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Le Chatelier's Principle to predict the change in equilibrium position

Submitted by Anonymous (not verified) on Wed, 07/30/2008 - 19:52

Here is another problem I had with Le Chatelier's principle:

"How would these equilibria be affected by increasing the temperature?
a) 4NH3 + 5O2 4NO + 6H2O + heat
b) heat + NaCl Na+ + Cl-"

I don't understand what they mean when they say it shifts according to the direction heat is used up, or how it depends on whether or not the reaction is exo- or endothermic, or how to tell which it is.
If that makes sense.

First of all, exothermic reactions are reactions in which energy is produced.  In exothermic reactions the heat term is on the product side (or delta H will have a negative value).  Thus
                      4 NH3  + 5O2 --> 4 NO + 6 H20 + heat
is an exothermic reaction in the forward direction.

However, the double arrow indicates that this is an equilibrium which means that the reaction also goes in the reverse direction.  If the reaction is exothermic in the forward direction it must be endothermic in the reverse direction.

An endothermic reaction is one in which heat is absorbed.  Therefore the heat term will appear on the reactant side (or delta H will have a + value).  The second reaction:
                            heat + NaCl ---> Na+ + Cl-
is endothermic in the forward direction.  However, it to is at equilibrium which means that it also goes in the reverse direction.  Therefore, in the reverse direction it must be endothermic.

LeChatlier's principle says that if heat is added (by raising the temperature), the reaction will shift in the endothermic direction.  In the case of the first equilibrium,
                        4NH3 + 5O2 4NO + 6H2O + heat
that means that it will "shift" to the left.

The term "shift" means that the reaction rates will change so that they are no longer equal (that is they will no longer be at equilibrium).  In the case of raising the temperature, the endothermic reaction is speeded up to a greater degree than the exothermic reaction so that there is a net reaction toward the left side (NH3 a O2 are being produced faster than they are being used up).  In general if the forward reaction becomes faster than the reverse reaction the equilibrium is said to "shift" toward the products or to the right.  If the reverse reaction becomes faster than the forward reaction the equilibrium is said to "shift" to the left or towards the products.

After the shift takes place the rates of the forward and reverse reactions eventually become equal to one another again (although they will not be equal to the rates in the earlier equilibrium).  Once this new equilibrium is reached the concentrations of the reactants and products will become constant.