of ∆G o against T on the grid. 12.5 10.0 7.5 5.0 2.5 0.0 –2.5 –5.0 –7.5 –10.0 –12.5 T / K G o / kJ mol–1  Calculate the gradient of your graph. Determine the ∆S o in J K–1 mol–1 for this reaction. Show all working. ∆S o = J K–1 mol–1 Group1 hydrogencarbonates, MHCO3, decompose on gentle heating to give the corresponding metal carbonate, carbondioxide and water vapour. Write an ionic equation for the decomposition of the hydrogencarbonate ion. The thermal stability of Group1 hydrogencarbonates increases down the group. Suggest an explanation for the trend in thermal stability of the Group1 hydrogencarbonates. The buffer system in seawater contains a mixture of HCO3 – and H2CO3. equilibrium 5 H2CO3 + H2O HCO3 – + H3O+ Define a buffer solution. Construct two equations to show how equilibrium5 acts as a buffer solution. The [HCO3 –] / ratio in a sample of seawater is 14.1. Calculate the pH of this sample. [pKa: H2CO3, 6.35]  pH = 
9701_s22_qp_42
THEORY
2022
Paper 4, Variant 2
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Carbon disulfide, CS2, is flammable and reacts readily with oxygen, as shown in reaction 1. reaction 1 CS2+ 3O2CO2+ 2SO2Table 3.1 shows the standard enthalpy of formation, ΔH o f , and the standard entropy, S o, for some substances. Table 3.1 CS2O2CO2SO2ΔH o f / kJ mol–1 116.7 0.0 –393.5 –296.8 S o/ J K–1 mol–1 237.8 205.2 213.8 248.2 Calculate the standard Gibbs free energy change, ΔG o, in kJ mol–1, for reaction 1 at 25 °C. ΔG o = kJ mol–1 Carbon disulfide reacts with chlorine to form tetrachloromethane, as shown in reaction 2. reaction 2 CS2 + 3Cl2 CCl 4 + S2Cl2 ΔH o = –261.6 kJ mol–1 ΔS o = –365.5 J K–1 mol–1 Calculate the maximum temperature, in K, for reaction 2 to be feasible. temperature = K
9701_s24_qp_42
THEORY
2024
Paper 4, Variant 2
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Propane, C3H8, and butane, C4H10, are components of Liquefi ed Petroleum Gas (LPG) which is widely used as a fuel for domestic cooking and heating. To which class of compounds do these two hydrocarbons belong? Write a balanced equation for the complete combustion of butane. When propane or butane is used in cooking, the saucepan may become covered by a solid black deposit. What is the chemical name for this black solid? Write a balanced equation for its formation from butane. Propane and butane have different values of standard enthalpy change of combustion. Defi ne the term standard enthalpy change of combustion. A 125 cm3 sample of propane gas, measured at 20 °C and 101 kPa, was completely burnt in air. The heat produced raised the temperature of 200 g of water by 13.8 °C. Assume no heat losses occurred during this experiment. Use the equation pV = nRT to calculate the mass of propane used. Use relevant data from the Data Booklet to calculate the amount of heat released in this experiment. Use the data above and your answers to and to calculate the energy produced by the burning of 1 mol of propane. The boiling points of methane, ethane, propane, and butane are given below. compound CH4 CH3CH3 CH3CH2CH3 CH3(CH2)2CH3 boiling point / K Suggest an explanation for the increase in boiling points from methane to butane. The isomer of butane, 2-methylpropane, (CH3)3CH, has a boiling point of 261 K. Suggest an explanation for the difference between this value and that for butane in the table above.
9701_w13_qp_21
THEORY
2013
Paper 2, Variant 1
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Propane, C3H8, and butane, C4H10, are components of Liquefi ed Petroleum Gas (LPG) which is widely used as a fuel for domestic cooking and heating. To which class of compounds do these two hydrocarbons belong? Write a balanced equation for the complete combustion of butane. When propane or butane is used in cooking, the saucepan may become covered by a solid black deposit. What is the chemical name for this black solid? Write a balanced equation for its formation from butane. Propane and butane have different values of standard enthalpy change of combustion. Defi ne the term standard enthalpy change of combustion. A 125 cm3 sample of propane gas, measured at 20 °C and 101 kPa, was completely burnt in air. The heat produced raised the temperature of 200 g of water by 13.8 °C. Assume no heat losses occurred during this experiment. Use the equation pV = nRT to calculate the mass of propane used. Use relevant data from the Data Booklet to calculate the amount of heat released in this experiment. Use the data above and your answers to and to calculate the energy produced by the burning of 1 mol of propane. The boiling points of methane, ethane, propane, and butane are given below. compound CH4 CH3CH3 CH3CH2CH3 CH3(CH2)2CH3 boiling point / K Suggest an explanation for the increase in boiling points from methane to butane. The isomer of butane, 2-methylpropane, (CH3)3CH, has a boiling point of 261 K. Suggest an explanation for the difference between this value and that for butane in the table above.
9701_w13_qp_22
THEORY
2013
Paper 2, Variant 2
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