Question Video: Single- and Triple-Bond Energies of Phosphorus

The diatomic molecule of phosphorus, P₂, contains a triple bond. The molecule is highly unstable and rapidly converts to molecules containing only single bonds, such as the pyramid-shaped molecule P₄. The equation for this reaction is given. This reaction is highly exothermic. a) How many single bonds are present in the P₄ molecule? b) Why is the conversion of P₂ to P₄ exothermic? c) Which bar chart illustrates the difference in bond energy between the single and triple bonds of phosphorus?

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Video Transcript

The diatomic molecule of phosphorus, P2, contains a triple bond. The molecule is highly unstable and rapidly converts to molecules containing only single bonds, such as the pyramid-shaped molecule P4. The equation for this reaction is given. This reaction is highly exothermic. How many single bonds are present in the P4 molecule?

We’re introduced to the diatomic molecule, that’s a molecule containing two atoms, of phosphorus, meaning both atoms are phosphorus atoms. In this molecule, there’s a triple bond, meaning six shared electrons between the two phosphorus atoms. We’ve been given an equation where we see two of these P2 molecules reacting to form one P4 molecule. And we’ve been told this reaction is highly exothermic, which means much more energy is released because of the reaction than is absorbed.

The first question is fairly simple. We just need to count the number of single bonds in a P4 molecule. Counting from the front, each phosphorus atom has three bonds to each other phosphorus atom. And being careful not to count the same bond twice, we can see overall there are six unique single bonds. For the next part, I’m going to summarize some of the information because it’s not all essential to the answer. So we’re dealing with the reaction of two P2 molecules reacting to form one P4 molecule. That’s a very exothermic reaction.

Why is the conversion of P2 to P4 exothermic?

A full understanding and explanation of why this reaction is exothermic is far beyond the scope of this video. But we can use some simple principles to understand where to look. When discussing energy and reactions, we need to remember that bond breaking requires energy and bond formation releases energy. An exothermic reaction releases more energy than it requires. So in this case, we definitely know that the total bond energy of our products, the P4 molecule, is greater than the total bond energy of our two P2 molecules. So our answer to why is the conversion of P2 to P4 exothermic is that two phosphorus-phosphorus triple bonds are weaker than six phosphorus-phosphorus single bonds.

And we can simplify a little and say that one phosphorus-phosphorus triple bond is weaker than three phosphorus-phosphorus single bonds. I’ll stall our information away for the next bit.

Which bar chart illustrates the difference in bond energy between the single and triple bonds of phosphorus?

Let’s have a quick recap of what we know. For covalent bonds between two atoms, a triple bond is stronger than the double, and it’s stronger than the single. Each graph has a bond energy for the single bond and the triple bond. The higher the bar, the stronger the bond. Bar chart (C) and (E) both have higher bars for the PP single bond, so they can’t be true. And when we look at bar chart (B), we can see the other piece of information we know from the previous parts isn’t true. The PP triple bond is much greater than three times the bond strength of the PP single bond. So this is not correct either.

This just leaves us (A) and (D), where we see a much more substantial difference between the bond energy of the PP triple bond and the PP single bond in bar chart (A). The difference in bar chart (D) is too small. It’s only (A) that shows roughly the relationship we’d expect between a single and triple bond strength.

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