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 C