Question

application of Hess's Law

Answer 1

Hess's Law of Constant Heat Summation (or just Hess's Law) states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function.

Answer 2

The heat of any reaction ΔH∘fΔHf°ΔHf° for a specific reaction is equal to the sum of the heats of reaction for any set of reactions which in sum are equivalent to the overall reaction.

Application

Hydrogen gas, which is of potential interest nationally as a clean fuel, can be generated by the reaction of carbon (coal) and water:

C(s)+2H2O(g)→CO2(g)+2H2(g)

Calorimetry reveals that this reaction requires the input of 90.1 kJ of heat for every mole of C(s)consumed. By convention, when heat is absorbed during a reaction, we consider the quantity of heat to be a positive number: in chemical terms, q>0 for an endothermic reaction. When heat is evolved, the reaction is exothermic and q<0 by convention.

It is interesting to ask where this input energy goes when the reaction occurs. One way to answer this question is to consider the fact that the reaction converts one fuel, C(s), into another, H2(g). To compare the energy available in each fuel, we can measure the heat evolved in the combustion of each fuel with one mole of oxygen gas. We observe that

C(s)+O2(g)→CO2(g)



produces 393.5kJ for one mole of carbon burned; hence q=−393.5kJ.