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
Why is the conversion of P2 to P4
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.