Hello students, this is Dr. Sansom here for your pre-lab lecture video for experiment 6, which is about equilibrium. Today in class you're going to do an experiment that will allow you to determine the value of the equilibrium constant for a reaction. And the goals for today are to explain why k is constant at a given temperature and also describe how k changes at different temperatures. So you'll collect data at different temperatures and use that to relate The changes in K, two changes in delta H and delta S and delta G. You're also going to use Le Chatelier's principle to predict what changes will happen if you stress the system by adding or removing a reactant or changing the temperature.
And our process skill for today will be critical thinking. And the reason you're doing critical thinking is because you're going to have to make a claim about which direction you think the reaction is shifting when you are looking at Le Chatelier's principle. And anytime you're constructing an argument, you should support your claim with evidence and reasoning. To do this, you have to put together different types of information.
In this experiment, you're going to be putting together ideas about temperature, and also ideas about spectroscopy. You'll be using the spectrometers to say what's happening if the absorbance increases or decreases. What does that mean about the concentrations of the products and reactants? So you'll be synthesizing information from different sources in order to come to your final conclusion. So today we're talking about dynamic equilibrium.
And we call this equilibrium because at equilibrium there's balance in the system. The concentration of reactants and products are constant. They don't change anymore once we've reached equilibrium.
Note that even though the word says equilibrium, it does not mean that those concentrations are equal, rather the forward and reverse reaction rates are equal. So we call it dynamic because if we could zoom in and look at the molecular level, we would still see the forward and reverse reactions occurring, but they occur at the same rate. So when we zoom out and look at just the beaker, we can't tell that anything is happening at all. It appears to not change.
So dynamic equilibrium is the word that we use to describe that. And You guys today are going to be calculating the equilibrium constant and it has the expression down at the bottom here that Kq is the concentrations of products raised to their coefficients divided by the concentration of reactants raised to their coefficients and it communicates the extent of the reaction to equilibrium or in other words the ratio of products to reactants. If K is small, less than 1, then we say the reactants are favored.
If it's greater than 1, we say the products are favored. And if it's somewhere in the middle, then usually it contains a decent mixture of both reactants and products. And the reaction system that you're going to look at today with iron and thiocyanate ions is in this category, where the equilibrium constant is kind of in the middle. And so you'll be able to see visually with your eyes how the concentrations change when the equilibrium shifts.
Today in class, you'll also be looking at the temperature dependence of the equilibrium. constant. And this is just a generic reaction coordinate diagram where we've got reactants on the left and products on the right. And essentially, when we raise the temperature, it's going to affect the rate of the forward reaction differently than the rate of the reverse reaction. If we think of this like the high jump, that's kind of like the reverse reaction.
Normal people can't jump over that huge. high bar, right? But normal people, probably a bunch of them, not all of them, but many of them can jump over, say, a hurdle.
And so that's like our forward reaction. It's easier to jump over that activation energy. So if we give people a lot of energy, maybe a running start with our hurdle, more people are going to be able to get over the hurdle going in the forward direction.
But even with a running start, it's still going to be pretty hard for people to get over that. a huge high jump in the back. So I want you to take a minute here and think about these questions.
How will heating the system affect the rate of the forward reaction, the rate of the reverse reaction, and then the equilibrium? Go ahead and pause the video and take a minute to think about those questions. So as we heat the system, the rate of the forward reaction and the rate of the reverse reaction will both increase.
but because we're starting out with such a small amount of molecules going over that reverse activation barrier, it's actually going to affect the rate of the reverse reaction more than the rate of the forward reaction. And because of that, our equilibrium will actually shift to the left, and equilibrium constants will actually change in this situation. And you will actually determine the way the equilibrium constant changes using your data from your experiment. And that will help you know if the reaction is endothermic or exothermic. You'll make some predictions about that in the pre-lab.
based on your molecular level understanding, but you'll be able to verify that with your data from the experiment. So this equation that we're going to use today, we can derive it from our delta G equals delta H minus T delta S and delta G equals negative RT ln K. Both equations, which hopefully you've seen before, and if we combine them together, we get this overall equation.
This exact equation is not on your equation sheet. You have a similar equation, but it doesn't include this intercept, which is our delta s over r. So you may want to just remember this one for when you are taking the exam. And here the natural log of keq equals the negative delta h over rt plus delta s over r.
And this equation describes the temperature dependence of the equilibrium constant in terms of thermodynamic quantities. So we can say how K will change based on the values of delta H and delta S for that reaction. This does assume that delta H and delta S stay constant over the temperature range that we're looking at, which is a pretty good assumption because we'll just be sort of between 0 and 100 degrees in our experiment today. Now, what does this mean about how our graph of ln Keq versus 1 over T will look for different types of reactions? Well, if we have an exothermic reaction, then the value of delta H will be negative, and so the slope will be positive.
And if we have a reaction that's endothermic, our delta H is positive, and our slope will be negative. Now, what about entropy? If our products...
are more ordered than the reactants, that will mean delta S is negative, so we'll have a negative y-intercept. And if our products are more disordered than the reactants, then we should have a positive delta S and a positive intercept. So for our reaction, what do we think it should look like? Well, we know that we've got iron and thiocyanate, and they're coming together to form a complex ion. They're forming a bond.
And if they're forming a bond, then we probably anticipate that our delta H should be negative because whenever we form bonds, it should be exothermic. So our slope is likely to be positive, indicating a negative delta H. Now in the reaction, you might also think you have a negative delta S because you have your free iron and free thiocyanate ions coming together to form a single complex ion, FeSCN2+. Thank you.
And that should be more ordered than the reactants, so then we would expect a negative delta S. Now you'll see in the reaction today whether these predictions are correct or incorrect based on your data. The last part of the lab that you're going to do is Le Chatelier's principle.
So you'll have a system at equilibrium. You'll mix up all of these test tubes with reactants in them, and they will establish an equilibrium. And then you're going to stress them. So you're going to add a little bit extra of the reactants or remove some of the reactants. By doing this, you'll be able to see what happens to the equilibrium.
It will shift to the right or to the left. Because your product is red and your reactants are yellow, if it's in the middle, it'll be sort of an orange color. And if it shifts more towards the products, it'll be a red color. If it shifts more towards the reactants, it'll be a yellow color. In lab today, you want to make sure that you wear long pants, closed-toed shoes, goggles, and gloves, and your lab coat.
And the solutions that you're working with today with iron-3-nitrate are corrosive and oxidizing. They're made with acid inside. So it's important that you avoid contact with your skin and eyes, and also avoid contact with any flames or sparks. You don't have any of those in the lab today, so that shouldn't be a problem, but just in case, just don't do that.
And then the other solutions that you're using are skin and eye irritants. So you should just use general precautions with them. The last thing I'll say is be careful today. You have some of the same chemicals, but in different concentrations. And you want to make sure that you're using the right concentration at the right part of the experiment.
Otherwise, you won't get any data that makes sense. So please read the bottles carefully and listen to your TA there to find the appropriate. concentration of the solution that you're looking for for each part of the experiment. Thanks very much and have fun in lab!