Experiment10 2April2020* ThermodynamicValuesforUrea

Experiment 10 2 April 2020

Thermodynamic Values for Urea

Photo credit: h,p://www.turchem.com/products/urea/

Yes, CO(NH2)2 for sure.

I am so curious about urea’s

thermodynamic values.

Objec8ve: To determine these thermodynamic values for urea: Kc, ΔGo

sol’n, ΔHosol’n, and ΔSosol’n

Overview: 1.  What is urea, anyway? 2.  Dissolving urea in water and making predic8ons. 3.  Determining ΔGo

sol’n and Kc. 4.  Determine ΔHo

sol’n. 5.  Calcula8ng ΔSosol’n. 6.  What we are doing in lab today. 7.  Your lab report. 2 2 2

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.

But, first, what is urea anyway?

1. What is urea, anyway?

So urea is CO(NH2)2 – a weak base…

...with lots of hydrogen bonding

It’s a very interesFng and important molecule

...Google it and you will be amazed.

Urea

H

O

N C H H N H

3 3 3

Here is the ball and sFck model of urea. What is the hybridizaFon at carbon? At nitrogen?

And here is the Lewis dot

structure of urea. Does every atom have an octet?

What are the bond angles at nitrogen? At carbon?

What sort of intermolecular forces are we talking about

here?

1. What is urea, anyway?

Pssst. Urea’s molar mass is 60.06 g/mol

4 4 4

Urea has four nitrogen-‐hydrogen bonds that can parFcipate in

hydrogen bonding, so it is very soluble

in water.

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.

Kc is stuff on the right divided by stuff on the le\, omi]ng solids. That leaves us with

Kc = [Urea]E.

Urea(s) Urea(aq) Kc = [Urea]E In the first part of the experiment, we will dissolve urea in

water. This is the equilibrium we will study today. It is simply the dissolving of urea in water – or the “dissoluFon” of urea.

5 5 5

2. Dissolving urea in water and making predic8ons.

With all its hydrogen bonding, urea

dissolves readily in water.

The lifle E means

equilibrium… Info for

IntroducHon

Over 1000 grams per liter!

However, we will be making smaller

soluFons.

We’ve predicted that urea dissolves rather

easily so Kc must have a reasonably large value. If 1 mole urea dissolves per liter, Kc = 1. We will see that it is bigger

than that.

Urea(s) Urea(aq) Kc = [Urea]E

6 6 6

2. Dissolving urea in water and making predic8ons.

What do you predict for ΔG? To predict, we ask ourselves, “Does it happen?” Yes means ΔG < 0.

The lifle E means

equilibrium…

Info for IntroducHon

What about ΔS? Is there more disorder or

less disorder?

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.

3. Determining ΔGosol’n and Kc.

First we will be given a saturated soluFon of urea – the equilibrium shown in the equaFon. If we know urea’s molar solubility, we know Kc.

þ  Kc = [Urea]E ☐ ΔGo

sol’n ☐ ΔHo

sol’n ☐ ΔSosol’n

From Kc we can calculate ΔGo – we’ll talk about that next. Then we do a second experiment for ΔHo...

…and finish up by calculaFng ΔSo.

7 7 7


We use the ΔGo and K formula* we talked about

in class...

ΔGsol’n = ΔGosol’n + RT ln(Qc)*

0 = ΔGo

sol’n + RT ln(Kc) ΔGo

sol’n = -‐RT ln(Kc)

*The ΔGo and K formula is used to calculate ΔG under non-‐standard condi8ons. See

Sec8on 18.10 in the book. 8 8 8



At equilibrium, ΔG = 0 and we get the ΔGo and Kc equaFon. Pssst, yeah you -‐ remember R = 8.314 J mol-‐1 K-‐1

9 9 9


Dr. Mafson will give you a saturated

soluFon of urea and all you have to do is

determine its density. Saturated soluFons are

at equilibrium.

Urea(s) Urea(aq) Kc = [Urea]E We will determine ΔGo differently from the procedure given in the lab manual on

page 66. What we will do is experimentally easier to do and gives

befer results. The calculaFons are given on the next four slides.

See where we are going with

this? Me neither.

10 10 10


Urea(s) Urea(aq) Kc = [Urea]E The density of pure water is basically 1.00 g/mL and pure water is zero mass % urea. Right? Are we good? Pure urea has a density of 1.335 g/L

and is… 100 mass % urea. If we knew the density of a saturated soluFon, we could

interpolate the results to get mass percent and from there we could convert to molarity!

0 % . . . . . . . . . . . Mass % Urea(s) . . . . . . . 100 %

1.00 g/mL . . . . . . . . . Density . . . . . . . . . 1.335 g/mL

Pure water . . . . . . . . . . . . . Pure urea

Ah…Got it. We’ve done stuff like this

before.

