The change in the state of equilibrium of a strained spring is an analogy (though rather a crude one) of the action of Le Chatelier's principle. 

    Here is how it is formulated in chemistry. Let an external force act on a system in equilibrium. Then the equilibrium will shift in the direction indicated by the external influence. It will shift until the reactive forces balance those applied externally.

    Reverting to the production of ammonia, the equation of its synthesis shows that four volumes of gases (three volumes of hydrogen and one volume of nitrogen) give two volumes of gaseous ammonia (2NH₃). Increasing the external pressure tends to reduce the volume. 

    In this case the influence is favourable. The "spring is compressed." The reaction proceeds mainly from left to right: 

3H₂  +  N₂    2NH₃, 

    Ammonia synthesis involves a release of heat. If we heat the mixture, the reaction will proceed from right to left, because heating increases the volume of the gases, and the volume of the reactants (3H₂ and N₂) is larger than the volume of the resultant (2NH₃). Hence, the back reaction will predominate over the forward one. The "spring" will stretch.

    Both influences result in a new state of equilibrium, but in the first case it corresponds to an increase in the ammonia yield, whereas in the second case the yield will decrease sharply.
 

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