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### 3 Methods

Posted: Sun Jan 14, 2018 11:57 pm
Can someone help clarify when each of the three methods discussed in class should be used. I understand that enthalpy being a state function is the basis of the methods but am not sure when to use them.

### Re: 3 Methods

Posted: Mon Jan 15, 2018 12:31 am
You use Hess law when they give you multiple reactions with the enthalpy of reaction values. You add up the bonds formed and broken when your given a table with bond enthalpies. And you use the enthalpy of formations when everything in the reaction is in standard state.

### Re: 3 Methods

Posted: Mon Jan 15, 2018 7:46 pm
How come with Hess' Law we subtract the enthalpies of the reactants from that of the products, but when calculating bond enthalpies we essentially subtract the products from the reactants?

### Re: 3 Methods

Posted: Mon Jan 15, 2018 8:53 pm
In terms of signs for bond enthalpies, breaking bonds always require energy (endothermic, and thus a positive value) and forming bonds always releases energy (exothermic, and thus a negative value). Therefore, it does appear that we are subtracting the enthalpies of the products from the enthalpies of the reactants.

### Re: 3 Methods

Posted: Mon Jan 15, 2018 8:58 pm
For Hess's Law the enthalpy of the reactants is subtracted from the products due to the general form of the change in enthalpy for the reaction being, simply, the final enthalpy of the reaction minus the initial enthalpy of the reaction.

For bond energies it's the opposite because of how energy flows when bonds are broken and formed. When bonds are broken, energy is released into the system which makes bond breaking an endothermic process (positive energy for the system). On the other hand, energy must be provided for bonds to form which means for the products to be formed, energy from the system must be used to do the work of forming those bonds which makes bond formation exothermic for the system (negative energy since the system loses that energy/heat to the formation of bonds).

Thus, the difference in these two methods is in their approach. Hess's Law uses enthalpies for the reaction as a whole, before and/or after, in order to find the change in enthalpy for a certain reaction. On the other hand, bond enthalpies are used to find the the energy required to break and form certain bonds that you then apply to the actual substances undergoing change/formation in the reaction.

Hope that helps!