Using appropriate data from Appendices 10 and 11 determine the average P-F bond enthalpies in (a

Using appropriate data from Appendices 10 and 11 determine...

Key Concepts

Bond Enthalpy

Bond enthalpy is the measure of the energy required to break a chemical bond in a molecule, generally expressed in kilojoules per mole (kJ/mol). It reflects the intrinsic strength of a bond and is fundamental in understanding the energetic stability and reactivity of molecules.

Average Bond Enthalpy Calculation

Average bond enthalpy refers to the energy required to break a specific type of bond in a compound, averaged over all such bonds in that molecule. This concept is particularly useful when a molecule has several bonds of the same type that might experience slight variations in their individual bond energies due to differences in molecular environment.

Molecular Structure and Geometry

The molecular geometry of a compound influences its bond enthalpies because the spatial arrangement of atoms affects the distribution of electron density and, consequently, the strengths of individual bonds. Variations in bond angles and positions (such as axial versus equatorial in polyatomic molecules) lead to different bond strengths even among similar bonds in one molecule.

Thermochemical Analysis Using Hess’s Law

Hess’s law provides a framework for calculating bond enthalpies via thermochemical cycles, which involve the use of heats of formation or reaction data. By applying Hess’s law, one can deduce the energy required to break bonds, even when direct experimental measurement is not possible, by combining various enthalpy changes from related reactions.

Transcript

00:01 Okay, so this question is really just asking if you know how to pull the right information from the appendices or from a sheet of data to answer the question.

00:14 Instead of just throwing the numbers at you and seeing if you can apply them to the right equation, it's sort of taking another step back and saying like, you know the equation.

00:24 Let's make sure you know which data actually goes in it if you look at a bunch of other data.

00:30 So it's asking to find the phosphorus fluorine bond enthalpy in two molecules, phosphorus trifluoride and phosphorus pentafluoride.

00:51 So you know that to find the bond association enthalpy, as we've done in previous questions, you set it up like this.

01:02 I'll do p, f, i'll do x here.

01:11 That's your target, and you know that it's gaseous.

01:16 And then over here, you have p gaseous plus, oops, plus x, f monomotomic, gaseous.

01:35 And then down here you have the starting ground states for phosphorus, and fluorine.

01:48 Let's move that over a little bit.

01:55 And so you look in the graph, you look in the appendices and you need to know what you're pulling out.

02:05 So first let's let's finish setting this up.

02:07 It's this is going to be the, this is going to be one enthalpy.

02:14 This will be another.

02:18 And go ahead and see if you can guess which enthalpy is which and what these ground states might be, you can pause the video if you need.

02:25 Give you a couple seconds.

02:28 Okay, so hopefully you got this right.

02:32 The enthalpy here, you have your ground states.

02:36 This is going to be the heat of formation and this is going to be the heat of atomization.

02:48 So now you know that you need both of those things.

02:53 So we'll make a little table in a second, but you know that you also need to find the ground state or the standard states of phosphorus and fluorine.

03:04 And you can look those up and see that that's p4 solid or white phosphorus and fluorine is a diatomic gas.

03:21 So it's p2 gashes.

03:25 And then you can just balance this in accordance with how many you need here.

03:31 So we'll go ahead and do that really quickly.

03:32 It's going to be x over 2 and here it will be 1 or 1 over 4 that's just in accordance with what we need here if these two things are different those numbers would be different so let's go ahead and set up our table we have we're really going to need two small tables so we have p f3 and p5 and we need their heat information and then over here we have, we're going to be getting the heat of atomization for phosphorus and fluorine.

04:24 And so between appendices 10 and 11, go ahead and write that out, 10 just gives you the heat of atomization, and so you can see that that is for fluorine, 79, and for phosphorus, it's 315, and then you can look up in appendix 11, the heat of formation.

05:01 And so you see that for pf3, it's 958.

05:14 And for pf5, it's negative 1594.

05:24 Okay.

05:24 And so all of these are in kilojoules per mole, and that's very important to remember, because without that, this calculation can be done.

05:41 So we have our data down here, and we have this setup.

05:47 So we can tell the general equation, which i'll write out in the second.

05:52 But first we know that the heat of atomization is going to go here...

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