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This guide explores the principles of calorimetry and applies Hess’s Law to construct enthalpy cycles. By utilizing enthalpy changes of combustion, you can calculate the enthalpy change for reactions indirectly. Discover how the enthalpy change from a state A to B can be determined by summing the enthalpy changes of intermediate reactions, even if the path taken differs. Learn to apply the relationship of enthalpies in both forward and reverse reactions to achieve accurate calculations of enthalpy changes in chemical processes.
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Enthalpy changes from DHc L.O.: Use Hess’ law to construct enthalpy cycles Use enthalpy changes of combustion to calculate an enthalpy change of a reaction indirectly.
A -> B DH = + 20 J mol-1 B -> A DH = - 20 J mol-1
HESS’S LAW “The enthalpy change is independent of the path taken” How The enthalpy change going from A to B can be found by adding the values of the enthalpy changes for the reactions A to X, X to Y and Y to B. DHr = DH1 + DH2 + DH3 If you go in the opposite direction of an arrow, you subtract the value of the enthalpy change. e.g. DH2 = -DH1+ DHr-DH3 The values of DH1 andDH3 have been subtracted because the route involves going in the opposite direction to their definition.
Enthalpy of reaction from enthalpies of combustion If you burned all the products you should get the same amounts of oxidation products such a CO2 and H2O as if you burned the reactants. Enthalpy of combustion is an exothermic process
