Chemical Equilibrium Theory, An Introduction

Hello,

This post covers the basics of chemical equilibrium.

Definitions

We say that a reaction is in a state of dynamic chemical equilibrium when two conditions are met: (1) The reaction is reversible and the forward and reverse reactions happen simultaneously and (2) The rate of the forward reaction equals the rate of the reverse reaction.  A solution with a reaction in equilibrium will appear to be static (unchanging): This is only an illusion, caused by the fact that an equilibrium reaction has constant reactant and product concentrations.  However, the forward and reverse reactions of a chemical equilibrium system are happening in a very dramatic way (at the molecular level), so we describe the system as dynamic.

Different chemical equlibria have varying relative (compared to each other) amounts (or concentrations) of reactants and products.  We use the term, position, to describe the relative amounts of equilibrium reactants and products.  Weak acids and "insoluble" ionic compounds (They're actually very slightly soluble) are equilibrium systems where the concentrations of products are very small compared to the reactant concentrations.  For such chemical equilibria we say that "the position lies to the left".  Still, other equilibrium systems consist of a relatively large product concentration and then we say, "the position lies to the right".

The Equilibrium Constant

There is a mathematical way to generate a number which will indicate the position of a chemical equilibrium system.  The Equilibrium Constant value is determined from a calculation using reactant & product concentrations and the coefficients of the balanced equilibrium equation.  The equilibrium constant has the symbol, Keq.  A large Keq indicates a position which lies to the right while a small Keq (usually much smaller than 1) tells us that the equilibrium position lies to the left.  The following figure provides a summary of the fundamentals of equilibrium theory covered in this post.

As the figure indicates, the equilibrium constant calculation is exponential, and so is dramatically affected by larger coefficient values and reactant/product concentrations.  This is why Keq values are often very much smaller than 1 or very much larger than 1.  For chemical equilibrium reactants and products which are pure liquids or solids, we exclude them from the equilibrium constant calculation.  This has to do with the fact that if reactant and product concentrations were to change (either by a temperature change or intentional change in a reactant or product concentration), any change in the concentrations of pure liquids and solids are very small compared with concentration changes in gaseous and solution reactants and products.  The issue of changing reactant and product equilibrium concentrations will be explored in my next post.

That's all for now.

Have a good one!

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