An example of an alternative conception:
It is common for learners to think that in a chemical equilibrium the reaction mixture contains equal amounts of reactants and products.
Often in introductory chemistry, learners are first introduced to reactions that are considered to go to completion, and only later meet chemical equilibria (so that it appears these are only an exceptional minority of chemical reactions *). Equilibria occur when the rates of forward and reverse reactions are the same and so cancel out. Learners however are more likely to focus on the amount of material present (somerthing that can often be directly perceived) rather than the rate of change of that amount (an abstraction).
* Arguably, a more authentic account would introduce all chemical reactions as potentially equilibria – where under particular condtions some effectively go to completion (or, effectivey do not occur at all).
Understanding how chemical equiloibrium can occur requires appreciating that
- both the 'forward' and 'reverse' reactions can potentially occur
- the tendency of these to occur (e.g., at some particular temperature) may be quite different, but
- the rate of reaction depends both on this tendency and the concentration of 'reactants' present
- (where 'reactants' for the 'reverse' reaction are the products for the 'forward' reaction)
- so: the rates of the forward and reverse reactions will come to balance at some point when the energetically more favourable reaction has occurred to such an extent that
- reactants are at a low concentration, but
- products are at a high concentration.
Clearly this requires considerable coordination of abstract ideas.
