Let’s imagine a very simple reaction:
A + B ⇄ C + D
We’ll start with a container that holds some A and some B (and nothing else). Assuming that A and B are not solids, we know that A and B will collide and that some of those collisions will result in reaction (those that have enough energy and correct orientation). That means that as time goes on the concentration of A and B will decrease. We know that the rate at which they react will slow down (as the concentrations go down) but, up to now, we’ve assumed that the reaction will continue until we run out of something (limiting reactant).
A graph of the concentration of A and B might look something like this over time (I have arbitrarily decided that we started with more B than A for this example).
On the graph above, the pink line shows the decreasing concentration of A and the green line shows the decreasing concentration of B. Since the reaction uses one A for every one B, they decrease together in "lock-step". When the amount of A reaches 0, the reaction would stop. That is simple stoichiometry.
That means that the graph of the concentration of A and B would look more like this:
But now that we know reactions are reversible, the picture gets more complicated. As our reaction runs forward it creates C and D. Again, assuming that they are not solids, they will collide with each other, and some of those collisions will result in a reaction that produces A and B. Of course at the beginning, the backward reaction will be VERY slow; we start with 0 C and 0 D, but as the forward reaction proceeds, there will be more and more C and D and therefore the backward reaction will get faster.
That means that we can NEVER run out of A or B because the reverse reaction is always making some. At the same time, we never run out of C or D because the forward reaction is always making more.
One more REALLY IMPORTANT idea. The forward rate is initially much faster than the backwards reaction. In other words, at the beginning we are using up A and B much faster than the backward reaction is making them. BUT, as the forward reaction slows down and the backward reaction speed up, there MUST come a point where the rates are equal. In other words, there must be a point when the rate forward and rate backward are the same. At that time, the concentration of each thing in the reaction (A or B or C or D) will stop changing — for every A the forward reaction uses, an A is created by the backward reaction.
That means that we can NEVER run out of A or B because the reverse reaction is always making some. At the same time, we never run out of C or D because the forward reaction is always making more.
One more REALLY IMPORTANT idea. The forward rate is initially much faster than the backwards reaction. In other words, at the beginning we are using up A and B much faster than the backward reaction is making them. BUT, as the forward reaction slows down and the backward reaction speed up, there MUST come a point where the rates are equal. In other words, there must be a point when the rate forward and rate backward are the same. At that time, the concentration of each thing in the reaction (A or B or C or D) will stop changing — for every A the forward reaction uses, an A is created by the backward reaction.
That means that the graph of the concentration of A and B would look more like this:
If we include the concentrations of C and D, the graph might look like this. Since C and D are produced together in a 1:1 ratio, I've shown them with a single blue line.
Or this
Or this
Which of these depends on whether the forward reaction or backward reaction is easier (lower activation energy and easier orientation). The math is coming later in this unit.
What is important about these graphs is what they have in common. ALL three of those graphs have concentrations that become constant at a given moment in time. That it, on all three graphs, there is a time after which the concentrations become horizontal lines — constant concentration.
What is important about these graphs is what they have in common. ALL three of those graphs have concentrations that become constant at a given moment in time. That it, on all three graphs, there is a time after which the concentrations become horizontal lines — constant concentration.
At that point, it will appear that the reaction has stopped (since the amount of product stops changing). However, nothing could be further from the truth. The reaction continues, but at equal rates in both directions, so there is no change in the amounts of the various components.
A note about closed and open systems:
All of this has been assuming that the system is closed, that is nothing is being added or taken away from the system.
A simple example of that idea is your water bottle. If you leave your water bottle sealed, it will never be dry. The water inside continues to evaporate, but some of the vapor will condense and the amount of liquid in the bottle will stop changing. If, however, you remove the lid, water vapor will leave the bottle and so the rate of condensation IN the bottle will never match the rate of evaporation. The bottle will eventually dry out. The matching of rates cannot be achieved in an open system - that is, a system where one of the components is continuously added or removed as the reactions proceed.
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