Chemical kinetics deals with the study of factors affecting the reaction rate or speed of motion of reacting molecules. Now when we talk about reaction rate, we're going to say reaction rate is just a measure of speed and change in concentration of reactions or products over time. Now to a shortwave writing concentration is to show it in brackets. So if you see a term in brackets, that means we're talking about its concentration. So if A reactants were in brackets, we'd be talking about reactant concentrations. If products were in brackets, then we're talking about product concentrations.
Now here we have an example of two chemical reactions. In the first one we have reactant A, which is represented by this red rectangle, and we have reactant B mixing with it. Together they give us product C. Now if we were to take a look, we see that in the beginning we have 5A molecules and 5B molecules. And then if we go to the other side, we can see that a good amount of product has been formed. We've formed 4 product Cs and there's still some A and B remaining. All this happens within 55 seconds.
If we take a look at the other reaction, we have AB plus Y gives us W. Here in the beginning we have only AB and Y because at the beginning all we have is reactants. If we look on the other side, we can see not much product is formed. There's only one W product that's formed right here. So if we were to compare these two reactions, we'd say that reaction one within the same amount of time, 55 seconds, we make way more product. And because we're making more product, we can say that this represents a faster rate. If we look at the other reaction, we can see that in the same 55 seconds we only make one product, so not as much. This would be our slower rate. OK, so less reactant is transformed into product.
Now here we're going to say for reactions that come to a completion, that means that almost all the reactants are broken down and products are formed. Remember, we break down reactants in order to form products. Now we're going to say for these reactions that go to completion, we use a single arrow to signify it. So if you have here a single arrow going forward, single arrow going forward. Now here within the given 55 seconds, not much product is forming here. But if we give it enough time, most of the product would be formed. It's just a little bit slower when compared to the first reaction over here.
Now besides that, we can say that reaction rates decrease with time due to reduction in concentrations of reactants. So this reaction can't go on forever. Eventually all your reactant will be gone, and once all your reactant is gone, the reaction stops. The reactant that's broken down has been transformed into product. So there's limits to this. There's a limit because all the reactants will disappear and therefore that sets a limit on how much product can be formed.