Procedure:
Obtain a reservoir of75mL of .17 M sodium thiosulfate. This amount will be enough for all five of your trials. For the hydrochloric acid, do not obtain a reservoir, but rather extract your 2.5mL of HCl for every reaction; measure 2.5 mL HCl, use in trial, then measure 2.5mL again from classroom reservoir/container.
Write a large black ‘X’ in thick marker or pen on a blank sheet of copy paper. You will place your reaction beaker on to this ‘X’ during testing.
For each trial, measure the amount of sodium thiosulfate needed (shown below) into your graduated cyclinder, and measure the 2.5mL of HCl in a separate cylinder. Each trial will have 27.5mL of substance total: a total of 25mL of thiosulfate and deionized water, and 2.5mL of HCl.
1: 25mL sodium thiosulfate 2.5 mL HCl
2: 20mL STS, 5mL DI water, 2.5mL HCl
3: 15mL STS, 10mL DI water, 2.5mL HCl
4: 10mL STS, 15mL DI water, 2.5mL HCl
5: 5mL STS, 20mL DI water, 2.5mL HCl
With the sodium thiosulfate and the deionized water in the reaction beaker on top of the X, quickly poor the HCl into the reaction. AS YOU DO this, immediately start the timing device. Stir the mixture slightly and briefly. Keep the device recording time, and qualitatively describe the reaction taking place. Look down at the reaction at the ‘X’ in the beaker, and stop the timer when the X is no longer visible.
All products of the reaction can be flushed into the sink, so do so at the end of each reaction and at the end of the lab. Clean your station and equipment.
25mL sodium thiosulfate 2.5 mL HCl
20mL STS, 5mL DI water, 2.5mL HCl
15mL STS, 10mL DI water, 2.5mL HCl
10mL STS, 15mL DI water, 2.5mL HCl
5mL STS, 20mL DI water, 2.5mL HCl
Data:
To find the concentration of thiosulfate ion within the reaction using first trial as example:
= .1545454545 = concentration of
Qualitatively, the reaction was obviously visible. You could notice that at first, it became cloudy very quickly, but its effect soon got slower and slower, until looking at the black ‘X’ took minutes and minutes. What’s to say here is that the reaction started off fast, and ended slowly. In the 25mL sodium thiosulfate solution, you could see change almost immediately and noticeably. The creamy white cloud appeared in every reaction, but as we lessened the concentration of sodium thiosulfate, the reaction took longer and longer, so to say that the cloud formed slower and slower.
Results/Discussion:
With the data projected above, we can use plots of certain variables over time in order to curves and, hopefully, a line. The plot forming a linear trend will portray the order of the reaction. This phenomenon is determined by the integrated rate laws of reactions and their orders, where in the rate law the slope forms a line.
The plots formed are the attached Graphs 1, 2, 3. Graph 1 displays a plot of concentration of thiosulfate over time, and projects a curve-like trend. This indicates, by understanding of integrated rate laws, the reaction is not a zero order reaction. If the trend were linear, then the reaction would be zero order with respect to thiosulfate.
Graph 2 displays a plot of the natural log of concentration of thiosulfate over time. This trend is also curve-like and thus is not the determining graph of the reactions order with respect to thiosulfate. If the trend were linear, then the reaction would be first order with respect to thiosulfate.
Graph 3, however, displays a plot of the inverse of concentration over time. This trend is linear, with a positive slope. This indicates that the reaction has an order of 2 with respect to thiosulfate.
Overall, with the data given and the data found experimentally, it is not possible to find the total order of the reaction, and thus the rate law. In the experiment, we only altered the concentration of hypo sodium thiosulfate, and did not alter the concentration of HCl. Changing experimentally the concentration of HCl would be necessary for finding the rate law because HCl is a reactant, and we unknown to its order. To find the order of HCl, we would have to do a process such as the one we committed to hypo sodium thiosulfate. And then, by noticing patterns in its concentration over time, we can determine its order, and then go on to find the rate law. The rate law in general consists of:
Where A and B are both reactants and each have a certain order, n and m. In this case we only know one of the reactants, A, and its order, n (hypo sodium thiosulfate with order of 2). We would have to find the order of HCl in order to progress in finding the rate law.
Error in this lab could have arisen as systematic error in measurements, such as measuring out correct volumes of reactants and deionized water. Even then systematic error can be added for the meniscus phenomenon. Error also could have risen from systematic incorrect recordings of time. The concentration graphs were dependent on time and could have been wrong because of it. Also a definite user error could erupt from one’s determination/definition of opacity; when the ‘X’ may have not been visible to some, others may have been still been able to see it as the reaction continued, and this could have largely affected time measurements. And, as always, random could have occurred in the instance of unwanted substances such as dust and excess water, which may have (but not likely) affected either the reaction or the concentrations or both.