h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why does it form this shape?

It is quite definitely something to do with collision theory (blindfolded people in a room)


Increase in heat, then reduction in number of unreacted particles?

Cannot be an increase in unreacted particles, so must be an increase in energy.

This energy must come from somewhere, and there is no external influence that could cause this. The reaction is slightly exothermic.

Therefore: The extra energy must come from the reaction itself. (in theory, with a large enough amount of magnesium and hydroclauric acid I could increase the rate of reaction to such a degree that I could cause enough heat to melt the earth)

HOWEVER: this does not explain the later slowing of the reaction.

The only two things that have an effect on the rate of reaction as far as collision theory is concerned are amount of energy and concentration of reactants. The higher of either of these is, the faster the rate of reaction. Consequently, the lower either of these is the slower the rate of reaction will be.

This means that to cause the later slowing of the reaction, one of these must reduce.

The amount of energy does not reduce, as more is produced by the reaction itself.

Therefore: The slowing must be due to the fact that at a certain point, enough of the reactants have been used up to cause the reaction to lose its 'momentum', and slow down.


SUMMARY:

The reaction starts relatively slowly due to the low amount of energy, but as more energy is released by the exothermic reaction the rate increases (then more energy released, faster, etc.) until a large enough amount of the reactants is used up to slow the reaction down (then more used, slower, etc.) until all reactants are used up, causing the reaction to stop. This is that traditional 's' shape you get with most exothermic reactions.