Equilibrium

When you see the word equilibrium, think "equal." That's essentially what it means. Equilibrium for a reaction is achieved when the rate of the forward reaction is equal to the rate of the reverse reaction. Okay, slow down. What does that mean? Think of it this way: at equilibrium, every time the reactants turn into products, some product turns into reactants. This is still too fuzzy, isn't it? Let's try it again. Each chemical reaction is reversible. Remember this graph, from the page on energy?

The reaction proceeds, as we say, in the forward direction when reactants come together to form products, as in:

2NO₂ (g) ------> 2NO (g) + O₂ (g)

But, if a system is given enough energy, it could just as easily go in the other direction:

2NO (g) + O₂ (g) ------> 2NO₂ (g)

As NO₂ decomposes to form the products, there will be a considerable accumulation of NO gas and O₂ gas. It is entirely possible that some of these molecules might react to reform the intial NO₂. When the reaction reaches the point where every time an NO₂ molecule decomposes, NO and O₂ come together to reform that NO₂ molecule, that is called equilibrium.

Changing elements such as temperature and pressure can also greatly affect the system. For instance, if you have the above reaction:

2NO₂ (g) ------> 2NO (g) + O₂ (g)

You can see that the side with the products has three total molecules (two NO and one O₂) and the side with the reactants has two molecules, both NO₂. If the pressure is simply the amount of time that molecules hit the sides of the container, you can see that the products side has more molecules, and thus will hit the sides of the container more frequently. We say that the pressure is greater on the products side. So, if we compress the cylinder, that is, if we squish the gas into a smaller sized container (decrease the volume), we in effect put a stress on the system. This stress will actually cause a change in the equilibrium. Now, there is less space for the molecules to be in, so they are constantly running into one another. What do you think will happen? Well, if you guessed that the NO and O₂ will react to reform NO₂, you'd be correct. Let's think about this. We have a system that for all intensive purposes was at equilibrium. We increased the pressure by decreasing the volume (we compressed the container). Now, the system is most stable at equilibrium. If it could reach equilibrium by reducing the pressure, then it probably will. If there are less total molecules present on the NO₂side, if the NO and O₂ react to reform the NO₂, the system will effectively reduce its pressure. If this doesn't make sense, think of it this way. Let's say you were in a room one day and the ceiling suddenly started moving slowly downward. Pretend that you cannot get out of the room. When the ceiling reaches your head, you have to make a choice: do you bend down to avoid being crushed, or do you continue to stand as tall as you can and pray that you will survive? Well, if you bend down, you give yourself some more room to move around. This is just like the above situation. By decreasing the number of molecules, or bending down, the system is able to "survive."

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