[Theseven]

Hello, I have just received my quiz on thermodynamics. I am very confused by one problem on bond enthalpy. Prof gave us the following data:
C(s)+2F2--> CF4(g)   H=-680kj/mole
F2(g)-->2F(g) H=158 kj/mole
C(s)-->C(g) H=715kj/mole
And we had to calculate the average bond enthalpy. From what I understand we have to reverse the first reaction and double the second then we cancel out 2F2 when adding first 2 reactions together. I guess we are not taking into consideration last reaction since it is elementary compound.

[mjc123]

The procedure you describe will not give you the bond enthalpy. Why? Why is the third equation relevant?

[Enthalpy]

Solid graphite is a molecule with many strong bonds. The heat of vaporization, which is in this equation an atomization (gaseous C₂ would exist too), tells you that a big action happens.

The bonds in graphite are so strong that they make it the best material to resist sublimation at heat, second only to tungsten and rhenium, but better than ceramics. Invaluable for technology.

Remember as well that the standard enthalpy of formation refers to the elements in their normal state - usually molecules. Mega-important to understand. In fact, writing just C or Al for a solid is misleading; something like C_N or Al_N would be safer.

Then you should be able to answer the quiz.

As you are there, you could compare the bond enthalpies with the standard enthalpy of formation of the compounds and meditate it a bit: most chemical reactions are only rearrangements of already formed molecules, individual atoms are absent from Earth except rare gas, and for accuracy, we try not to compute a heat of reaction from the bond energy but rather from enthalpies of formation.

[Theseven]

Thank you for your responses!