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General Chemistry

Learn the toughest concepts covered in Chemistry with step-by-step video tutorials and practice problems by world-class tutors

15. Chemical Kinetics

Intro to Chemical Kinetics

Chemical Kinetics is the study of factors affecting reaction rates, which examines the speed by which reactants are consumed to make products.

Chemical Kinetics

Intro to Chemical Kinetics Concept 1

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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 see a reaction rate is just a measure of speed and changing 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 for reactors were in brackets, we'll be talking about reacting concentrations. If products were in brackets, then we're talking about product concentrations. Now here we have an example of to chemical reactions. In the first one, we have reacted a, which is represented by red, this red rectangle, and we have react and be mixing with it. Together, they give us products seat. Now, if we were to take a look, we see that in the beginning we have four A molecules well, five molecules and five B molecules. And then if we go to the other side, we can see that a good amount of product has been formed. We formed four products sees, 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 a B plus Y gives us w Here in the beginning, we have only a B. And why? Because at the beginning, all we have is react. It's 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 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. Okay, so less reaction is transformed into product. Now here we're going to say, for reactions that come to a completion. That means that almost all the reactions are broken down and products are formed. Remember, we break down reactions 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. Okay, so if you have here a single arrow going forward, single arrow going forward Now here, uh, within the given 55 seconds, not much product is forming here, but if we give it enough time, most of the product would be for me. 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 reactions. So this reaction can't go on forever. Eventually, all your reaction will be gone. And once all your reaction is gone, the reaction stops. The reaction that's broken down has been transformed into product. So there's limits to this. There is a limit because all the reactions will disappear, and therefore, that sets a limit on how much product can be formed.

Intro to Chemical Kinetics Example 1

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here It says formation of carbon tetrachloride from chloroform is given by the reaction. So here we have one mole of chloroform reacting with one mole of chlorine gas to produce one mole of carbon tetrachloride gas plus one mole of HCL. Now here it says which graph accurately shows the concentration of CCL four as a function of time for this reaction. So remember, as time progresses, we expect our reactions to decrease in amount and we break down these reactions in order to form product. So we expect these to be increasing at us over time. Now here we're looking at, uh, the concentration of CCL four, which we expect to be increasing so they can't work. So the answer is either B, C or D. Now, remember, your reaction does not go on forever. Eventually, all your reactions we broken down and therefore your product reaches a limit of how much of it can be made. Because of this, B and C don't work because in B, it's saying that CCL four grows exponentially that can't happen eventually run out of reactivates C is not as steep of an increase, but it's still an increase forever. This doesn't work. D is the best answer because indeed, we can see that our product is increasing over time. But eventually it reaches a plateau because we've run out of reactant. Once we've run out of reactant, we can no longer make any more product. So it's going to reach a value or plateau like we see an option. D. So out of all the choices, Option D is the correct answer.