Question

Problem 9.10 Trimethylpentane The standard heat of combustion (AH) of liquid 2,3,3-trimethylpentane (CgH18) is reported in a table of physical properties to be - 4850.0 kJ/mol. A footnote indicates that the reference temperature for the reported value is 25°C and the presumed combustion products are CO2(g) and H20(9). The heat capacity of water is approximately 75.40 J/(mol K) over the temperature range of interest. Physical Property Tables Heat of Combustion There is some question about the accuracy of the reported value, and you have been asked to determine the heat of combustion experimentally. You burn 2.010 grams of the hydrocarbon with pure oxygen in a constant-volume calorimeter and find that the net heat released when the reactants and products [CO2(9) and H20(9)] are all at 25°C is sufficient to raise the temperature of 1.000 kg of liquid water by 20.84 °C. Write an energy balance to show that the heat released in the calorimeter equals ngHix All , and calculate all (kl/mol). All = 1 kJ/mol Then calculate GH, . (See Example 9.1-2.) Aî - 1 kJ/mol By what percentage (absolute value) of the measured value does the tabulated value differ from the measured one?

Answer 1

0 diven mass of colto burned = 2.019 8° 18 molar imass of Cotlig = 114.20/mol Moles of C& His burned mass of Ca His burned molar mass of Cat is burned 2.01 = 0.0176 moles Mass of water 114.2 molar mass of water = 18.02 kg/kmol moles of water (n) = mass of water molar mass of water 18.02 = 0.055494.1mol = 55.494 moles specific heat of water (Co) = 75.4 J/ molik Temperature rise of water (AT) = 20.84c Heat relired to heat water (water) = n XÇXAT = 55.494x75.4720.84 = 87199.9 T = 87.1197 KJ Heat released by combustion 2 Heat rewired to heat water - 87.1197 kv for constant volume Colorimeter, heat released by combustion = -1x change in internal energy

· change in internal energy = -1.X87.1197 = -87, 1197 kJ Molar change in internal energy change in internal energy Moles of Cg His burned -87:1197 0.0176 Allc =-4949.9 kJ/mol combustion reaction of Catig is Cshig (2) + 12-50 (3) →800,(9)+94,20(a) Here an Vi Product E Vi reactant where V = stoichiometric coefficients of only gases (8+9) -(12.5) = 4.5 Alte Auct on RT T: Standard temp 1000 = -4949.9+(4.5*8.314x298) -25°C = 298 € -49387.50.926 t/mor Atte -4938.75 kJ/mol Actual AH, - calculated 4H % difference (absolute) Actual Alte Xloo

3 (-4938.75) - (+4850) X100 (-4938.75) %difference - 1.8% combustion reaction of Cg His is Cs Hig(44+ 1a-501(8) →800,(9)+9H0 (9) Standard enthalply of formation of components () 0 kJ/mol -393.5 kJ/mol 0 2 (9) Co (9) H2O(9) - 241.83 kJ/mol AHE (3 044.p.) product ( V4HP) reactants where V = Stoichiometric coefficient Ale = [9 x Hico.com +9XAHP AH " Cg Hig(e) +12.5%AH -4938.75 = = {8x(-393.5j+9x(-241.83] [1x2 mt 2:98] Hico Hell -5324.47 +4938.75 alte egh -385.72 kJ/moj C& Hg (1)