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Le Chatlier's Principle refers to chemical reactions that are in equilibrium. It holds that if an equilibrium reaction is stressed, the reaction will adjust in such a way as to remove the stress. So given the following reaction we can look at how stressing in different ways effect it.
2NO <---> N2 + O2 + heat
Note that as written, the forward reaction is exothermic. If additional heat is added to the reaction to stress the system, the equilibrium will shift in such a way as to remove the extra heat. In this case, the extra heat will increase the rate of the reverse reaction and we will see a decrease in N2 and O2 and an increase in NO.
Note that the number of moles of gas on each side is equal. That is that there are two moles of NO on the left and a total of 2 moles of gas on the right. This is telling us that if we increase (or decrease) the pressure on this system there will be no change in either the forward or reverse reaction. Thus the amount of each substance will be unchanged.
If we increase the amount of NO, the equilibrium will shift to the right and consume the NO to create more N2 and O2 (and heat).
If more N2 or O2 are added (and assuming there is enough of the other present) the equilibrium will shift toward the left and consume them to produce more NO.
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