Collision theory and mechanisms – linking it together

I’ve just started teaching the kinetics topic to my students and so far we have focussed on the collision theory. Have you heard of this?

The collision theory explains to us what happens during chemical reactions in terms of the collision of particles.

In order for a chemical reaction to occur, particles need to come together (collide) with sufficient energy (speed). This is called the activation energy (and is defined as the minimum amount of energy required for a chemical reaction). If the activation energy is high, the reaction is hard to start (it may need to be heated up to very high temperatures).

However, sufficient activation energy is not enough – particles also need to collide in the correct orientation. A bit like putting a key in a lock, it is no good having the key and lock, the key must be put in the lock in the correct orientation (this was a new idea to my students).

However, what was a big eye opener for my students was the ideas that most (all?) chemical reactions involve one or two reactants.

For example, if the following reaction is considered:

N₂ + 3H₂ –> 2NH₃

The equation implies that one nitrogen molecule and three hydrogen molecules simultaneously collide to produce two ammonia molecules.

However, the probability of four molecules coming together with sufficient activation energy and in the correct orientation is very very small (if not impossible).

What actually happens in chemical reactions is that they much smaller – often one reactant breaking down (step 4) or two reactant coming together (steps 1-3).

1, N₂ + H₂ –> HNNH

2, HNNH + H₂ –> H₂NNH₂

3, H₂NNH₂ + H₂ –> H₃NNH₃

4, H₃NNH₃ –> 2 NH₃

The reactants and products in steps 1 – 4 can cancel out if they appear as both reactants and products. If you do this you will end up with the overall equation (N₂ + 3H₂ –> 2NH₃). The species are also referred to as intermediates and only exist for a small instant in time. It is not possible to isolate or extract them. For example, if the product of reaction 1, (HNNH) does not find another H₂ to react with almost straight away it would decompose back into N₂ and H₂.

By Tweenk – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=10587531

What these series of steps are is the ‘mechanism’. You may have come across this in organic chemistry. The mechanism shows how a reaction proceeds and one of the ways in which it is determined is through kinetic (rates of reaction) data.

The order of reaction can also be determined from kinetic data and this can also work towards providing a mechanism for a reaction.