CHEM 1050 Lecture Notes - Lecture 5: Joule

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Recall: h for a reaction is equal in magnitude but opposite in sign for reverse reaction. Do not forget: state of reactants and products should be specified. 2h2o(g) 2h2o(l) h = -88 kj (reaction is exothermic) **must put the coefficient 2 because oxygen and hydrogen are diatomic. On friday we saw combustion of ch4 when h2o was liquid instead of gas h = - 890 (88 kj difference) Therefore more heat available for transfer to surroundings because 88 kj is released when 2 moles of gaseous water is condensed to liquid. You can get h from experiments or from calculations from h associated with other reactions using first law. Because of first law and state function concepts. H depends on amount of material: sum of h of all individual steps must be same as that of the final one step equation. H = -890 kj this was calculated from h for condensation of h2o(l) plus h for ch4 .