Now here is what this "arithmetic" looks like in practice. At room temperature K for the synthesis of ammonia is about 100,000,000. It would seem that under such conditions a mixture of nitrogen and hydrogen should change instantly into ammonia. But this does not happen. The forward reaction is too slow. Would raising the temperature help? 

    We heat the mixture to 500°C. . .

    But here the chemist would check us: "What the deuce are you doing? You'll not get anywhere that way!" Indeed, he stopped us just in time, this chemist with his calculations. Here is what they show: at a temperature of 500°C, K is only six thousand, 6 x  10³! The "green light" for the back reaction

2NH₃    3H₂  +  N₂. 

    The most favourable conditions for ammonia synthesis are as low a temperature and as high a pressure as possible. This is the domain of another law acting in the realm of chemical reactions. This law is known as Le Chatelier's principle, after the French scientist who discovered it.

    Imagine a spring built into a fixed support. If it is neither compressed nor stretched it may be said to be in equilibrium. 

    But if it is compressed or stretched the spring comes out of its state of equilibrium. Simultaneously its elastic forces, those that counteract compression or stretching of the spring, begin to increase. Finally, there comes a moment when both forces again balance each other. The spring is once more in a state of equilibrium, but not the same as it was in initially. Its new equilibrium is displaced towards compression or stretching.
 

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