C1210 lecture 16

C1210 lecture 16 - Hesss Law suggests that large databases...

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Unformatted text preview: Hesss Law suggests that large databases of heats of reaction are useful for predicting heats of reaction for new reactions. Databases include combustion reactions, phase changes, and formation of compounds from elements. Definition : The standard heat of combustion, of a substance is the amount of heat released when one mole of a fuel substance is completely burned in pure oxygen gas and all reactants and products are brought to 25 C and one bar pressure. All carbon in the fuel becomes CO 2 gas and all hydrogen becomes liquid H 2 O. Combustion reactions are always exothermic, so is always negative for combustion reactions. c H c H Writing an equation for a standard heat of combustion: A power plant burns methane, CH 4 ( g ), for which the standard heat of combustion is -890 kJ/mol. For every 1.0 MJ of energy produced, how many mol CO 2 are produced? We want mol/MJ. First we need a balanced combustion equation. Carbon CO 2 Hydrogen H 2 O The other reactant is O 2 . Burning one mol of CH 4 produces one mol CO 2 and releases 890 kJ of energy. The reaction must be CH 4 + 2O 2 CO 2 + 2H 2 O = -890 kJ. One mole of fuel One mol CH 4 1 mol CO 2 890 kJ released c H /MJ CO mol 1.12 MJ 1 J 10 1 J 10 1 kJ 1 released kJ 890 CO mole 1 2 6 3 2 = The heat of combustion, H , of acetone, C 3 H 6 O, is 1790.4 kJ/mol. How many kJ of heat are evolved in the combustion of 12.5 g of acetone? How many g of acetone constitute a mole? 1 mol acetone = 3 12.01 + 6 1.01 + 1 16.00 = 58.09 g What part of a mole is 12.5 g acetone? mol 0.215 g 5 . 12 g 58.09 acetone mole 1 = The heat of combustion of one mole of acetone is 1790.4 kJ, so: kJ 385 mol 215 . mol 1 kJ 4 . 1790 = n-Octane, C 8 H 18 ( l ), has a standard heat of combustion of 5450.5 kJ/mol. a 15-gallon fuel tank can hold about 480 mol of n-octane. How much heat can be produced by burning a full tank of n-octane? kJ 10 62 . 2 mol 480 mol 1 kJ 5 . 5450 6 = Until now, we have only been concerned with enthalpy differences. H = H products H reactants We can mark an enthalpy zero by defining a Standard Heat of Formation ....
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This note was uploaded on 12/10/2009 for the course CHE 1220 taught by Professor Jespersen during the Spring '09 term at St. John's.

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C1210 lecture 16 - Hesss Law suggests that large databases...

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