1. Hess’ Law: Adding Reaction Enthalpies Some chemical reactions can not be carried out easily in order to determine the amount of heat or energy that can be obtained. Eg: Mg (s) + 1/2O 2(g)  MgO (s) + light ?

2. G.H. Hess recognized a relationship between algebra and chemical equations. Chemical reactions can be added, subtracted, multiplied, and divided like math equations. For any chemical reaction that can be written in a series of steps, the overall enthalpy of the reaction is the result of adding up the sum of all the individual steps of the reaction.

3. Eg:Calculate the enthalpy of reaction for the formation of rust. 2Fe (s) + 3/2O 2(g)  Fe 2 O 3(s) Given: 1)Fe 2 O 3(s) + 3CO (g)  2Fe (s) + 3CO 2(g)  H o = -26.7 kJ 2)CO (g) + 1/2O 2(g)  CO 2(g)  H o = -282.7 kJ We can rearrange the given equations in a manner that will yield the overall reaction.

4. 1)X-1 2Fe (s) + 3CO 2(g)  Fe 2 O 3(s) + 3CO (g)  H o = +26.7kJ 2)X3 3CO (g) + 3/2O 2(g)  3CO 2(g)  H o =-848.1kJ _____________________________________________ 2Fe (s) + 3/2O 2(g)  Fe 2 O 3(s)  H o =-821.4kJ