a question related to bond energy !!!

[peacefulheart]

---

Members do not see this ad.

Which of the following reactions is likely to give off the most heat per gram of reactants? The reaction goes in the right direction

A. 2H------H2

B. 2O------O2

C. 3O-------O3

D.Xe+3F2------XeF2

The answer is A.

Thanks a lot.

 

[gettheleadout]

Moving to MCAT Q&A.

 

[image187]

hmm... I'm guessing a covalent bond gives off a certain amount of energy during these reactions. Since the question asks about "grams" it probably has everything to go with molecular weight here. I'd think there are just so many more hydrogen atoms in a gram of reactant than for the other answer choices

 

[LoLCareerGoals]

i think answer is A even if we are asked about per mole heat.

I eliminate D because Xe is a ng.
I eliminate C because:
(i) ozone formation generally requires heat input
(ii) 3 molecules collapsed into 1 a large enropic decrease
Now down to A&B.
(i) O=O of O2 is a double bond, so its formation is more expensive then H-H single bond
(ii) Generally the larger/more complex the molecule the greater the enropy increase for a generalized reaction:
M2 -> M + M (M is just molecule, not metal)
I am not sure if it applies to single atoms, but I am guessing it might (O atom is larger could be polarized in more ways so it is more "chaotic" *waves hands*), so O+O -> O2 is a larger entropic decrease then H+H-> H2.

 

[Laureatebarrel]

C and D is eliminated like you said, but I think A is right because it has smaller molecular weight thus more atoms and more energy released.

 

[peacefulheart]

i think answer is A even if we are asked about per mole heat.

I eliminate D because Xe is a ng.
I eliminate C because:
(i) ozone formation generally requires heat input
(ii) 3 molecules collapsed into 1 a large enropic decrease
Now down to A&B.
(i) O=O of O2 is a double bond, so its formation is more expensive then H-H single bond
(ii) Generally the larger/more complex the molecule the greater the enropy increase for a generalized reaction:
M2 -> M + M (M is just molecule, not metal)
I am not sure if it applies to single atoms, but I am guessing it might (O atom is larger could be polarized in more ways so it is more "chaotic" *waves hands*), so O+O -> O2 is a larger entropic decrease then H+H-> H2.

For your explanation of C, it should give off heat since it is form a bond.

thanks

 

[LoLCareerGoals]

What??? Bonds store energy. To form one it takes input of energy.

 

[sazerac]

What??? Bonds store energy. To form one it takes input of energy.

no

If atoms could release energy by breaking their bonds, then the atoms would drift apart and release their energy. The only reason a bond exists at all is because it released energy to form the bond, and therefore it is going to take an input of energy from somewhere else to break it.

 

[gettheleadout]

What??? Bonds store energy. To form one it takes input of energy.

Wrong. Breaking bonds requires input of energy, bond formation releases energy.


Sent from my iPhone using SDN Mobile