Experiment 10 5 April 2018 Thermodynamic Values for Urea

Transcription

1 Experiment 10 5 April 2018 Thermodynamic Values for Urea Yes, CO(NH 2 ) 2 for sure. I am so curious about urea s thermodynamic values. Photo credit: h,p://

2 Objec8ve: To determine these thermodynamic values for urea: K c, ΔG o sol n, ΔHo sol n, and ΔSo sol n Today we are going to learn more about urea than you thought possible. We will apply skills we ve learned this year to predict properfes and thermodynamic values. Overview: 1. What is urea, anyway? 2. Dissolving urea in water and making predic8ons. 3. Determining ΔG o sol n and K c. 4. Determine ΔH o sol n. 5. Calcula8ng ΔS o sol n. 6. What we are doing in lab today. 7. Your lab report. But, first, what is urea anyway? 2

3 1. What is urea, anyway? Here is the ball and sfck model of urea. What is the hybridizafon at carbon? At nitrogen? What are the bond angles at nitrogen? At carbon? H O N C H N H H And here is the Lewis dot structure of urea. Does every atom have an octet? What sort of intermolecular forces are we talking about here? Urea 3

4 1. What is urea, anyway? Urea has four nitrogen- hydrogen bonds that can parfcipate in hydrogen bonding, so it is very soluble in water. Pssst. Urea s molar mass is g/mol 90% of the urea manufactured is used in ferflizers. It is a solid pellet ferflizer that is easy to use. Urea is also the main nitrogen- containing component in animal urine. So I think we can say this tree has been ferflized with urea. 4

5 2. Dissolving urea in water and making predic8ons. Urea(s) Urea(aq) K c = [Urea] E This is the equilibrium we will study today. It is simply the dissolving of urea in water or the dissolufon of urea. K c is stuff on the right divided by stuff on the le\, omi]ng solids. That leaves us with K c = [Urea] E. Info for IntroducJon The lifle E means equilibrium Over 1000 grams per liter! With all its hydrogen bonding, urea dissolves readily in water. However, we will be making smaller solufons. 5

6 2. Dissolving urea in water and making predic8ons. Urea(s) Urea(aq) K c = [Urea] E We ve predicted that urea dissolves rather easily so K c is must have a reasonably large value. If 1 mole urea dissovles per liter, K c = 1. We will see that it is bigger than that. What do you predict for ΔG? To predict, we ask ourselves, Does it happen? Yes means ΔG < 0. What about ΔS? Is there more disorder or less disorder? Info for IntroducJon What about ΔH? Is heat given off or does it take heat? The answer is: We can t predict ΔH from what we know so far. 6

7 3. Determining ΔG o sol n and K c. Urea(s) Urea(aq) K c = [Urea] E First we will make a saturated solufon the equilibrium shown in the equafon. If we know urea s molar solubility, we know K c. þ K c = [Urea] E ΔG o sol n ΔH o sol n ΔS o sol n From K c we can calculate ΔG o we ll talk about that next. Then we do a second experiment for ΔH o... and finish up by calculafng ΔS o. 7

8 3. Determining ΔG o sol n and K c. Urea(s) Urea(aq) K c = [Urea] E We use the ΔG o and K formula* we talked about in class... At equilibrium, ΔG = 0 and we get the ΔG o and K c equafon. ΔG sol n = ΔG o sol n + RT ln(q c )* 0 = ΔG o sol n + RT ln(k c ) ΔG o sol n = - RT ln(k c ) We predicted K c and the ΔG o. How are we doing? *The ΔG o and K formula is used to calculate ΔG under non- standard condi8ons. See Sec8on in the book. 8

9 3. Determining ΔG o sol n and K c. Urea(s) Urea(aq) K c = [Urea] E The literature value for the molar solubility of urea is 15.9 mol/l. This is also K c. Most of us will not get such a high number. If K c comes out less than 10, perhaps this part of the experiment should be repeated. A K c value of 15.9 gives ΔG o = kj/mol. þ K c = [Urea] E þ ΔG o sol n ΔH o sol n ΔS o sol n 9

10 4. Determining ΔH o sol n. Urea(s) Urea(aq) ΔH sol n =? To determine ΔH, we ll repeat the coffee cup calorimeter experiment from last semester. See how she is swirling the solufon during data But urea chills the solufon as it collecfon? dissolves last semester the reacfons were exothermic. 10

11 4. Determining ΔH o sol n. Urea(s) Urea(aq) ΔH sol n =? From the LoggerPro data, we get the inifal temperature and the final temperature. T i Read T i from the numerical data collected. Where the horizontal blue trendline and the Fme of addifon intersect, we have T f. Pink dot! 11

12 4. Determining ΔH o sol n. Urea(s) Urea(aq) ΔH sol n =? Be sure to use the mass of the solufon in the calorimeter calculafon and the mass of the urea to get moles of urea in the ΔH o calculafon. q cal = Spec Heat x Mass Sol n x ΔT = (4.023 J g - 1 deg - 1 )(mass Sol n )(T f - T i ) Then ΔH o sol n = (- q cal /mol Urea ) The literature value is ΔH o = kj/mol urea.! 12

13 5. Determining ΔS o sol n. Urea(s) Urea(aq) ΔS sol n =? ΔG o sol n = ΔHo sol n - TΔSo sol n The change in entropy is obtained by calculafon using the Gibbs- Helmholtz equafon. Watch the units (kj/mol and J/mol K). The literature value for ΔS o = 69.5 J/mol K. How close did you get? þ K c = [Urea] E þ þ ΔG o sol n ΔH o sol n ΔS o sol n zzzzzzz 13

14 5. Determining ΔS o sol n. Urea(s) Urea(aq) A final note. The values we determined today are probably not under truly standard condifons, 298 K and 1.0 atm and 1.0 M. So they are our esfmates of ΔG o sol n, ΔHo sol n, and ΔS o sol n. The lab manual leaves off the lifle naughts. þ þ þ þ K c = [Urea] E ΔG o sol n ΔH o sol n ΔS o sol n We keep them (the naughts) in here because they are necessary when we calculate ΔG o sol n by relafng it to the equilibrium constant K c because at equilibrium, ΔG = 0 and then: ΔG o sol n = - RTlnK c 14

15 6. What we are doing in lab today. 1 Wear your safety glasses. Dress for a mess. 2 The cover sheet summarizes everything that you need to include with your report. 3 Take Fme wrifng an introducfon in your own words before lab. 4 Record observafons and details as carefully as possible. 5 Use the analyfcal balance for massing out the urea for the ΔH part. Making 60 ml of 1.0 M urea takes about 3.6 g. Do not try to get exactly g! Get close, but record exactly how much you used. Examples: g or g. 6 LoggerPro gives temperatures to nearest 0.1 degree. This sets the significant figures for your q and ΔH calculafons. 7 In your conclusions, calculate percent error from the literature values shown below. These values came from the interweb or your textbook ΔH o = 14.0 kj/mol urea ΔG o = kj/mol urea ΔS o = 69.5 J/mol K 15

16 7. Your lab report. 1 First, the cover page with TA ini8als. 2 Next, the trimmed copy pages from your lab notebook stapled together. 3 A,ach your calorimetry chart, including the data at the end of of your report. Staple all together. 4 On- line results due at the end of class today. Late submissions are not graded see the syllabus. 5 Turn in lab report today or before the start of class tomorrow. Late labs may not be graded see the syllabus. S8ck people inspired by xkcd cartoons by Randall Munroe ( 16

17 Ummm How much urea did we use anyway?!?! WOW! Are these things safe? 17

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