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
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This note was uploaded on 04/12/2011 for the course CHEM 1010 taught by Professor Marshall during the Spring '11 term at North Texas.