AH Values of Reactants and Products

By applying Hess s law, we can use AH values to determine AH for any reaction. All we have to do is view the reaction as an imaginary two-step process. 

Step ]. Each reactant decomposes to its elements. This is the reverse of the formation reaction for each reactant, so each standard enthalpy change is —AH. Step 2. Each product forms from its elements. This step is the formation reaction for each product, so each standard enthalpy change is AH.  

We write this equation as though it were the sum of four individual equations, one for each compound. The first two of these equations show the decomposition 

CHAPTER 6 Thermochemistry Energy Flow and Chemical Change 

It s important to realize that when titanium(IV) chloride and water react, the reactants don t actually decompose to their elements, which then recombine to form the products. But that is the great usefulness of Hess s law and the state-function concept. Because A//°x is the difference between two state functions, minus A//°eaciams it doesu t matter how the change actually occurs. We simply add the individual enthalpy changes to find A//°x  

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