8 methane 555 ethane 519 propane 5035 butane 495

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Unformatted text preview: the energy balance with Q0 ;v ¼ À282.0 kJ, rxn ^0 ^ DhH2 OðlÞ ¼ Q0 ;v þ DN Ru T0 ; rxn ¼ À282:0 kJ/mol Á 1 mol þ ðÀ1:5 mol Á 8:314 J/mol À K Á 298K Á 0:001 kJ/JÞ ¼ ðÀ282:0 À 3:72ÞkJ ¼ À285:7 kJ The error is (285.8 À 285.7)/285.8 ¼ 0.03%. In this case, more heat is given off ^ if the reaction is carried out at constant pressure, since the P-V work (1.5Ru T0 ) due ^0 to the compression of 1.5 mol of gases in the reactants would contribute to DhH2 OðlÞ . However, this difference is only about 1–2% of the enthalpy of formation. The enthalpy of formation for gaseous H2O is obtained by adding the latent heat ^0 to DhH2 OðlÞ : ^0 ^0 ^ DhH2 OðgÞ ¼ DhH2 OðlÞ þ hfg ¼ À241:88 kJ/mol; Table 2.3 Heat values of various fuels Heating value Fuel MJ/kg Hydrogen 141.8 Methane 55.5 Ethane 51.9 Propane 50.35 Butane 49.5 Gasoline 47.3 Paraffin 46 Diesel 44.8 Coal 15–27 Wood 15 Peat 6–15 Methanol 22.7 Ethanol 29.7 Propanol 33.6 Acetylene 49.9 Benzene 41.8 Ammonia 22.5 Hydrazine 19.4 Hexamine 30.0 Carbon 32.8 BTU/lb 61,100 23,900 22,400 21,700 20,900 20,400 19,900 19,300 8,000–14,000 6,500 2,500–6,500 9,800 12,800 14,500 21,500 18,000 9,690 8,370 12,900 14,100 kJ/mol 286 890 1,560 2,220 2,877 ~5,400 16,300 ~4,480 200–350 300 726 1,368 2,020 1,300 3,270 382 622 4,200 393.5 2.4 Adiabatic Flame Temperature 31 where ^ hfg ¼ 43:92 kJ/mol: Example 2.5 The heat released by 1 mol of sugar in a bomb calorimeter experiment is 5,648 kJ/mol. Calculate the enthalpy of combustion per mole of sugar. Solution: The balanced chemical reaction equation is C12 H22 O11 ðsÞ þ 12O2 ðgÞ ¼ 12CO2 ðgÞ þ 11H2 OðliqÞ Since the total number of moles of gas is constant (12) in the products and reactants, DN ¼ 0. Therefore, work is zero and the enthalpy of combustion equals the heat transfer: À5,648 kJ/mol. 2.4 Adiabatic Flame Temperature One of the most important features of a combustion process is the highest temperature of the combustion products that can be achieved. The temperature of the products will be greatest when there are no heat losses to the surrounding environment and all of the energy released from combustion is used to heat the products. In the next two...
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This document was uploaded on 01/20/2014.

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