Intro to Chemical Kinetics - Video Tutorials & Practice Problems
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Chemical Kinetics is the study of factors affecting reaction rates, which examines the speed by which reactants are consumed to make products.
Chemical Kinetics
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concept
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 gonna say reaction rate is just the measure of speed and change in concentration of reactions or products over time. Now to a short way of 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 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 2 chemical reactions. In the first one, we have reactant a, which is represented by red, 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 4 a molecules well, 5 a molecules and 5 b 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 c's, 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 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 1, 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. Okay? So less reactants 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 gonna 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, within the given 55 seconds, not much pocket 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 is a limit because all the reactants will disappear, and therefore that sets a limit on how much product can be formed.
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Intro to Chemical Kinetics Example 1
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It says formation of carbon tetrachloride from chloroform is given by the reaction. So here we have 1 mole of chloroform reacting with 1 mole of chlorine gas to produce 1 mole of carbon tetrachloride gas plus 1 mole of HCl. Now here it says, which graph accurately showed the concentration of c c l 4 as a function of time for this reaction. So remember, as time progresses, we expect our reactants to decrease in amount, and we break down these reactants in order to form product, so we expect these to be increasing at us, over time. Now here we're looking at, the concentration of c c l 4 which we'd expect to be increasing. So a can't work. So the answer is either b, c, or d. Now remember, your reaction does not go on forever. Eventually, all your reactants will be 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 c c l four grows exponentially. That can't happen. Eventually, you run out of reactants. C is not as steep of an increase, but it is still an increase forever. This doesn't work. D is the best answer, because in d, we can see that our product is increasing over time, but eventually it reaches a plateau, because we've run out of reactants. Once we've run out of reactants, we can no longer make any more product. So it's gonna reach a value or plateau like we see in option d. So out of all the choices, option d is the correct answer.