Excel at equilibrium

How to make a simulation

The aim is to help your students set up a spreadsheet that allows them to change the different starting conditions of a reaction, for example the concentrations or reactants, the rate constants of forward and backward reactions, and even whether a system is closed or open. These parameters are levers that learners can tweak with the consequences immediately evident to them.

Learners produce a personal, virtual playground where they can test different scenarios quickly and easily

To do this, use the simplest possible equilibrium:

$$A \underset{k_b}{\stackrel{k_f}{\rightleftharpoons}} B$$
$$\text{Rate}_f = k_f[A]$$
$$\text{Rate}_b = k_b[B]$$

 At equilibrium:

$$\text{Rate}_f = \text{Rate}_b = k_f[A] = k_b[B]$$

$$K = \frac{k_f}{k_b} = \frac{[B]}{[A]}$$

My students all have Microsoft surfaces, but this would work equally well in a computer lab or IT suite. I have created a video taking the students step by step through the process of programming the simulation. Students can pause the video and rewatch sections to take the activity at their own pace while I circulate the room offering support where needed. Learners produce a personal, virtual playground where they can test different scenarios quickly and easily. They get a real sense of pride as they create their own simulation.

How to use the spreadsheet

There is so much learning that your learners can pull from the simulation, but some of the most valuable is the resolute unchangingness of the equilibrium constant (K_eq) while they tweak the original concentrations of A and B. However, when you change either k_f or k_b it immediately affects the value that K_eq settles on.

Another lovely learning point is to add in a leaky pot and allow some particles of B to escape each second. Seeing the graphs unfold in front of your very eyes is powerful: the two rate lines begin to converge but then tantalisingly never meet, as the reverse rate drops away from the forward rate and both continue downwards to hit zero. The students do this themselves when distilling off the alkene when dehydrating an alcohol.

$$\text{Rate}_b = k_b[\text{Alkene}][\text{Water}]$$

If they remove the alkene each second, the rate of the backwards reaction has its proverbial shoelaces tied together and can never rise enough to meet that of the forwards rate.

Wins for everyone

This activity allows both you and your students to bring together the ideas little k and big K of kinetics and equilibria and develop an overarching understanding of how all chemical reactions progress. One of my students said he had not realised before why the equilibrium constant was only affected by temperature and nothing else. Using the simulation helped him to see how the reaction evolved over time to find an equilibrium and always settled on concentration values that combine to give the ratio of the two rate constants, unless the rate constants themselves are changed (crucially by different degrees) by altering the temperature.