11 11 11


0 % . . . . . . . . . . . Mass % Urea(s) . . . . . . . 100 %

1.00 g/mL . . . . . . . . . Density . . . . . . . . . 1.335 g/mL

Pure water . . . . . . . . . . . . . Pure urea

Suppose you determined the density of your saturated soluFon to be 1.143 g/mL.

The interpolaFon formula to convert density into mass percent is:

Mass % = 100 % X Measured density – 1.00 g/mL

1.335 g/mL – 1.00 g/mL

Mass % = 100 % X = 42.7 mass % 1.143 g/mL – 1.00 g/mL 1.335 g/mL – 1.00 g/mL

Repeat this calculaFon with your density of

soluFon.

3. Determining ΔGosol’n and Kc. þ  Kc = [Urea]E

þ  ΔGosol’n

☐ ΔHosol’n

☐ ΔSosol’n


Remember how we converted mass percent into molality back in

January? We made a concentraFon grid. 42.7% urea means 42.7 g urea

for every 100 g of soluFon.

mass moles Vol Urea(s) 42.7 g H2O Sol’n 100 g

?

?

12 12 12

So I know you can convert mass to moles and mass to volume using the density.

You got this. Go!

Oh! One more thing – knowing moles and volume, you can calculate

molarity. Then you’ll have Kc!

13 13

3. Procedure for [Urea]E, Kc and ΔGo

So, we are not doing the procedure on the top of page 66 Ftled DeterminaFon of ΔGo.

Instead, you will determine the density of a saturated urea

soluFon and do it as described on the previous slides. Obtain a 10.00 mL volumetric pipet from

your TA.

With the proper balance, we can get four significant

figures!

The rest of this alternate procedure

is up to you.

To determine ΔH, we’ll repeat the coffee cup calorimeter experiment from last

semester.

But urea chills the soluFon as it dissolves – last semester the reacFons were exothermic. 14 14 14

4. Determining ΔHosol’n.

See how she is swirling the soluFon

during data collecFon?

Urea(s) Urea(aq) ΔHsol’n = ?

Ti

15 15 15


From the LoggerPro data, we get the iniFal temperature and the final

temperature.

Where the horizontal blue trendline and the Fme of addiFon intersect, we have Tf. Pink dot!

Read Ti from the numerical data

collected.


þ  Kc = [Urea]E þ  ΔGo

sol’n ☐ ΔHo


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 ΔHo calculaFon.

qcal = Spec Heat x MassSol’n x ΔT

Then ΔHosol’n = (-‐qcal/molUrea)

= (4.023 J g-‐1 deg-‐1 )(massSol’n)(Tf -‐ Ti)

! 16 16 16



The literature value is ΔHo = +14.0 kJ/mol

urea.

Heads-‐up! Read this again.


sol’n þ  ΔHo


17 17 17

5. Determining ΔSosol’n.

Urea(s) Urea(aq) ΔSsol’n = ?

zzzzzzz

ΔGosol’n = ΔHo

sol’n -‐ TΔSosol’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 ΔSo = 69.5 J/mol K. How close

did you get?


sol’n þ  ΔHo

sol’n þ  ΔSosol’n

18 18 18

5. Determining ΔSosol’n.

Urea(s) Urea(aq)

A final note. The values we determined today for ΔGo are not under standard condiFons, 298 K and 1.0 atm and 1.0 M. How we determined ΔHo

sol’n comes prefy close to being

standardish.

We keep them (the naughts) in here because they are necessary when we calculate ΔGo

sol’n by relaFng it to the

equilibrium constant Kc because at equilibrium,

ΔG = 0 and then: ΔGo

sol’n = -‐RTlnKc

Note: the lab manual leaves off the lifle naughts.

6. What we are doing in lab today.

ΔHo = 14.0 kJ/mol urea ΔGo = -‐6.86 kJ/mol urea ΔSo = 69.5 J/mol K

19 19 19

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 and Conclusion in your own words. Also, sources of error

④  Record observaFons and details as carefully as possible.

⑤  Use the analyFcal balance for massing out the urea for the ΔHo part. Making 60 mL of 1.0 M urea takes about 3.6 g. Do not try to get exactly 3.600 g! Get close, but record exactly how much you used. Examples: 3.604 g or 3.597 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… The value for ΔGo will be different because these are for

1.0 M soluFons.

20

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 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.

7. Your lab report.

Ummm… How much urea did we use anyway?!?!

WOW!

Are these things safe?

Chem Lab with the S1ck People and Bird was created and produced by Dr. Bruce Mafson, Creighton Chemistry. Enjoy it and share it if you wish.

S8ck people inspired by xkcd cartoons by Randall Munroe

(www.xkcd.com)

Documents

Experiment10 2*April*2020* ThermodynamicValuesforUrea

Experiment10 2April2020* ThermodynamicValuesforUrea