NJC H1 Chemical Energetics Tutorial

National Junior College SH1 H1 Chemistry 1 Chemical Energetics Tutorial 1 Write thermochemical equations to represent the following statements: (a) Standard enthalpy change of formation of hydrogen bromide gas is –36.2 kJ mol−1 (b) Standard enthalpy change of combustion of liquid propanol (CH3CH2CH2OH) is – 2017 kJ mol−1 (c) Standard enthalpy change of neutralisation of hydrochloric acid and sodium hydroxide is −57 kJ mol−1 (d) Lattice energy of calcium chloride solid is –2237 kJ mol−1 (e) Bond energy of I−I (refer to the Data Booklet for the bond energy) 2 [N2008 P1 Q32] Which reactions represent standard enthalpy changes? 1 NH3(g) + HCl(g) → NH4Cl(s) 2 C(g) + 6H(g) → C2H6(g) 3 CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) A 1 only B 1 and 2 only C 2 and 3 only D 1, 2 and 3 3 The heat liberated in the neutralisation given below is −114 kJ mol−1. 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) By using this information, what is the most likely value for the heat liberated in the following neutralisation? Ba(OH)2(aq) + 2HCl(aq) → BaCl2(aq) + 2H2O(l) A −57 kJ mol−1 B −76 kJ mol−1 C −114 kJ mol−1 D −228 kJ mol−1 4 The enthalpy change of neutralisation of ethanoic acid with sodium hydroxide can be found experimentally by mixing known volumes of 1.0 mol dm –3 ethanoic acid, CH 3COOH, and 1.0 mol dm–3 NaOH. The following results are obtained. Volume of CH3COOH used = 40.0 cm3 Volume of NaOH used = 30.0 cm3 Initial temperature of mixture = 28.0 C Final temperature of mixture = 32.6 C (a) Use the data given to calculate the standard enthalpy change of neutralisation of CH3COOH with NaOH. [–44.9 kJ mol–1] (b) The experiment is repeated with NaOH and HCl and it is found that the enthalpy change of reaction between NaOH and HC l is more exothermic than that calculated in (a). Explain why this is so.

National Junior College SH1 H1 Chemistry 2 5 When 1.00 g of ethanol in a spirit lamp was burned under a container of water, it was found that 100 cm 3 of water was heated from 15 C to 65 C. The process was known to be only 70% efficient. (a) Suggest reasons why only 70% of heat released by combustion is transferred to the water. (b) Calculate the standard enthalpy change of combustion of ethanol. [–1370 kJ mol–1] (c) By using the value you have obtained in (b) and the following data: Enthalpy change of combustion of carbon −393.5 kJ mol−1 Enthalpy change of combustion of hydrogen −285.8 kJ mol−1 Calculate the enthalpy change of formation of ethanol. 2 C(s) + 3 H2(g) + ½ O2 (g) ⎯→ CH3CH2OH(l) ∆Hf =? [–271 kJ mol–1] 6 When 6 g each of carbon, hydrogen and methanol, CH 3OH (l), are completely burnt in oxygen, 196.8, 857.7 and 136.2 kJ of heat are evolved respectively. Calculate the enthalpy change of formation of liquid methanol. [− 239 kJ mol−1] 7 The chemical equation for the combustion of propene is as shown below. C3H6 (g) + 4½ O2 (g) → 3CO2 (g) + 3H2O (l) The table below shows the standard enthalpy changes of formation of the compounds involved in the reaction. Compound C3H6(g) CO2(g) H2O(l) O2(g) ΔHfꝋ / kJ mol–1 +20 –394 –286 0 (a) Explain why the standard enthalpy of formation, ΔHfꝋ, of oxygen is zero. (b) Use the data from the table above to calculate the standard enthalpy of combustion of propene. [–1930 kJ mol–1] 8 The yellow chlorine dioxide gas, ClO2, has been used for many years as a flour-improving agent in bread-making. It can be made in the laboratory by the following reaction: 2AgClO3(s) + Cl2(g) ⎯→ 2AgCl(s) + 2ClO2(g) + O2(g) Hrꝋ = 0 kJ mol–1 Given that Hfꝋ of AgClO3(s) = –25 kJ mol–1 and Hfꝋ of AgCl(s)= –127 kJ mol–1. Calculate Hfꝋ of ClO2(g). [+102 kJ mol–1]

National Junior College SH1 H1 Chemistry 3 9 One of the most important uses of alkanes is as fuels. In some countries, where crude oil is either scarce or expensive, biofuels such as ethanol are increasingly being used as fuels instead of hydrocarbons. (a) Define the term “bond energy”. (b) (i) Write an equation that represents the standard enthalpy change of combustion of octane, C8H18(l). (ii) Use the bond energies given in the Data Booklet to calculate a value for the standard enthalpy change of combustion of octane. [–5130 kJ mol–1] (c) The standard enthalpy changes of combustion of three hydrocarbons are given in the table below. Alkane formula Hco / kJ mol–1 Heptane C7H16 – 4817 Octane C8H18 – 5470 Nonane C9H20 – 6125 (i) Suggest a reason for the discrepancy between the Hco for octane you calculated in (b)(ii) and that given in the table. (ii) Suggest what the regular increase in the values of Hco given in the table corresponds to. 10 Sulfur hexafluoride can be made by reacting sulfur tetrafluoride with fluorine in the gas phase: SF4(g) + F2(g) ⎯→ SF6(g) Hrꝋ = − 434 kJ mol−1 Using relevant data from the Data Booklet, calculate an average value for the S−F bond energy. 11 (a) Define, with the aid of an equation and using magnesium nitride, Mg 3N2 (s) as an example, what is meant by the term lattice energy. (b) How would you expect the magnitude of the lattice energy of Mg3N2 to compare with that of MgO? Explain your answer. 12 [N99 P3 Q31] The diagram illustrates the energy changes of a set of reactions. Which statements are correct? 1 The enthalpy change for the transformation U → R is +42 kJ mol−1. 2 The enthalpy change for the transformation T → S is endothermic. 3 The enthalpy change for the transformation R → T is −33 kJ mol−1 A 1 only B 1 and 3 only C 2 and 3 only D 1, 2 and 3 R S H = −134 kJ mol−1 T U H = −75 kJ mol−1 H = +92 kJ mol−1