1

Done by: Malik Mohammed AL-Nabhani

ID#: 89770

Date: 24/March/2012 Submitted to: Dr: Emad Khudaish.

Sultan Qaboos University

Collage of science

Department of chemistry

Enthalpy of protonation of

glycine

Physical chemistry laboratory I CHEM3335

Lab report (7)

2

)(

)2(..........)()(

)(

)1......(..........

2222223

223223

mediabasicincidaasbehavesglycine

OHCOCHNHOHCOCHNH

mediaacidicinbaseaasbehavesglycine

HCOCHNHHCOCHNH

objective

The main aim of this experiment is to determine the enthalpy of glycine

using equilibrium constant measurement.

Introduction

Glycine is an amino acid and its formula is (NH2CH2CO2H). it has both

basic group and acidic group. Whereas amino (-NH2) group behaves as

a base, (-COOH) behaves as an acid. Basic group is stronger than acidic

group because its )100.5( 4bK ,whereas the Kb of the acidic group

)100.5( 10 . Glycine molecules have a very large dipole moment when

they are dissolved in water because of significant charge separation in the

molecules. Reaction of glycine takes place in both acidic media and basic

media. That is, it reacts as a base in acidic solution and vice versa. Both

reactions are proceeding as follow:

It is clear from the previous equations that both reactions are reversible.

That is, after a certain time the reaction will be proceeded in both

direction. The speed of the direction depends on the value of equilibrium

constant (K). To determine the value of (K), we shall titrate the glycine

with a strong base like NaOH. The chemical equation we need is shown

below:

Where (NH2CH2CO2-) is a conjugate base of (

+NH3CH2CO2

-). Then, the

equilibrium constant can be presented by this equation.

)3...(..........)()( 223222

COCHNHHCOCHNH

)4...(..........222

223

HA

HA

HCOCHNH

COCHNHK

3

If the titration is carried out at two different temperatures the enthalpy

)( H change reaction (3) and its value can be evaluated using van't Hoff

equation.

If we assume that ( H ) is independent on temperature in the range

(20 to 50 Co), equation (5) can be integrated to be like that:

Where,

- ( H ) enthalpy of protonation of glycine.

- K1 equilibrium constant at (T1).

- K2 equilibrium constant at (T2).

- R 8.314J/mol.k .

Experimental equipments and material

Material and equipments used in the experiment:

- Jacketed cell.

- Pipette 25ml.

- pH meter.

- Burette.

- 2.0 KCl.

- Volumetric flask 100ml.

- Distilled water.

- 0.10M NaOH and 0.300g of Glycine.

)5(.............)/1(

ln

R

H

Td

Kd

)11

(lnln22

12TTR

HKK

4

Procedure

1) About (0.300g ± 0.010) of glycine is weighed and then transferred

quantitatively to a 100ml volumetric flask.

2) 10ml of 2.0 M KCl is added by a pipette to the volumetric flask.

3) The resulted solution is diluted to the mark using distilled water.

4) Take 25ml of glycine solution and place it in the jacketed |(titration)

cell connected to the circulating water bath and set the temperature of

water bath to 25Co.

5) Place the magnetic stirrer bar.

6) Titrate the glycine solution against (0.10 M NaOH/ 0.10 M KCl)

solution.

7) Record the pH of titration at each set of adding (0.10 M NaOH/ 0.10 M

KCl) solution.

8) Construct a pH titration curve.

