help with thermodynamics

[thechairman]

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33. What is the enthalpy of formation, ΔHf, of hydrogen
iodide at 298 K? (Note: The bond energies of H2(g),
I2(g), and HI(g) at 298 K are 436 kJ/mol, 151 kJ/mol,
and 298 kJ/mol, respectively.)
A. 9 kJ/mol
B. 4.5 kJ/mol
C. –4.5 kJ/mol
D. –9 kJ/mol

I am in disagreement with the book's answer, which is C. but if you use the change of reaction, isn't it products minus reactants (which gives B as answer), to obtain the change in enthalphy, and not the other way around?

The book says to use:

I would use 436/2 + 151/2 - 298 = -4.5

but aren't you supposed to use:

I would use 298 - 436/2 - 151/2 = +4.5?

please help, thanks

 

[DrMT]

i must be ******ed..... i did (2*(298)) - (436+151) = 9

 

[eg77]

i must be ******ed..... i did (2*(298)) - (436+151) = 9

that's what i would've done too

 

[lookuphere]

the answer depends on whether 1 or 2 molecules of HI are being formed. if its just HI, then i would say its 4.5kJ. 9kJ would be if H2+ I2---> 2 HI (but it would be for 2 molecules of HI)

 

[thechairman]

both answers derive from the number of mols taken into account. so numerically, they are both correct.

but the problem here, is not the value of the Hfusion, but the sign. i don't see why the answer should be negative, when products minus reactants give a positive value

 

[DrMT]

both answers derive from the number of mols taken into account. so numerically, they are both correct.

but the problem here, is not the value of the Hfusion, but the sign. i don't see why the answer should be negative, when products minus reactants give a positive value

pretty much


where is this problem from? i cant think of any thermo questions where ive done "reactants - products"

any SDN mcat super-ballers care to help out?

 

[thechairman]

this was from problem #33 in Kaplan FL 5

 

[RoadRunner17]

think bonds broken - bonds formed

if the energy of the bonds formed were greater than that of broken, then you have a negative enthalpy which means its exothermic. this makes sense because when you form bonds, it will release energy. bonds require energy to break, and thus its endothermic. so in order for these relations to hold true, it should be bonds broken - bonds formed... which it is.

when you do this, you will get -9kJ/mol... but remember you calculated this for 2 moles in the correct stoichiometric ratio, so you have to divide by two to get the correct quantity.

 

[DrMT]

ahhhh so true.

i am ******ed

 

[chad5871]

Don't forget that on the MCAT, "bond energies" actually refer to "bond dissociation energies," or the energy released when the bond is broken. This was stated in ExamKrackers.

The way you calculated it (298 - 436/2 - 151/2 = +4.5) would mean that the HI bond was broken and the H2 and I2 bonds were formed.

H2 - bond dissociation E (H2 -> 2H) = 436
I2 - bond dissociation E (I2 -> 2I) = 151
HI - bond dissociation E (HI -> H + I)= 298; bond formation E (H + I -> HI) = -298

So (436/2) + (151/2) + (-298) = -4.5