2023 NJC Energetics I Tutorial (Student)

National Junior College SH1 H2 Chemistry 1 Chemical Energetics Tutorial Definition of Standard Enthalpy Change 1 With the aid of thermochemical equations, define the following terms. (a) standard enthalpy change of combustion of methane, ∆Hc ꝋ (CH4(g)) (b) standard enthalpy change of atomisation of hydrogen, ∆Hat ꝋ (H2). (c) standard enthalpy change of formation of liquid methanol, ∆Hf ꝋ (CH3OH(l)) (d) bond energy of H−H bond (e) standard enthalpy change of neutralisation of reaction between H 2SO4 and NaOH, ∆Hn ꝋ (f) lattice energy for sodium oxide (g) 1st ionisation energy of sodium (h) 2nd ionisation energy of sodium (i) 1st electron affinity of fluorine (j) standard enthalpy change of solution of sodium hydroxide NaOH(s), ∆Hsol ꝋ (NaOH(s)) (k) standard enthalpy change of hydration of Mg2+(g), ∆Hhyd ꝋ (Mg2+(g)) (l) standard enthalpy change of vaporisation of liquid bromine, ∆Hvap ꝋ (Br2(l)) Experimentally Determined Enthalpy Change 2 Concepts: Experimentally Determined Enthalpy Change (Mixing 2 solutions) + Experimentally Determined Enthalpy Change (Cooling Curve) Hydrogen carbonate ions react with acids as follows. HCO3–(aq) + H+(aq) → H2O(l) + CO2(g) Two different experiments are conducted to determine the standard enthalpy change of the above reaction. (a) In experiment 1, known volumes of 0.150 mol dm –3 NaHCO3(aq) and 0.1 50 mol dm –3 HCl(aq) are mixed and stirred in order to obtain the maximum change in temperature of the reaction mixture. The following results were obtained: Volume of NaHCO3(aq) used 25.0 cm3 Volume of HCl(aq) used 25.0 cm3 Maximum change in temperature –0.8 °C Using the data given and assuming that the specific heat capacity of all solutions are 4.2 J K –1 g–1, calculate the standard enthalpy change for the reaction. (+44.8 kJ mol−1)

National Junior College SH1 H2 Chemistry 2 2 (b) In experiment 2, 25.0 cm 3 of 0.150 mol dm –3 NaHCO3(aq) and 25.0 cm3 of 0.150 mol dm −3 HCl(aq) are mixed at time = 2 min, and several temperature readings were taken at fixed intervals. The results are shown in the graph below. (i) Explain why method to obtain the standard enthalpy change for the reaction in experiment 2 is expected to be more accurate than that obtained in experiment 1 in (a). (ii) Using the information in the graph and assuming that the specific heat capacity of all solutions are 4.2 J K–1 g–1, calculate the standard enthalpy change for the reaction using experiment 2. (+56.0 kJ mol−1)

National Junior College SH1 H2 Chemistry 3 3 Concepts: Experimentally Determined Enthalpy Change (Combustion, including heat capacity of calorimeter) An experiment was conducted to determine the enthalpy change of combustion of ethanol (C2H5OH) using the apparatus setup below. The following results were obtained when the ethanol is burnt completely in excess oxygen. initial mass of burner with wick containing ethanol/ g 60.0 final mass of burner with wick containing ethanol/ g 57.6 initial temperature of water in calorimeter/ °C 25 final temperature of water in calorimeter/ °C 60 heat capacity of calorimeter/ J °C−1 883 Mr of ethanol 46.0 (a) Using the data given, calculate the standard enthalpy change for the reaction. (−592 kJ mol−1) (b) The actual enthalpy change of combustion of ethanol is found to be −1370 kJ mol−1. Suggest two reasons for the deviation of the enthalpy change of combustion of ethanol calculated in (a) from the actual value. Hess’ Law 4 Concept: Hess’ Cycle (Given Enthalpy Change of Formation Data) The chemical equation for the combustion of propene is as shown below. C3H6(g) + 9 2 O2(g) → 3CO2(g) + 3H2O(l) Standard enthalpy change of formation of C3H6(g)/ kJ mol−1 +20 Standard enthalpy change of formation of CO2(g)/ kJ mol−1 −394 Standard enthalpy change of formation of H2O(l)/ kJ mol−1 −286 Using the data given in the table above, calculate the standard enthalpy change of combustion of propene. (−2060 kJ mol−1) thermometer metal can containing 200 g of water burner with wick containing ethanol

