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Unformatted text preview: Chapter 6: Energy and Chemical Reactions 273 Chemical Fuels 103 . Answer: 0.78 g C 3 H 8 Strategy and Explanation: Determine the mass of fuel required to generate the heat energy to melt a mass of ice and raise its temperature to a specified value. Use Equation 6.2 and the molar enthalpy of fusion to calculate the heat energy needed. Then balance the equation describing the combustion of the fuel. Use Equation 6.11, a table of molar enthalpies of formation, and the stoichiometry of the balanced equations to determine the molar enthalpy change for each combustion reaction. Then use that enthalpy and the molar mass as conversion factors to determine the mass of fuel needed. To change phase, use q = m × Δ H fus ; to change the temperature of the water, use q = c × m × Δ T. q total = (m ice × Δ H fus,ice ) + (c water × m water × Δ T water ) To simplify the calculation, factor out the common mass term m ice = m water = m: q total = m × [( Δ H fus,ice ) + (c water × Δ T water )] Table 6.1 gives the specific heat capacity of water to be 4.184 J g –1 °C –1 . Question 46 gives the molar enthalpy of fusion of ice as 333 J/g. q total = 56.0 g × [(333 J/g) + (4.184 J g –1 °C –1 ) × (75.0 °C – 0. °C)] = 3.6 × 10 4 J The reaction describing the combustion of propane is similar to those described in Section 6.11. The fuel reacts with oxygen and making gaseous carbon dioxide and water, as shown in the equation: C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(g) Balance the chemical equation: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) Δ H° = (3 mol) × Δ H f o (CO 2 ) + (4 mol) × Δ H f o (H 2 O(g)) – (1 mol) × Δ H f o (C 3 H 8 ) – (5 mol) × Δ H f o (O 2 ) Look up the Δ H f o value in Table 6.2. Δ H° = (3 mol) × (–393.509 kJ/mol) + (4 mol) × (–241.818 kJ/mol) – (1 mol) × (–103.8 kJ/mol) – (5 mol) × (0 kJ/mol) = –2044.0 kJ Now, we’ll figure out the mass needed for the melting and warming of the water sample: 3.6 " 10 4 J " 1 kJ 1000 J " 1 mol C 3 H 8 2044.0 kJ " 44.0953 g C 3 H 8 1 mol C 3 H 8 = 0.78 g C 3 H 8 Reasonable Answer Check: The molar enthalpy of C 3 H 8 is large and negative because propane is a good fuel. The energy needed for the sample is relatively small, so the mass of fuel is also small. 104. Answer: 70. g CH 4 Strategy and Explanation: Determine the mass of fuel required to raise the temperature of the air in a house to a specified value, given the dimensions of rooms in the house, the molar heat capacity of air, the average molar mass of air and the density of the air. Use the dimensions of the house, the molar mass, and the density of the air to determine the mass of air being heated. Then use Equation 6.2 to calculate the heat energy needed to raise the temperature of the air. Get the balanced equation describing the combustion of the fuel. Use Equation 6.11, a table of molar enthalpies of formation, and the stoichiometry of the balanced equations to determine the molar enthalpy change for each...
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 Fall '08
 Kerber
 Chemistry, Thermodynamics, Enthalpy, Reaction, Energy, mol, Reasonable Answer Check, Energy and Chemical Reactions

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