9) Repeat steps (4 to 8) but at 45C instead of 25C.

results

Volume of

titrant (mL)

Total volume

of titrant (mL) pH at 25 0C [H] [OH]

0 0 6.83 1.47911E-07 1.1899E-07

0.5 0.5 8.17 6.76083E-09 2.6032E-06

0.5 1.0 8.52 3.01995E-09 5.8279E-06

1.0 2.0 8.89 1.28825E-09 1.3662E-05

1.0 3.0 9.14 7.24436E-10 2.4295E-05

1.0 4.0 9.33 4.67735E-10 3.7628E-05

1.0 5.0 9.51 3.0903E-10 5.6952E-05

1.0 6.0 9.68 2.0893E-10 8.4239E-05

1.0 7.0 9.85 1.41254E-10 0.0001246

1.0 8.0 10.02 9.54993E-11 0.00018429

1.0 9.0 10.22 6.0256E-11 0.00029209

1.0 10.0 10.48 3.31131E-11 0.00053151

Table.1: Values obtained from the experiment at T= 25C

5

TG [NaOH] TH [HA] [A] k

0.040109 0 0.040109 0.040109 2.89202E-08 9.3765E+12

0.039323 0.001961 0.037362 0.037365 0.001958188 2.8223E+09

0.038567 0.003846 0.03472 0.034726 0.003840329 2.9943E+09

0.037138 0.007407 0.029731 0.029744 0.007393747 3.1228E+09

0.035812 0.010714 0.025098 0.025122 0.010689992 3.2439E+09

0.034577 0.013793 0.020784 0.020821 0.013755476 3.2362E+09

0.033424 0.016667 0.016758 0.016815 0.016609714 3.2759E+09

0.032346 0.019355 0.012991 0.013076 0.0192706 3.2476E+09

0.031335 0.021875 0.00946 0.009585 0.021750402 3.1198E+09

0.030386 0.024242 0.006143 0.006328 0.02405813 2.7541E+09

0.029492 0.026471 0.003021 0.003314 0.026178501 2.1006E+09

0.028649 0.028571 7.8E-05 0.00061 0.028039917 6.5648E+08

Volume of

titrant (mL)

Total volume of

titrant (mL) pH at 45 0C [H] [OH]

0 0 6.58 2.63027E-07 2.7184E-07

0.5 0.5 7.85 1.41254E-08 5.0618E-06

0.5 1.0 8.2 6.30957E-09 1.1332E-05

1.0 2.0 8.58 2.63027E-09 2.7184E-05

1.0 3.0 8.86 1.38038E-09 5.1797E-05

1.0 4.0 9.06 8.70964E-10 8.2093E-05

1.0 5.0 9.24 5.7544E-10 0.00012425

1.0 6.0 9.41 3.89045E-10 0.00018378

1.0 7.0 9.58 2.63027E-10 0.00027184

1.0 8.0 9.77 1.69824E-10 0.00042102

1.0 9.0 9.97 1.07152E-10 0.00066728

1.0 10.0 10.25 5.62341E-11 0.00127147

Kw 1.76E-14

mglycine 0.3011

nglycine 0.004

Cglycine 0.04

Average K1 7.8393E+11

Ln k1 27.3875

Table.2: Values obtained from the experiment at T= 25C

Table.3: Values obtained from the experiment at T= 25C

Table.4: Values obtained from the experiment at T= 45C

6

TG [NaOH] TH [HA] [A] K

0.040109 0 0.040109 0.040109 8.809E-09 1.7312E+13

0.039323 0.001961 0.037362 0.037367 0.001956 1.3526E+09

0.038567 0.003846 0.034720 0.034732 0.003835 1.4354E+09

0.037138 0.007407 0.029731 0.029758 0.007380 1.5330E+09

0.035812 0.010714 0.025098 0.025149 0.010662 1.7087E+09

0.034577 0.013793 0.020784 0.020866 0.013711 1.7473E+09

0.033424 0.016667 0.016758 0.016882 0.016542 1.7735E+09

0.032346 0.019355 0.012991 0.013175 0.019171 1.7665E+09

0.031335 0.021875 0.009460 0.009732 0.021603 1.7127E+09

0.030386 0.024242 0.006143 0.006564 0.023821 1.6227E+09

0.029492 0.026471 0.003021 0.003689 0.025803 1.3342E+09

0.028649 0.028571 0.000078 0.001349 0.027300 8.7904E+08

0

2

4

6

8

10

12

0 0.01 0.02 0.03

pH

[NaOH/KCl]

pH at 25 C

pH at 45 C

Kw 7.15E-14

mglycine 0.3011

nglycine 0.004

Cglycine 0.043

Average K2 1.4440E+12

Ln k2 27.9984

Table.5: Values obtained from the experiment at T= 45C

Table.6: Values obtained from the experiment at T= 45C

graph.1: [NaOH/KCl] at T= 25 0C and 45

0C

7

Sample calculation

Several equations are used in order to get the value of ∆H.

