Collision theory - a few questions

[lboogy]

Hi all,

 

I've got a few questions about collision theory.......

 

1)re: activation energy, do some particles collide but because they have an insufficient amount of energy they do not react (would these be known as ineffective collisions as opposed to effective collisions)?

 

2)what exactly is colliding? Electrons or full atoms?

 

thanks very much!

[woelen]

Collisions usually occur in the liquid state or the gas state. Indeed, when collisions are not strong enough, then no reaction occurs. For some reactants, even a soft touching is sufficient to get a reaction, for others a hard collision is needed. The higher the temperature, the harder the collisions.

 

Colliding is when the electrons of different molecules (or atoms) interact with each other. Collisions are not like inelastic collisions, such as pool-balls, but the electromagnetic forces gradually become stronger when atoms/molecules are getting closer to each other. So, collisions are more like elastic collisions. Think of it as if we have strong magnets, with only a single pole (in reality such magnetic monopoles do not exist) and these are randomly shaken through each other. The magnets repel each other and only when they are strongly forced into each other they actually touch, otherwise the collision is elastic and soft.

[lboogy]

thanks for that. But one thing that I still don't understand is that at a lower temperature, where not as much energy is passed to the particles, and the reaction is slower, how do the particles eventually acquire enough energy as time goes on? I would have thought that if the heat is too gentle to provide sufficient energy for strong collisions then as time goes by the particles would not acquire the required energy to cause a successful collision resulting in a reaction, but it seems that the particles will eventually acquire the energy to collide and then react, but it will just take longer. Hope I'm clear enough!

[John Cuthber]

The temperature is related to the average speed; when it's cold there are a few fast movers which have enough energy to react and a lot of slow ones that don't. When you heat things you get a bigger proportion of the molecules moving fast enough to react so the rate of the reaction increases. If you want to know about the maths you need to find out about Boltzmann distributions .

[timo]

SLIGHTLY OFF-TOPIC:

Collisions [between atoms] are not like inelastic collisions, such as pool-balls, but the electromagnetic forces gradually become stronger when atoms/molecules are getting closer to each other. So, collisions are more like elastic collisions.

I am not sure if you are aware of it but just in case: The usage of the terms "elastic collision" and "inelastic collision" you used is that which you´re used from everyday-life. However, the scientific meaning of the terms is different, at least in physics. There, "elastic collision" refers to a collision in which the kinetic energy is conserved. An inelastic collision hence is one, where some of the kinetic energy is transformed into some other form of energy. A typical meter scale of an inelastic collision is a car crash, since cars are explicitely designed to absorb a lot of the energy by deforming. On the atomic scale, an inelastic collision would probably be activiting one of the colliding molecules/atoms to an exited state (vibrational modes, excited electron states) - or in a broader sense also by destroying the bound state. So if you ever happen to run across the term "deep inelastic scattering" - it has nothing to do with picking very hard targets (like very hard protons ), but with studying the target by activating some internal degrees of freedom and observing the results.

[woelen]

Atheist, good that you pointed out. I was not aware of this. In my context, elastic does not mean "loss of energy", but just that the particles feel an increasing force, when they approach each other.