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 + O2→CO2(pg. 165)The book tells us that this can occur as 2 stepsC + ½O2→CO ΔH°= – 110.5 kJCO + ½O2→CO2ΔH°= – 283.0 kJC + CO + O2→CO + CO2ΔH°= – 393.5 kJI.e. C + O2→CO2ΔH°= – 393.5 kJ•Hess’s law allows us to add equations.•We add all reactants, products, & ΔH°values.•We can also show how these steps add together via an “enthalpy diagram” …Steps in drawing enthalpy diagrams1.Balance the equation(s).2.Sketch a rough draft based on ΔH°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 awayFinish: two pointing to finishIntermediate: one to, one away6.Look at equations to help complete balancing (all levels must have the same # of all atoms).7.Add axes and ΔH°values.C + O2→CO2ΔH°= – 393.5 kJReactantsIntermediateProductsC + O
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