Bond Breaking and Forming
As we saw in the last section, for a reaction to occur several conditions have to be met. So let's assume that we have a reaction between two molecules; the molecules have collided, their energy is sufficient, and they are in the correct orientation. (Here we'll stick to covalent molecules to make this easier to understand but the details here apply to all types of bonds). So what happens next?
We have two molecules. Within each of these molecules there are bonds which exist between the atoms. Remember that a bond is just a sharing of electrons between neighbouring atoms. In order to form new molecules, these bonds need to break and then new ones need to form between different atoms. What happens at the microscopic level is that the electrons need to be swapped to new atoms.
Breaking of bonds does not happen spontaneously
What this means is that there has to be some reason for the electrons to swap positions and for the old bonds to break. A good example is if you have two magnets joined together. Think about when you want to pull them apart. If you want them to come apart it will not happen automatically, you have to move them apart with your hands. You have to put in energy. The same thing with bonds. To encourage the electrons to move, and the bond to break, we need to put energy into the molecule.
Whenever a bond is broken, energy is used.Forming new bonds
Once the old bonds have been broken, the electrons are temporarily free from the bond but the atoms no longer have full outer shells. This means the atoms are not stable and they want to get to a more stable state again. Going back to our analogy with the magnets, imagine if we pull the magnets apart but there are stronger magnets near by. What happens when you take your hands away? The magnets will now move (on their own) to link with the stronger magnets. They may even come together with a snap (noise is a release of energy). So now instead of putting in energy, energy is released. This is the same situation when new bonds are formed between atoms.
When a bond is formed, energy is given out.Endothermic vs Exothermic
Bringing this all together, in a reaction:
- Energy is needed to break the existing bonds.
- Energy is released when new bonds are formed.
It is very unlikely that the amount of energy required to break the bonds is going to be exactly the same as the energy released from forming the new bonds. So what we get is a net difference between the energy in vs energy out
If the energy going in is greater than the energy coming out, then we call this an endothermic reaction.
If the energy going in is less than the energy coming out, then we have an exothermic reaction.