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 given to calculate the standard enthalpy change of neutralisation of CH3COOH with NaOH. [–44.9 kJ mol–1] Amount of water formed = Amount of NaOH reacted (since NaOH is the limiting reagent) = 30.0 1000 × 1.0 = 0.0300 mol Enthalpy change of neutralisation of CH3COOH with NaOH ∆Hn ꝋ = − mc∆T nH2O = − (40+30) × 4.18 × (32.6 - 28) 0.0300 = − 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. CH3COOH(aq) + NaOH(aq) → CH3COO−Na+(aq) + H2O(l) HCl is a strong acid that ionises completely in aqueous solution. CH3COOH is a weak acid that ionises partially in aqueous solution. Since ionisation of CH3COOH(aq) is an endothermic process, some heat released from the neutralisation is absorbed to further ionise the weak acid completely. Therefore, the enthalpy change of neutralisation of CH 3COOH(aq) with NaOH is less exothermic than that of HCl with NaOH.

National Junior College SH1 H1 Chemistry 3 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. Heat lost to the surrounding air and to the container Presence of draught (wind currents) (b) Calculate the standard enthalpy change of combustion of ethanol. [–1370 kJ mol–1] Hc = − mcT nethanol burnt × 𝟏𝟎𝟎 𝟕𝟎 = − 𝟏𝟎𝟎 × 𝟒.𝟏𝟖 × (𝟔𝟓–𝟏𝟓) 𝟏.𝟎𝟎 𝟒𝟔.𝟎 × 𝟏𝟎𝟎 𝟕𝟎 = –1373  − 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] Hrꝋ =  n Hcꝋ (reactants) –  m Hcꝋ (products) ∆Hf(CH3CH2OH)= 2(−393.5) + 3(−285.8) − (−1373) = −271.4  −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] Hc = – q nsubstance burnt ∆Hc(C) = − 196.8 6 12.0 = −393.6 kJ mol-1 ∆Hc(H2) = − 857.7 6 2.0 = −285.9 kJ mol-1 ∆Hc(CH3OH) = − 136.2 6 32.0 = −726.4 kJ mol-1 C(s) + 2 H2(g) + ½ O2 (g) ⎯→ CH3OH(l) ∆Hf =? Hro =  n Hco (reactants) –  m Hco (products) ∆Hf(CH3OH) = −393.6 + 2(−285.9) − (−726.4) = −239 kJ mol− 1

National Junior College SH1 H1 Chemistry 4 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. O2(g) → O2(g) Initial state = final state  there is no change in the energy level, hence ΔHfo = zero . ΔHfo = zero for any elements in their standard states and most stable allotrope. (b) Use the data from the table above to calculate the standard enthalpy of combustion of propene. [–2060 kJ mol–1] Hco = 3(–394) + 3(–286) – (20) – 4½ (0) = –2060 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] Hrꝋ = 2(–127) + 2 × Hfꝋ of ClO2 – 2(–25) = 0 2 × Hfꝋ of ClO2 = 254 – 50 Hfꝋ of ClO2 = +102 kJ mol–1

National Junior College SH1 H1 Chemistry 5 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”. Bond energy is the amount of energy absorbed to break 1 mole of covalent bonds between atoms in a gaseous molecule to form gaseous atoms. (b) (i) Write an equation that represents the standard enthalpy change of co