Estimate the bond energy' S-F' in SF6.the standard
What is the S-F bond energy in SF6 given DHfo for each of the reaction components?
For this problem you must first calculate the change in enthalpy (heat transfer, delta H under standard conditions) for the conversion, S(g) + 6F(g) => SF6(g) OR for the reverse reaction, it really doesn't matter, because the numerical value will be the same, regardless. Once you calculate the heat transferred, you will be able to say that the amount of heat transferred was the amount of potential energy trapped in all of the S-F bonds, all six of them in the molecule. So, to obtain the answer asked for (the energy content on one S-F bond), all you will have to do is divide by 6.
The above is a logical approach, because all six of the S-F bonds are identical. This is so, because, according to VSEPR theory (look it up for more background on that, if you are interested), you can predict that its molecular geometry is "octahedral" with the central sulfur atom having "sp3d2 hybridization."
Here is how you set up the problem:
First write the balanced chemical equation with the given heats of formation (in kJ) written under each of the reaction components:
S(g) + 6F(g) => SF6(g)
275 6(80.) -1100.
Note: I am assuming that each of the above quanties is good (i.e., known) to at least the unit's place; that is, + or - 1 kJ. This reasonable assumption allows me to unambiguously indicate the number of sig figs in each quantity - an important consideration for proper rounding off of the final answer.
Recall that the sum of the product values minus the sum of the reactant values, each component multiplied by its corresponding coefficient will give the net enthalpy ("reaction enthalpy") of the reaction as written. In this case, there being only one product, the reaction enthalpy is:
-1100 - (480 + 275) = - 1855 kJ. From this, we can see that as S and six Fs are combined, 1,855 kJ of heat are released into the surroundings (that is, an exothermic reaction). The amount of heat released informs you of the combined bond energy of ALL six S-F bonds.
A good rule to remember: As bonds are formed, energy is always released (an exothermic process). As bonds are broken (as in the reverse reaction), the same amount of energy is being absorbed (an endothermic process).
Therefore, in conclusion, one S-F bond has a bond energy of 309.17, which is more properly rounded off to 309 kJ.
Recall that the rule for rounding off when adding or subtracting is to make sure that the final answer has the same precision as the values used to calculate it. Since each given value was good only only to the last unit
on Jan 05, 2011