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Determining Order of a Reaction Using a Graph - Concept
M.Ed.,San Francisco State Univ.
Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.
In this particular segment, we'll take a look at tips and tricks for determining the order of a reaction using a graph. So you can always use experimental data that you have. So for your graphs, the orders of the reaction that we're going to take a look at are; zero order, first order, and second order.
So in a zero order, if I graphed my data, then on my y axis, I'll have the concentration of whatever my reactant is. That'll be graphed versus time that I have there. So based on your concentrational increase here. Basically, if I end up with a straight line with a negative slope, then it would be a zero order reaction.
Now if I have a first order reaction, then I would graph a natural log of A versus time. So the concentration of my A versus time. If I graph that, and I end up with a straight line for this, then it would be first order reaction.
Then for a second order reaction, if I graph the inverse of the concentration A, so basically 1 over the concentration A versus time. I graph that, and I end up with a straight line with a positive slope. Then I know it's second order.
Our keys here are only one of these graphs would be true for your data. Now when you plot some of the graphs, it might look like this, or it might look say this. Or it might look say this. That's not a straight line. So only one of these three sets of data that will get you a straight line.
If you graph the first order reaction. The first order reaction basically ends up with a straight line with a positive slope. Then that's not right, because it has to be a negative slope. So it will have to be either zero first for second order, assuming that it's either zero, first or second. It has to be only one of these three.
Now the last clue is the slope of your line. When you figure out what the order is, the slope would equal negative of your k. So negative of your rate constant if it's a zero order reaction. If it's a first order reaction, your slope is also equal to -k. So negative of your rate constant.
Then if it's the second order reaction, the slope of your line equals k. So that's equal to your rate constant. So it's only one of these three will end up with a straight line that looks like how it's supposed to. Zero order, you have a straight line with a negative slope only when you graph concentration A versus time.
First order, would be natural log of concentration A versus time. If you get a straight line with a negative slope, then that would be first order. For second order, if you graph the inverse of the concentration A versus time, you get a positive straight line with a positive slope, then you know it's second order.
The other thing; is you graph 3 or 4 points, and you notice they're straight, then that would be plenty to help you figure out. Two points not enough. You can't tell if you're going to have a straight line, even though in Math class they say that two points make a line. So in Chemistry when you're graphing these, you want to make sure you have at least 3 or 4 points. 4 points just to make sure. So if you have a set of 10 points, only graph 4 of them, then you're set. You'll know if it's a straight line or not.
Hopefully these tips and tricks will help you in determining the order of a reaction using a graph. Have a good one.
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