NJC 6 H1 Chemical Energetics Tutorial Solutions

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 ½H2(g) + ½Br2(l) → HBr(g) Hof = –36.2 kJ mol−1 (b) Standard enthalpy change of combustion of liquid propanol (CH3CH2CH2OH) is – 2017 kJ mol−1 CH3CH2CH2OH(l) + 2 9 O2(g) → 3CO2(g) + 4H2O(l) Hoc = –2017 kJ mol−1 (c) Standard enthalpy change of neutralisation of hydrochloric acid and sodium hydroxide is −57 kJ mol−1 NaOH(aq) + HCl(aq) → NaCl (aq) + H2O(l) Hon= –57 kJ mol−1 (d) Lattice energy of calcium chloride solid is –2237 kJ mol−1 Ca2+(g) + 2Cl ̶ (g) → CaCl2(s) H = –2237 kJ mol−1 (e) Bond energy of I−I (refer to the Data Booklet for the bond energy) I2 (g) → 2I (g) BE = 151 kJ mol−1 1 mole of covalent bond in gaseous molecules 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 (A) 1. ✓ (True) The physical state of reactants and products given are their corresponding physical states under standard conditions. 2.  (False) Carbon is in solid state and hydrogen exists as H 2(g) under standard conditions. 3.  (False) H2O is in liquid state under standard conditions. 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 (C) The standard enthalpy change of neutralisation of an acid with an alkali is the energy evolved when one mole of water is formed from the reaction of the acid and alkali under standard conditions. Both reactions involve mixing of a strong base and a strong acid and each gives two moles of water. Therefore, the heat liberated for both reactions is the same.

National Junior College SH1 H1 Chemistry 2 H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) H = –114 kJ mol–1 2HCl(aq) + Ba(OH)2(aq) → BaCl2(aq) + 2H2O(l) H = –114 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 give