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hess-law-handout

# hess-law-handout - Hesss law Hesss Law states that the heat...

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Hess’s law Hess’s Law states that the heat of a whole reaction is equivalent to the sum of it’s steps. For example: C + O 2 CO 2 (pg. 165) The book tells us that this can occur as 2 steps C + ½O 2 CO Δ H ° = – 110.5 kJ CO + ½O 2 CO 2 Δ H ° = – 283.0 kJ C + CO + O 2 CO + CO 2 Δ H ° = – 393.5 kJ I.e. C + O 2 CO 2 Δ H ° = – 393.5 kJ Hess’s law allows us to add equations. We add all reactants, products, & Δ H ° values. We can also show how these steps add together via an “enthalpy diagram” … Steps in drawing enthalpy diagrams 1.Balance the equation(s). 2.Sketch a rough draft based on Δ H ° values. 3.Draw the overall chemical reaction as an enthalpy diagram (with the reactants on one line, and the products on the other line). 4.Draw a reaction representing the intermediate step by placing the relevant reactants on a line. 5.Check arrows: Start: two leading away Finish: two pointing to finish Intermediate: one to, one away 6.Look at equations to help complete balancing (all levels must have the same # of all atoms). 7.Add axes and Δ H ° values. C + O 2 CO 2 Δ H ° = – 393.5 kJ Reactants Intermediate Products C + O
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