Bond Enthalpy Thermochemistry help needed

[dorjiako]

A second metabolic process involves the net production of ATP and 2 ADP and 2 NAD+. If the conversion of these molecules is endothermic and adds 443.5KJ to the overall enthalpy of the reaction, find the enthalpy for a "high energy" phosphate bond.
c=0 is 805kj/mol, 0=0 is 497, 0-H is 464, c-c is 347, c-H is 413.
The answer is 338.5KJ/mol but I just can't seem to get the answer. I don't even know what the overall enthalpy that they are talking about in the question is ? Help is needed as I am literally drowning in this question!!!!!

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[cycloethane]

I forget, but I will tell you that your use of the word "literally" is wrong. Keep that in mind for the writing section. To literally drown means exactly that--you are physically drowning. I don't think you mean that.

 

[Integrated MCAT Course]

In a chemical reaction you have state A transformed into state B. Old bonds are broken and new bonds are formed. With constant volume, the overall enthalpy change resolves to the internal energy change, basically the electrostatic potential energy difference of the bonding electrons reagent versus product.

The process of using standard bond enthalpies involves an imaginary pathway, which is allowed by Hess' Law of Heat Summation, of breaking the bonds of the reagent, forming the standard states of the elements, and then going from the standard states to the products. Reference to the standard state gives a common reference point to compare the product and reagent. (The standard state of carbon is graphite, of hydrogen, H2 gas, etc.)

In problem solving, you have to count the number of bonds there are of each type in the reagent and the product. Sum the enthalpies of the bonds on the left side and the bonds on the right side. The net enthalpy change is the enthalpy of the products minus the enthalpy of the reagents. In this case, you are already given the total enthalpy change, so I imagine the missing variable is the high energy phosphate bond that is formed. It's hard to know exactly without seeing the stoichiometry of the reaction. I hope this helps.