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What is the Equilibrium Constant?

Equilibriam Constant

Screen grab showing equilibrium using interactive animation (with audo explanation) from chapter 4 (click to enlarge)

Equilibrium is certainly one of the most interesting topics for a chemist! However, it seems that to study it is neither easy (nor enjoyable) for our students. This is probably due to the inherent difficulty in handling a system not using a state function corresponding to an observable; using rather, this strange equilibrium constant, a “practical” interpretation of which may seem unclear.

In the Physical Chemistry textbook, Chemical kinetics follow (Chapters 9 and 10) the equilibrium (Chapter 4), however I believe that they can give some useful insight to understanding equilibrium itself – or, to more specific,  dynamic equilibrium (see Page 4-28, to find an easy and clear correlation between kinetics and equilibrium).

Given a reversible reaction, as the direct route (Reactants → Products) proceeds [R] decreases and [P] increases. Consequently, velocity (R→P) decreases and v (P→R) increases. At a certain state, the two velocities will become equal: that is, the same quantity of reactants which are consumed by the direct reaction is gained, at the same time, by the inverse reaction.

From a certain point of view, we can imagine this process like two cities full of cars, a certain number of which are moving from one to the other. If the number of cars leaving City 1 is equal to the number of cars leaving City 2, at any moment we will count so many cars in each city. The amount of cars in City 1 is not necessarily equal to the amount of cars in City 2; simply, their number in each city is the same, at any time.

The same can be said about reactions (the animation at Page 4-28, see screen grab above, is inspiring in this context)! Do not believe equilibrium to mean [R] = [P], but rest assured that, given enough time, the concentration (or the pressure, for gases) of the species in the reactor will find a fixed value, to be kept indefinitely.

So, what can the main problem for the student trying to understand chemical equilibrium be? Probably, the difference between chemical and mechanical equilibria: the latter is defined by a situation where “all is fixed”, while the former is a situation where “all MACROscopic variables are fixed”, but nothing is said about MICROscopic ones, which in turn can (and, probably, have to) be in constant motion.

Does it sound so difficult? I hope not, otherwise… well, you need to find equilibrium between your hate for this topic and your need to understand it! 🙂

Written by: Gaetano Giammarino

Gaetano Giammarino has authored 11 more articles.

He has received a PhD in Computational Chemistry at Università degli Studi della Basilicata, where he teaches General Chemistry in the Faculty of Engineering.

One Response to What is the Equilibrium Constant?

  • Bryan says:

    Thanks again Gaetano. Indeed equilibrium is a concept we teach and try to define as you say.

    When I teach chemical kinetics, I tell my students that we live in a dynamic world and equilibrium is a condition of no noticeable change. However I think it important to tell them that most reactions are constantly fed (We put food in our mouths and gas in our cars). We do not want equilibrium in manycases: our hearts and cars will stop!

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