when we change pressure, why do the concentrations of reactants and products sharply increase then follow le chatelier's principle to go back into equilibrium? like an explanation of how this works, not just why- thanks 
edit: why do concentrations change, more so, than just increase- thanks (this related to equilimbrium conc./time graphs)
When you change the pressure, you change the volume. The lower the volume (higher pressure), the more likely particles are to react. Consider putting many people in a large room then moving all of them to a smaller room, they'd bump into each other more often. So basically, the more particles you have, the more likely for that reaction to occur. Hence why increased pressure favours the side with fewer particles.
So considering the reaction: A + D --> 2C
If I have A + D in a container, and change the pressure (eg increase), then A + D have less room to move around and more chance of colliding with the right orientation. This would result in an increase in [C] and a decrease in [A] and [D]. Over time, as [C] increase, it's more likely to react, 2 C's would collide to make A + D, and then A and D would react again and then this would continue until they reached equilibrium. This part corresponds to the flattening out. Overall though, a new equilibrium is established. That's because, with lower pressure, you have more [C] and less of [A] and [D].
You can see this through the equilibrium constant as it stays...constant.
K= [C]^2/[A][D] --> If C increases, A and D must decrease.
Alternatively, if you decrease pressure, C decreases and the concentration of reactants will increase accordingly.
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