Chemical kinetics: notes for teachers
This page is a list of notes and websites that may be useful for teachers planning lessons on chemical kinetics, the branch of physical chemistry that studies the rates of chemical reactions.
Animations and presentations
YouTube clips
Interactive simulations
Animations
Crosswords
The best of the web
RSC webpages
Wikipedia articles
Explosions
Often exciting to watch, explosions are an interesting 'application' of kinetics.
Types of explosion
Explosions fall into two categories based on their speed of propagation:
- Detonation – faster-than-sound explosion (caused by high explosives)
- Deflagration – slower-than-sound explosion (caused by low explosives)
Dust explosions
Substances that aren't normally considered highly flammable can explode when their surface area per unit mass is much higher than normal.
My thoughts on teaching kinetics
Relationships between kinetics and thermodynamics
I agree with Dave Lock that it is best to think of the thermodynamics (energy changes) and the kinetics (rates) of chemical reactions separately, although there are a few overlapping areas.
Here are a few important definitions:
- According to the second law of thermodynamics, a thermodynamically favourable reaction is one that increases the energy of the universe, ie. ΔSuniv > 0
- A thermodynamically unfavourable reaction cannot happen, unless it is somehow coupled to another reaction that is thermodynamically favourable and the two reactions together result in ΔSuniv > 0
- A thermodynamically stable chemical system is one in its lowest (Gibbs) energy state. It is necessary to include Gibbs energy in the definition in order to fully take the effects of entropy into account.
- A kinetically favourable reaction is one whose activation energy is low enough that it can proceed.
- A kinetically unfavourable reaction can only proceed if at least some of the reactant molecules have more energy than the activation energy. The more of them that do, the faster the reaction proceeds.
It's a good idea to consider energy and rates separately because they do operate independently most of the time:
- Kinetics considers how fast reactions happen.
- Thermodynamics considers how much energy redistribution takes place and which energy goes where.
- There are many hypothetical reactions that would be thermodynamically favourable but do not happen because they are kinetically inaccessible (the activation energy, Ea, is too high).
- The classic example is the metastability of diamond with respect to graphite - at room temperature and pressure, graphite is lower in energy than diamond, so the reaction of diamond to graphite is thermodynamically favourable. However, the activation energy for the conversion is very high (because the structure of diamond and the structure of graphite are very different), so the reaction does not proceed.
- By definition, kinetically favourable but thermodynamically unfavourable reactions do not exist (except ones coupled to other reactions so that the two are thermodynamically favourable overall, as mentioned above). This is because the thermodynamic unfavourability (uphill energy struggle) is itself a kinetic barrier - an activation energy.
To use a gravitational analogy for Gibbs energy, there's no way of getting from the bottom of a hill to the top (overall change in height is positive) without climbing as high as the hill. The only difference is this:
- in a thermodyamically unfavourable reaction with an activation energy barrier, the energy change of reaction is less than the activation energy
- a thermodynamically unfavourable reaction without a separate activation energy barrier, the energy change of reaction is equal to the "activation energy"
- There are also thermodynamically favourable reactions with no activation energy barrier, i.e. downhill all the way. These are called activationless reactions. The only thing stopping them from happening is the need for the reacting species to collide.
Synthetic chemists can often control reaction conditions so that either kinetic or thermodynamic effects dominate the key steps in a reaction. This strategy is called thermodynamic versus kinetic reaction control.