chemistry  decomposition of water(help)
posted by Farah on .
Hydrogen is a very appealing fuel, in part because burning it produces only nonpolluting water. One of the challenges that researchers face in making hydrogen fuel a reality is how to produce hydrogen economically. Researchers are investigating methods of producing hydrogen indirectly. The following series of equations represent one such method.
3FeCl2(s) + 4H2O(g) → Fe3O4(s) + 6HCl(g) + H2(g) ΔH° = 318 kJ
Fe3O4(s) + 3/2Cl2(g) + 6HCl(g) → 3FeCl3(s) + 3H2O(g) + 1/2O2(g) ΔH° = −249 kJ
3FeCl3(s) → 3FeCl2(s) + 3/2Cl2(g) ΔH° = 173 kJ
(a) Show that the net result of the three reactions is the decomposition of water to produce hydrogen and oxygen.
(b) Use Hess’s law and the enthalpy changes for the reactions to determine the enthalpy change for the decomposition of one mole of water. Check your answer, using the enthalpy of formation of water.
how do i show the deomcposition of water and for b) how do i start?

Add the equations and cancel molecules that are the same on both sides.(It may b necssary to reverse some of the equations.) For example, you have 3 FeCl2 in equation 1 on the left side and 3 FeCl2 in equation 3 on the right side. They cancel. Continue and see what is left.I didn't go through all of it but the problem suggests that everything except H2O ==> H2 + O2 cancels. This is not balanced as is but the coefficients probably will balance in the end.
For the b part, add the equations (reverse some if necessarychange the sign of delta H if reversed), then add all of the delta H values together to arrive at a final delta H for the series. 
Thank you, helped, the equations do balance out, and there are more than one ways to solve this either by multiplying the first equation by 2 and then manipulating further as well as just canceling out and adding all by hess's law.