To appreciate the concept of it is helpful to begin with a , i.e., a . Ignoring the up and down motion, instead consider just the balance between the two sides. That balance, when achieved with neither person's feet on the ground, is an example of an equilibrium. Note that there are two aspects to this equilibrium, the weight of each individual involved and the distance between each of them and the balance point (there is also a third component, that of the expenditure of energy, which we'll get to a bit later).

On a teeter totter, if two individuals do not have the same (i.e., ), then to balance the heavier one needs to sit closer to the balance point. This has to do with physics and , but the important point for us is that the two sides don't have to be identical in all the same ways to be at equilibrium. Instead they can vary in more than one way with those multiple ways adding up to generate an overall balance between the two sides.

In chemical equilibrium the equivalent concepts are concentration (as the weight/mass equivalent) and rate constant, which is analogous to distance from the balance point. Thus at chemical equilibrium, rather than weight × torque arm length balancing, it instead is concentration × rate constant that balance. Another way of saying this is that if things take a long time to get going from place A to place B, then they will tend to build up in place A, and particularly so if those same things find it easy to get going from place B back to place A.

These ideas are also equivalent to the concept of in sports, where performance is evened out by some manner of penalizing the generally better performing individual. Make them carry weights while running, ride a bicycle with slower tires, or have to defend a larger goal, whatever. The important point is that you don't have to change both sides equivalently to create a balance between the two sides, that is, to establish an .

And so, what about energy? An has the property of not requiring inputs of energy for its maintenance. Instead, an equilibrium can be viewed as an energy low, which also represents a stable configuration. Systems tend to move towards equilibrium because the inherent energy found within the system pushes them there, and in the course of this pushing that energy is lost to the larger environment (i.e., as they say in physics, to the ). The less energy a system possesses, then the less it is able to do something, particularly all on its own. The less able something is to change, relying on whatever energy is available to it, then the more stable that something is. Particularly without inputs of additional energy, systems that are found at equilibrium consequently are stable systems.

Alternatively, it is also possible to have balances that do require an input of energy to maintain, and we call those balances steady states. If we return to the teeter-totter analogy, consider two sides that balance not because they have the same weight or have adjusted their positions relative to the balance point but instead because the heavier or more distant individual (or both!) balances their side by pushing against the ground with their . In this case what matters are masses, torque arm lengths, and the amount of being applied to the ground. But since the latter requires an ongoing input of energy, the result is a steady state rather than an equilibrium. For more on these latter ideas, see my essay, Equilibrium and Energy.

Here we consider in particular the concept of chemical equilibrium.

First we have to consider how to balance . For instance, the following chemical equations is not balanced

+ → + (this is not yet balanced)

To properly balance it, we need to discuss the concept of stoichiometry.

The above video provides a quick introduction to the concept of stoichiometry – hilarious, I think…

Then we turn to reactions rates, which are studies within the context of what are known as , a.k.a., .

The above video provides an introduction to the concept of reactions rates.

Next we turn to the issue/concept of reversible reactions such as

2NaCl + CaCO₃ → Na₂CO₃ + CaCl₂ ("Forward")

Na₂CO₃ + CaCl₂ → 2NaCl + CaCO₃ ("Reverse")

Note, by the way, that the above reactions are now properly balanced. We also will consider what are known as .

Click on this link.

The above video considers reversible reaction equilibria, if we were going to consider the math!

In the above video the is modified using .

In this video another view of the same reaction is presented.

The associated with a hypothetical reversible reaction can be described using two rate constants:

A + B → AB (at rate k₁)
AB → A + B (at rate k₂)

The overall equilibrium constant equals k₁/k₂ (or k₂/k₁ if viewed with the reaction drawn going from right to left rather than left to right).

Related to the idea of reversible reactions, and very important to the functioning of , is the phenomenon of . Particularly, we can distinguish between reversible binding with high versus low between substances. Unfortunately, I have not been able to find any short videos on this subject.