More Ben's Chemistry

Equlibria in organic reactions

Having progressed thus far in A-level chemistry, I shall assume that you are aware that not all reactions follow through to completion, some will form a dynamic equilibrium. For example,

CH3COOCH2CH3 (l) + H2O (l)      CH3COOH (l)  +  CH3CH2OH (l)

As a result of this, we can write an equation for the equilbrium constant Kc of the reaction. This is calculated by multiplying the concentrations of the products, and then dividing by the concentrations of the reactants multiplied together.

In the above example,

Kc= [CH3COOH]  [CH3CH2OH] /  [CH3COOCH2CH3]  [H2O]

Remember that the square brackets represent the concentration, i.e. [CH3COOH] is the concentration of CH3COOH.

It is also worth noting that the units of Kc will depend upon the reaction, in the example above, the equilbrium constant is unitless as you are dividing two values with the same unit by two more values, also with the same unit. To emphasise the point, a few generic examples follow.

A   +    B                       C

Kc =[C] / [A] [B] 

[A], [B] and [C] each have units of mol dm^-3, hence the overall units for Kc are         dm³ mol^-1.

I    +     J               2 K

Kc =   [K]² / [I] [J]

Here again, Kc would be unitless. As seen on this reaction, the number of molecules of each product and reactant is the power to which the concentration of that species is raised. For I and J, only one molecule of each is present, so they are raised to the power one, which we can ignore. However, there are two molecules of K produced, so we square [K] to give find the equilbrium constant.