Experiment April 2018 Determina3on of the Standard Reduc3on Poten3al of Cu 2+

Transcription

1 Experiment April 2018 Determina3on of the Standard Reduc3on Poten3al of Cu 2+ Today we will make copper using zinc. How will that work? Will we be using a supernova? Got it. I think he meant to say making Cu by reducing Cu 2+ with zinc.

2 Objec1ve: To determine the standard reduc1on poten1al, E o, for Cu 2+ (aq) We will determine E o for this half- reacgon. And we shall see how we could do the same for any half reacgon. There is nothing special about copper. Cu 2+ (aq) + 2 e - à Cu(s) E o =? Overview: 2. Making the solu1ons 3. Making the graph 4. Procedure: What we are doing. 5. Your lab report This is your basic copper- colored tulip 2

3 We start with the ΔG and Q c equagon we learned about in the thermo chapter. The formula is used to calculate ΔG under non- standard condigons. We used it previously in the urea experiment. ΔG sol n = ΔG o sol n + RT ln(q c ) When we used it in the urea experiment, we uglized the fact that ΔG = 0 at equilibrium, so the equagon becomes: 0 = ΔG o sol n + RT ln(k c ) See SecGon in our book. Info for Introduc3on 3

4 ΔG = ΔG o + RT ln(q c ) - nfe = - nfe o + RT ln(q c ) These relagonships between ΔG and E and ΔG o and E o are subsgtuted in: ΔG = - nfe ΔG o = - nfe o Follow the math! E = E o - RT ln(q c ) nf E = E o RT log(q c ) nf Info for Introduc3on 4

5 y This experiment lets us determine E o by using the equagon of a straight line, y = mx + b. b x m E = E o RT log(q c ) nf E o is b. That s what we are trying to determine today. y = mx + b Info for Introduc3on 5

6 E = E o RT log(q c ) nf But first, why not add a few more simple connector lines y = mx + b Obviously. E E o m log(q c ) So if we graph E vs. log(q C ), we will know E o! Info for Introduc3on 6

7 In our experiment, we study the reducgon of Cu +2 by zinc. Cathode: Cu e - à Cu Anode: Zn à Zn e - Overall: Cu 2+ + Zn à Cu + Zn 2+ [Zn 2+ ] Cu 2+ + Zn à Cu + Zn 2+ Q c = [Cu 2+ ] By design for this experiment, [Zn 2+ ] = 0.10 M and never changes, so Q c = 0.10 [Cu 2+ ] Zinc is gray, so this is a zinc- colored tulip. 7

8 2. Making the solu1ons 0.10 Cu 2+ + Zn à Cu + Zn 2+ Q c = [Cu 2+ ] E = E o RT 0.10 log( ) nf [Cu 2+ ] E E o m log( 0.10 ) [Cu 2+ ] Great. Let s get We need to make a started! I ll bunch of solugons with bring the tulips! varying concentragons of Cu 2+ so we can have a variety of values along the x- axis. For each we will measure E and that should make a plot like this one. 8

9 2. Making the solu1ons Progressive dilu1ons Stock first second third fourth Eo E m 0.10 log( 2+ ) [Cu ] The stock Cu2+ solugon is about 0.10 M use the exact concentragon provided. Each dilugon contains 20.0 ml of the previous solugon and 30.0 ml water. 9

10 3. Making the chart Progressive dilu1ons Set up your Excel spreadsheet Stock first second third fourth * Remember, you can copy and paste formulas. *Stock solu1on concentra1on will vary. 10

11 3. Making the chart Do first Make all five solugons before measuring any E values. Start with the most dilute solugon. Measure E for the 4 th solugon first, then the 3 rd, then the 2nd, and the 1 st, and lastly for the undiluted solugon. 11

12 3. Making the chart Graph the stuff in the box. The x- axis is log(0.10/[cu 2+ ]) and the y- axis is E. log( ) 12

13 3. Making the chart Add the trendline, display the formula and R2. This example has an excellent R2 value, the closer to 1.00, the beier. And the y- intercept is E o = V log( ) The literature value is 1.10 V 13

14 4. What we are doing today 1 Wear your safety glasses. 2 The cover sheet summarizes everything that you need to include with your report. 3 We will follow the procedure preiy closely today. In the analysis part, we do print the Excel data and chart. 4 Take Gme wrigng an introducgon in your own words before lab. 5 Record observagons and details as carefully as possible. 6 You can record data directly into Excel or you can copy the data into your lab notebook. Aiach your Excel data and graph 7 Set the voltmeter to read DC Volts. 8 Red wire always goes to the copper wire 9 Values jump around a bit. Take a reading ayer 10 seconds or so. 10 In your conclusions, calculate percent error from the literature value, E o = 1.10 V. 14

15 5. Your lab report 1 First, the cover page with TA ini1als. 2 Next, the trimmed copy pages from your lab notebook 3 Next, aiach your Nernst chart, including the data. 4 Staple everything together. 5 There is no on- line component this week. 6 Turn in lab report today or before the start of class tomorrow. Late labs may not be graded see the syllabus. Woo- hoo! Out early! S1ck people inspired by xkcd cartoons by Randall Munroe ( 15