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5 Thermochemistry Solutions to Exercises (b) Use the specific heat of water, 4.184 J/g-°C, to calculate the energy required to heat the water. Use the density of water at 25°C to calculate the mass of to be heated. (The change in density of going from 21°C to 79°C does not substantially affect the strategy of the exercise.) Then use the 'heat stoichiometry' in (a) to calculate mass of Mg(s) needed. 75 mL mL g-°C 58° J = 18.146 = 18 required 18.146 kJ 1 mol Mg 24.305 1 mol Mg Mg = 1.249 = Mg needed 5.103 (a) For comparison, balance the equations so that 1 mole of CH₄ is burned in each. = -496.91 kJ AH° = AH° = (b) = - = + - (-74.8) - = = + - = = -393.5 + 2(-285.83 - = kJ (c) Assuming that O₂(g) is present in excess, the reaction that produces CO₂(g) represents the most negative AH per mole of burned. More of the potential energy of the reactants is released as heat during the reaction to give products of lower potential energy. The reaction that produces CO₂(g) is the most "downhill" in enthalpy. 5.104 (a) B₂O₃(s) = -(-2147.5kJ) B₂H₆(g) (b) If, like B₂H₆, the combustion of B₅H₉ produces B₂O₃ as the boron-containing product, the heat of combustion of B₅H₉ in addition to data given in part (a) would enable calculation of the heat of formation of B₅H₉. The combustion reaction is: 131