Chapt 13 Kinetics Lecture Part 4

this is our fourth lecture video on kinetics and I wanted to start by looking at what you're going to be doing lab because it applies so much to what we are talking about in lecture So in lab you are going to be looking at three different variables which affect the rate of the reaction in one part you are going to be looking at how concentration affects their reaction rates and another section you're going to be looking at how temperature affects the rate of reaction and we are also going to be looking at how a catalyst affects the reaction rate so in this lab you will be determining the orders of a reaction the rate constant and the energy of activation so the reaction that we're going to be looking at is the reaction of iodide reacting with bromate and acid to produce iodine bromide and water so the rate law expression said the radar expressions this reaction is the rate equals rate constant times the iodide concentration raised to some power and the bromate concentration raised to some power and the proton concentration raised to some power. we're going to be actually determining those orders of the reactions of the m value the n value and p value but we're not going to be directly measuring the rate of every action instead we will be recording the time it takes to consume a fixed amount of sodium thiosulfate Basically analogy of running a race the sodium thiosulfate represents the distance and we will record the time it takes for the reaction to run the race that's will be determined the relative reaction rates. We won't actually be measuring the reaction rates themselves but we're going to be comparing how long it takes the reaction to complete by changing the initial concentrations. So the sodium thiosulfate will be our limiting reagent and that is going to determine how long the reaction is going to occur so the molecular iodine that is produced will almost instantaneously be consumed by thiosulfate ions as described in equation three in equation one we're looking at the iodine, bromate, and the acid coming together to make the I2, the bromide, and the water and then the iodide that's produced i t that's produced it's going to immediately react with the thiosulfate mites and then once all of the thiosulfate ions are consumed then the I2 molecules then react with the starch that's added to the solution to make a blue-black complex and when we see the blue-black complex that tells us that all the thiosulfate has been reacted and this signals us when to stop the time measurement All the reactions are going to run the same distance. They are all going to use up the same amount of thiosulfate It is just that we are going to time to see how long each reaction takes place to reach that same point and that change in color is going to help us know. So this will be representative of some of the data that you collect so you will determine your initial concentration of your iodide (I-), your bromate, and your acid concentration, and you've got a different concentrations or different trials route so you'll see with different concentrations and the temperature needs to be the same for all of these and you measure the time it takes for the reaction to occur and we're going to take one thousand divided by that time to get the relative rates. Once we have our rates are reaction, then we can start setting up our ratio that we looked at in ou rprevious video lecture and we can choose from the orders of the reaction with respect to each reactant now also in that lab we're going to be measuring the energy of activation so remember that this is our Arrenhius equation where k equals A times e raised to the negative Ea over RT algebraically the arrhenius equation can be arranged in the following form so basically what we've done is taken the natural log of both sides So I take the natural log of k. So the ln of k equals the ln A he wouldn't i take the other both sides in these are multiplied is to be the same thing saying the ln of A plus in a class p_l_o_ you have this term and since we're taking the ln of the natural number e that disappears and we so we just get the negative Ea over RT and when it's arranged like this this should look familiar to you. This should look like an equation for a line if we plot the ln of k versus the inverse temperature So in this case the ln of k would be on our y axis and inverse temperature would be on our x-axis and from the slope of the line we can get the energy of activation. So we take our slope multiply it by our R one we can get the energy of activation we can also get the frequency factor from the y-intercept Now, alternatively we can replace the ln of K with the ln and about to break and use this on the graph to determine Ea this is actually what you are going to be doing in lab we're going to be plotting the ln of the relative rate vs the inverse temperatures and this gives you an idea to get some of that representative data that you will be collecting so from this equation of the line so the slope is minus two thousand seven hundred nine if we multiply the slope by the gas constant and by negative one we can get the energy of activation for this reaction Now sometimes when we're trying to figure out the energy of activation we don't always have enough data points to do a graph or graphing may take too long. We can do an approximation if we just have two data points but experimentally it's best to plot the data but has many data points as possible so that you can get a best-fit line but sometimes you don't always have that much information sometimes you need to do a two point version of the arrhenius equation So this is how we're going to get the two point form had a basically who tried to so we've got two different samples. We've got one with that one of one to thirty one and the other at another temperature remember changing the temperature changes the rate of any action to this is why we have 1 k value and another k value Now the frequency value, the A value, here is a constant will be the same in both reactions So what i've done here is I have solved both sides for A and set them equal to each other now i have just isolated the k value on just one side subdivided both sides by the k2 and have taken this nominee here multiply both sides and I moved it to the top here. so this gives me this equation here now I am going to take the ln of both sides so the ln that you wanted to meet with the elena this entire function Well the natural log of this numerator denominator i can't separate these out personnel in the event numerator my a c l_ in an effort to nominate ur Then what I am going to do i think that that the alan lipke ag disappears and i just had the negativity evade over party one equals maybe annual party to you not factory not energy of activation in the gas constant he's remembered the whole point in this at least equations and i can find the energy of activation so now that i've back to this out comic it better than negative ones soon and as for the position of t one minus to teach each year in which is reversed the springtime eight of them minus one infront so this is the two point form have the every seek ways in and you only use this when you only had two data points the temperature and every constant when you're trying to find the energy of activation citizens are force video and in the next video will be looking at your reaction mechanisms and catalyst to finish up are lectures on genetics