Chemistry 30

2.2 The Meaning of K_eq

What can the value of K_eq tell us about a reaction?

• If K_eq is very large, the concentration of the products is much greater than the concentration of the reactants. The reaction essentially "goes to completion"; all - or most of - of the reactants are used up to form the products.
  
  
• If K_eq is very small, the concentration of the reactants is much greater than the concentration of the products. The reaction does not occur to any great extent - most of the reactants remain unchanged, and there are few products produced.
  
  
• When K_eq is not very large or very small (close to a value of 1) then roughly equal amounts of reactants and products are present at equilibrium.

Here are some examples to consider:

the decomposition of ozone, O3	2 O3 (g) 3 O2 (g)	Keq = 2.0 × 1057
	Keq is very large, indicating that mostly O2 is present in an equilibrium system, with very little O3
production of nitrogen monoxide	N2 (g) + O2 (g) 2 NO(g)	Keq = 1.0 × 10-25
	Very little NO is produced by this reaction; N2 and O2 do not react readily to produce NO (lucky for us - otherwise we would have little oxygen to breath in our atmosphere!)
reaction of carbon monoxide and water	CO(g) + H2O(g) CO2 (g) + H2 (g)	Keq = 5.09 (at 700 K)
	The concentrations of the reactants are very close to the concentrations of the products at equilibrium

There are some practice questions to do next, followed by Assignment 2 . Complete both of these before moving on.