TG represents the mass balance of the total glycine concentration.

TG can be calculated from the mass of glycine used (stock solution) and

after the addition of titrant (dilution)

Mass of glycine used = 3.011g

Molar mass (NH2CH2CO2H) = 75.07g/mol

lmolL

titrantofadditionanybeforeglycineofionconcentratfirst

molmolg

g

massmolar

massglycineofmoles

/04.01.0

004.0

004.0/07.75

3011.0

Its molarity after adding 0.5 ml of NaOH/KCl :

The concentration of glycine, TG at 25C and 45C are the same.

Initial concentration of NaOH is 0.1 M

After adding the first portion af NaOH/KCl to the titration cell, its

concentration can be calculated by applying this equation:

After we obtain the value of [NaOH] and TG , the value of (TH) which is

the mass balance for the total available acid form can be easily calculated

e.g: TH = 0.039323 M – 1.961×10-3

M = 3.72E-02 M

HAATG

Mml

mlMTG 0.039323

5.25

2504.0

Mml

mlMNaOH

V

VMM

VMVM

3

2

112

2211

10961.1)5.25(

)5.0()1.0(

NaOHTT GH

8

[H+] can be calculated from pH.

pH = -log [H+]

e.g: pH =8.17 [H+] = 10

-8.17 = 6.76083E-09 M

Since Kw at 25C = 1.76×10-14

M 06-2.6032E 09-6.76083E

1076.1 14

H

KOH w

[HA] can be calculated now:

To find [A], we apply this equation:

[A] = TG - [HA] = 3.92E-02 - 3.73E-02M = 1.95E-03 M

After all these equations we can calculate the value of equilibrium

constant (∆H) at 25C.

09+2.8223E)0947.3()0395.1(

0273.31

EE

E

HA

HAK

We do the same calculation to get (K2, K3 ………………) at 25C.

The same calculations are also carried out to get the value of equilibrium

constant at 45C. Then, we take the average of K1 & K2.

M 02-E 3.7365 09-6.76083E 06-2.6032E 02-3.72E

MMM

HOHTHA H

molkj

molJH

kkkmolj

TTR

HaverageKaverageK

/0.24

/0636.24

)298

1

318

1(

./314.8

H11)+ln(7.8393E-12)+1.4440Eln(

)11

()(ln)(ln12

12

9

Discussion

The literature value of enthalpy of protonation of glycine is -44.2Kj/mol,

and that one was taken in this experiment is -24.0 Kj/ mol. It's significant

that how far our value from the literature. This difference is as a rustle of

errors due the experiment as systematic errors or either random errors.

pH meter is considered as systematic error source where is the pH reading

was fluctuated. That is, the obtained readings from pH meter may not

accurate. Moreover, getting the reading of the pH directly without waiting

good pH reading and this could be consider as random errors

Error analysis

1) In this experiment, it may not be that accurate due to some

systematic errors and human errors:

2) Standardization error (in preparing of the glycine solution)

3) Systematic error in the instrument that we used it in the experiment.

4) Pipetting errors.(adding more or less volume)

5) Errors in the pH meter.

6) Errors in the concentration of titrant solution & KCl solution.

References

1) Experimental physical chemistry, A. M Halpern, 2nd ED,1988.

2) Handout of enthalpy protonation of glycine.

3) Physical chemistry, Ira N .Levine, 6th ED.