# Why and when are bonds broken

I am new to chemistry (I have only done a GCSE in it) so forgive me if I am missing something obvious. But I am trying to very roughly simulate some chemical reactions. My background is in Artificial Life and Adaptive Systems rather than computational chemistry or chemistry in general. My stumbling block is that I am unsure exactly why bonds break and when they break.

Here are the things I am confused about:

Is the breaking of bonds by using energy a prerequisite for new bonds to be made with the involved atoms? If so, is this because the more energy there is, the more the atoms will move/vibrate, eventually pulling the bond apart? Otherwise, can bonds be broken by a new bond sort of pulling it apart in some way?

Is the strength/length of the bond what determines the amount of energy needed to break it? I'm also unsure of what the strength or length of a bond is based on. Am I correct in thinking that bond order is about the number of shared electrons in a covalent bond? But what about ionic bonds? Is it the amount of electrons "taken" by the more electronegative atom?

I know that there are multiple questions here. But they all answer the bigger question. Why and when do bonds break exactly? I have had a hard time trying to figure this out by searching on the internet alone. Often I just get pages explaining the energy requirements of a reaction. Thanks in advance, any help would be appreciated.

Is the breaking of bonds by using energy a prerequisite for new bonds to be made with the involved atoms?

A: No, it is possible for a bond to form between two species without first breaking a bond. But it is more common for reactions to occur with a tradeoff between breaking old bonds and forming new ones.

If so, is this because the more energy there is, the more the atoms will move/vibrate, eventually pulling the bond apart?

A: Adding energy in the form of heat can lead to breaking of bonds, which in turn can lead to formation of new bonds.

Otherwise, can bonds be broken by a new bond sort of pulling it apart in some way?

A: You can have a "displacement" reaction where one atom replaces another in a structure: the bond to the new atom forms at the same time as the other bond is broken. That process has a lower energy barrier between reactants and products, relative to breaking an existing bond first and then forming a new bond later.

Is the strength/length of the bond what determines the amount of energy needed to break it? I'm also unsure of what the strength or length of a bond is based on.

A: The strength of a bond is the same as the amount of energy needed to break it (in the absence of other reactants). Bond length tends to correlate inversely with bond strength, as for example in going from a single to a double to a triple bond between carbon atoms. Bond strength depends on a number of factors--bond strength tends to be greater between two atoms of about the same size. Bonds between two electronegative atoms like oxygen or chlorine are weaker than a bond between two carbon atoms.

Am I correct in thinking that bond order is about the number of shared electrons in a covalent bond? But what about ionic bonds? Is it the amount of electrons "taken" by the more electronegative atom?

A: You are correct here in your understanding of bond order. But the term "bond order" does not apply to so-called ionic bonds -- for example in sodium chloride -- where an electron is completely donated from sodium to chlorine and does not participate in keeping the atoms together. Rather, it is the electrostatic attraction between the sodium and chloride ions that keep them together.

• Actually, I find it somewhat hard to come up with an example where a bond would form without first breaking a bond. See, here is your newly formed molecule, now let's point at any atom in it and ask: where did this atom come from? It must have originated from another molecule, or maybe bulk metal, or ionic crystal - in short, something held together by bonds, which had to be broken. Atoms do not float around all by themselves. Well, noble gases do, and some of them also form compounds ($\ce{XeF_2}$, etc.) But then there was that bond in $\ce{F_2}$, which we broke to make this... – Ivan Neretin Sep 5 '15 at 8:54
• So let's ask the question, what two chemicals, each of which can be stored in a bottle in pure form, will combine together via a process of only bond formation and not breaking. Like boron trifluoride and trimethylamine. – iad22agp Sep 5 '15 at 12:31
• Boron trifluoride has bond order 4/3, for each fluorine is linked to boron with 1 regular sigma bond and 1/3 dative bond. In the resulting complex, the B-F bond order is 1, as illustrated by pyramidal geometry and increased bond length. Hence that 1/3 is broken. Is we disregard bond orders altogether, then the examples are plenty. Any Diels-Alder addition would do. Also, trimerization of acetylene to benzene, why not. – Ivan Neretin Sep 5 '15 at 15:14
• So, Ivan, what you are holding out for is a reaction between two atomic species that results in bond formation. For a reaction between two molecules, the formation of a new bond will cause a change in the existing bonds, no matter how slight. – iad22agp Sep 5 '15 at 20:41
• Exactly, and that's the problem with molecules. As for atomic species, there are not too much of them around. Well, we might imagine how a lone hydrogen atom, lost in the immense vastness of outer space, meets another such atom. Not that it does not happen in nature, but the example looks kind of far-fetched. – Ivan Neretin Sep 5 '15 at 21:09