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I'm having some trouble with part a) on problem 8.67. It's asking to find the enthalpy of formation of H2O in the liquid state, but in the solution you have to factor in breaking the bonds between H2 and O2. If we're trying to find the enthalpy of formation, why do we have to calculate the energy to break the bonds?
The way the reaction is balanced gives one mole of H20(l), which is helpful because when we find the enthalpy change for the reaction, we are also finding the enthalpy of formation of H20(l). There are a few ways to do this, but for this question we are asked to use bond enthalpies to calculate the change in enthalpy for the reaction. Adding all bond enthalpies broken and subtracting all bond enthalpies formed gives both the enthalpy change for the reaction and the enthalpy of formation for H20, but in gas phase (because of what is given in the tables). We are asked to find enthalpy of formation of H20 in liquid phase. Subtracting the enthalpy of vaporization from our value calculated for the enthalpy of formation of H20(g) gives us the enthalpy of formation of H20(l).
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