The equilibrium law is a tool used by chemists to help them work out how best to manipulate chemical reactions to achieve the greatest yield of the desired product.

For a reaction mixture at equilibrium at a certain temperature, the ratio of the concentration of products to reactants will remain constant. We call this ratio the equilibrium constant, Kc, and can quantify this using the following protocol:

reactants ⇋ products             Kc = [products] / [reactants]

Something that students of chemistry often struggle with is the relevance of this value. It is, therefore, useful to split the teaching and learning of Kc into two distinct but connected ideas:

  1. The determination of Kc for a particular reaction at known conditions (known temperature and pressure)
  2. The utilisation of the Kc value to select optimum conditions for a given reaction that gives the greatest yield of desired products.
Figure 1: What are two key strands in teaching the equilibrium law and its applications

The determination of Kc for a particular reaction at known conditions (known temperature and pressure)

This process can be explained in the following steps:

  1. Firstly, we need to allow a reaction the time to reach equilibrium. This involves taking sensible amounts (achieved through trial and error) of reactants or products and allowing them to react in a closed system. We can tell if a reaction has reached equilibrium if there are the macroscopic properties, such as concentration and colour, remain the same.
  2. The next step is to determine the concentration of the reactants and products in the equilibrium mixture. How this is done is beyond the scope and time of the IB course and this post. Just be comforted in the fact that it can be done (often by indirect means).
  3. These values can then be plugged into the equilibrium expression for this reaction to give a value for Kc for that reaction under the temperature and pressure conditions that it was carried out in.
  4. This value for Kc, along with the temperature and pressure, can then be recorded in a database for use by scientists in the future.

Point 3 above is where a lot of time is spent in equilibrium. This is fine, but it is just crunching numbers. The most important conceptual understandings when looking at equilibrium comes before and after this. So what does come next? Well, it’s this. How can we use Kc to determine the optimal conditions for a reaction? Look out for the next blog post to see my thoughts on this.

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