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Bond enthalpy P1 question

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Hello,

Can someone explain why the answer is A? I understand why the others are wrong, but can't seem to grasp why it's 1/4th CF4 and C, please explain.

,Screen Shot 2016-05-25 at 23.08.35.png

 

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Well in order to get an average bond enthalpy of C-F bondyou need the energy needed to break one bond and in CF4 you have four C-F bonds. So you divide it by four to get the energy needed for one mole of C-F bonds.

Since now you have 1/4 CF4, you need to balance out the equation, which means you get 1/4 C and F. 

It goes from the definition of the bond enthalpy which you should be familiar with :)

Hopefully that helped!

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The enthalpy of this reaction is the average C-F bond enthalpy. 
Also note that there are two conditions that must be satisfied
1) Bond enthalpy is calculated for gaseous states
2) No more bonds are present in products of bond enthalpy. 
Only (A) satisfies both.

When you learn about organic chemistry reactions, you will learn more about about breaking bonds evenly between the two nuclei (each gets half the electrons from the bond). I point this out because it may be weird to see F(g) but it does exist. 

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