Back to Thermodynamics

Why is equilibrium such a central concept in chemical thermodynamics? Well, equilibrium describes what happens to reversible reactions, and reversible reactions are prevalent in God's world. We need useful tools to describe what we observe. Among other things, this helps us predict and even control outcomes. Molecules and particles are constantly moving at the temperatures of our experiments and life around us. Their collisions can lead to reaction. Once product is formed, it doesn't just sit there. Product substances collide, too, and can result in re-forming reactant. There they are -- reactants and products co-existing in a mixture, and still undergoing changes. This can keep going on... to a point. Once the rate at which reactant forms product matches the rate at which product changes back to reactant (rate slower than the initial activity of the reaction), the dynamic mixture achieves a balance point. Ongoing reactions, but no increased amounts of anything produced. The composition of the mixture is static. Good, then we can measure or calculate how much of any reactant or how much of any product is in the equilibrium mixture! K helps us. K is the ratio of products over reactants. Multiply ALL the product amounts and divide that by the multiple of ALL the reactant amounts. So, since products are in the numerator of this ratio, then a large K value (> 1) means there are more products in the mixture than reactants! It is useful to know the value of K for any equilibrium reaction. Ka is the value give to an acid in water equilibrium reaction. Different acids will have different Ka values. In class examples (and in lab), we will calculate K given the equilibrium amounts of reactants and products. We will also do the opposite -- calculate equilibrium amounts by using a known K value -- this type is more involved (and often needs the wonderful quadratic formula :-).