National Junior College SH1 H2 Chemistry 4 5 Concept: Hess’ Cycle (Given Enthalpy Change of Combustion Data) Using the following data, construct an energy cycle to calculate the enthalpy change of formation of liquid ethanol, C2H5OH(l). enthalpy change of combustion of carbon/ kJ mol−1 −393 enthalpy change of combustion of hydrogen/ kJ mol−1 −286 enthalpy change of combustion of ethanol/ kJ mol−1 −1368 (−276 kJ mol−1) 6 Concept: Experimentally Determined Enthalpy Change + % Efficiency + Hess’ Cycle (Given Enthalpy Change of Combustion Data) + Hess’ Cycle (Given Bond Energy Data) Propan−2−ol is a colourless liquid usually used as a solvent and antiseptic. Like any other alcohol, it may be used as a fuel. The complete combustion of propan−2−ol is given in the equation below. CH3CH(OH)CH3(l) + 9 2 O2(g) → 3CO2(g) + 4H2O(l) (a) When 1.00 g of propan−2−ol was burned, it was found that 100 g of water was heated from 30 C to 89 C. The process was known to be only 75 % efficient. Calculate and show that the enthalpy change of combustion per mole of propan−2−ol is −2030 kJ mol−1. Assume the specific heat capacity of water to be 4.3 J g –1 K–1. (−2030 kJ mol−1) (b) Using bond energy data given in the Data Booklet, calculate the enthalpy change of combustion of propan−2−ol. (−1888 kJ mol−1) (c) Suggest a reason for the difference between the values calculated in (a) and (b). (d) Using the value given in (a) and the following data, construct a suitable energy cycle and calculate the enthalpy change of formation of propan−2−ol from its elements. enthalpy change of combustion of carbon/ kJ mol–1 −393.5 enthalpy change of combustion of hydrogen/ kJ mol–1 −285.8 (−294 kJ mol−1) 7 Concept: Hess’ Cycle (Given Bond Energy Data) One of the most important uses of alkanes is for fuels. In some countries, where crude oil is either scarce or expensive, biofuels such as ethanol are increasingly being used for fuels instead of hydrocarbons. (a) Define the term bond energy. (b) (i) Write a chemical equation for the complete combustion of liquid octane, C 8H18. (ii) Use bond energies in the Data Booklet to calculate a value for the enthalpy change of combustion of octane. (−5130 kJ mol−1)

National Junior College SH1 H2 Chemistry 5 (c) The accurate experimental enthalpy changes of combustion of three liquid hydrocarbons are given in the table below. alkane formula ∆Hc / kJ mol−1 heptane C7H16 –4817 octane C8H18 –5470 nonane C9H20 –6125 (i) Suggest a reason for the discrepancy between the ∆H c for octane you calculated in (b)(ii) and that given in the table in (c). (ii) Suggest what the regular increase in the values of ∆H c in the table in (c) represents. 8 Concept: Hess’ Cycle (Mix) (Challenging) During the industrial manufacture of nitric acid, NO2 is hydrolysed by passing the gas through water. 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g) Hr The following enthalpy changes are given: H / kJ mol−1 N2(g) + O2(g) → 2NO(g) +183 2NO(g) + O2(g) → 2NO2(g) −116 2N2(g) + 5O2(g) + 2H2O(l) → 4HNO3(aq) −256 (a) Using relevant data from the table above, calc