1 Meeting 19 Last time we saw how to measure “heats” of reaction, ΔHrxnusing calorimetry. For some reactions it is impossible to use this technique. Example: 2C(Gr)) + O2(g) → 2CO(g); ΔH= ? The reaction cannot be stopped at CO, and CO2(g) forms. What to do? Calculate ΔH from reactions whose ΔHrxns are known.Use Hess’s Law of Constant Heat Summation:The heat of a reaction is constant, whether the reaction is carried out directly or via a number of steps.ΔHoverall= ΣΔH’s of individual reactionsReverse an equation, change the sign of ΔH.\ Multiply an equation’s coefficients, multiply ΔHby the same coefficient for that reaction
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