Winter 2015 Midterm Question 2 and 3 D

[ali_boutrosdisc3H]

p. 149 of the course reader:
For part D, i am confused on how to set up this equation, could someone explain how you know which bond enthalpies to subtract and add?

[Rosaline_Chow_2L]

In calculating using bond enthalpies, the basic equation is enthalpy of rxn = bonds broken(reactant side) - bonds formed(product side). I find it easier to see exactly which bonds are broken if you draw out the structure of each of the molecules. Once you've done that, you can see that a C-I and H-O bond broke on the reactant side in order to form a C-O and H-I bond on the product side. That means deltaHrxn = [deltaH(C-I) + deltaH(H-O)] - [deltaH(C-O) - deltaH(H-I)]. Then, you can plug in the values provided to get deltaH(C-O).

[Jata_Kavya_3A]

for every question with bond enthalpies vs enthalpies of formation, will the question specify what they want?

[MariaReina_Garcia_3E]

for every question with bond enthalpies vs enthalpies of formation, will the question specify what they want?

The question usually specifies it. The pattern I have encountered is that when they ask for bond enthalpies, they generally come with a table of bonds (H-O, C-C, etc). and when they want enthalpies of formation, the delta Hf is usually given for the molecules.

[Temidayo Adegbenro 1C]

Hello, just to add on to the questions here, for 2 &3 part D, how do you know to ADD the summation of broken bonds to the summation of formed bonds. Usually the equation is summation of delta-H (broken bonds) minus the summation of delta-H (formed bonds)? And I also asks this because the two formed bonds are C-O and H-I, which releases energy, (so I am thinking of negative energy value), but the answer is +360kJ/mol.

[Noelle Min-1N]

Why is the value of standard enthalpy of reaction 42.00 kJ/mol for part C and D, but 84.00 kJ for part E? How do you know when to use the enthalpy value per mole?