What is Le Chatelier's principle used to explain?

2 Answers
Oct 8, 2016

How systems react to changes that affect chemical equilibrium.


In reversible reactions, the reaction does not go to completion, but rather reaches a point of stability known as the point of chemical equilibrium. At this point, the concentration of products and reactants do not change, and both products and reactants are present.

E.g. #CO_(2(g))+H_2O_((l)) rightleftharpoons H_2CO_(3(aq))#

Le Chatelier's principle states that if this reaction, at equilibrium, is disturbed, it will readjust itself as to oppose the change .

For example:
Change in concentration

  • If the concentration of reactants increase, the equilibrium will shift to the right and favour the forward reaction, converting more reactants into products as to oppose the change
  • If the concentration of products increase, the equilibrium will shift to the left and favour the reverse reaction, converting more products into reactants as to oppose the change

Other changes include temperature and pressure.

Oct 8, 2016

#"Le Chatelier's principle"# explains the behaviour of chemical systems at equilibrium.


#"Le Chatelier's principle"# explains that when a system at equilibrium is subject to an external perturbation or stress, the equilibrium will move so as to offset the external perturbation. Note here that #"offset "!=" counteract"#.

There should be several examples of such changes to equilibria on these boards. The difficulty is in finding the one you need. I can give you one, well-studied example:

#2 NO_2 (g) ⇌ N_2O_4 (g)# # " "# #;DeltaH = -57.2* kJ* mol^-1#

Nitric oxide is a red-brown gas; whereas dinitrogen tetroxide is colourless. The colour change is important, because it means we have a macroscopic observable to study how the equilibrium evolves. We could take a sealed tube of the given reaction at equilibrium, and subject it to stress: i.e. heating or cooling by placing the bulb in a hot water bath or an ice bath.

Upon heating, the equilibrium moves to the left, i.e. towards the reactants, as evidenced by the deepening of colour. On cooling it moves to the right, i.e. the colour dissipates.

And if we write the exothermic reaction this way:

#2 NO_2 (g) ⇌ N_2O_4 (g)+Delta#,

we can rationalize the behaviour of the equilibrium on the basis of Le Chatelier's principle. That is addition of heat shifts the equilibrium to the reactant side; whereas cooling the reaction shifts the equilibrium to the right hand side. Le Chatelier's principle can also be illustrated with regards to changes in concentration.

With regard to the dimerization reaction, can you give a simple reason why the FORWARD reaction should be exothermic?