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le Châtelier's principle describes qualitatively how a reaction responds to changes. However, it tells us nothing about the amount of the change. Sometimes it may be good enough to know that a change will occur. Often though, we would need to know how great the change will be.
For example, we know that for the Sabatier process:
CO2 (g) + 4 H2 (g) 
CH4 (g) + 2 H2O
(g) + 165 kJ
increasing the pressure will cause a shift to the right, and so more products. But how much more? If we double the pressure do we get twice as much methane, or only one percent more? Doubling the pressure will cost a great deal in weight, since using higher pressures will require much thicker walls on our reactor. If the yield only goes up by a small amount, it will not be sensible to make the necessary equipment modifications to do so. Or how about temperature? Exactly what does changing the temperature do to the yield? Since le Châtelier's principle only tells us that there is a change, but not how big the change is, it is not good enough, especially since the lives of astronauts are at stake.
We need a quantitative way of describing equilibrium. This is the equilibrium constant law, usually referred to as the equilibrium constant expression, or just the equilibrium constant.
Your goal in this chapter should be to understand how to work
with and do mathematical calculations with K, the equilibrium constant expression.By the time you have completed this chapter you should be able to:
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