Nur Syazwana Sharim

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123.220 Experiment 7

Enthalpy of Protonation of Glycine

Introduction

Glycine, the smallest amino acid is known to be ambivalent (able to be either inside or outside of the protein molecule) due to the presence of amine (-NH2) and carboxylate (-COOH) groups in the molecule. Glycine can behave as base to produce (+NH3CH2COO) ion in acidic solution and able to act as acid to produce (NH2CH2COO-) ion in basic solutions.

In aqueous solution, glycine may be present as a dipolar ion or better called zwitterion (+NH3CH2CO2-). The equilibrium constant of the protonation of the amine functional group is shown below:

(NH2CH2CO2-) + H+ → (+NH3CH2CO2-)

From the reaction, K can be expressed as,

Based on the titration of this species with a base, the enthalpy change for the reaction can be determined using the van’t Hoff equation,

and by assuming that ∆ H ° is independent of T, integrating the above equation gives,

−ΔH∘

R( 1T 2

− 1T 1

) = ln k2 − ln k1

Where:K1is the equilibrium constant at T 1,K2 is the equilibrium constant at T 2,R is a constant = 8.314 J/mol K.

Nur Syazwana Sharim

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123.220 Experiment 7

Procedure

0.3043 g of glycine was weighed and transferred into 100cm3 volumetric flask. 10cm3 of 2.0 mol/

d m3 KCl was then added and made to the mark using distilled water. The water bath was set to 25 C. The apparatus was set up as Figure 1.

25cm3 of the glycine/KCl solution was pipetted into clean dry titration cell. The solution was titrated

using 0.10 mol/dm3 NaOh/0.10 mol/dm3 KCl solution. pH of titrant volumes of 13 values up to 10

c m3was recorded and the pH titration curve was constructed. The steps were repeated with the water bath is set to 43.5 ᵒC.

Further explanation of the procedures may be referred to Advanced Chemistry Lab Manual page 47 and 48.

Nur Syazwana Sharim

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123.220 Experiment 7

Results and Discussions

The data collected were tabulated and treated according to both temperatures 25°C and 43.5ᵒC set during the experiment. Using the values calculated, the Kc value for each temperature was averaged and the standard deviation was obtained.

In Table 1 and Table 2, the Kc values for NaOH volume of 0 mL gave negative values of -4.05E+12 and -2.53E+12 respectively. Inconsistencies of the Kc values for both experimented temperatures were also notified in both tables. These may be due to improper cleansing of the titration cell giving inaccurate pH readings.

Vol/mL pH [H+] tvol TA alka [-OH] TH para-1 para-2 Kc0 7.06 8.70964E-08 25 0.040537 0 0.040537 4.05E-02 -1.15E-07 -4.05E+12

0.5 8.03 9.33254E-09 25.5 0.039742 0.0018431 0.037899 3.79E-02 1.84E-03 2.21E+091 8.65 2.23872E-09 26 0.038978 0.0036154 0.035363 3.54E-02 3.61E-03 4.38E+09

1.5 9.01 9.77237E-10 26.5 0.038243 0.0053208 0.032922 3.29E-02 5.30E-03 6.36E+092.05 9.26 5.49541E-10 27.05 0.037465 0.0071238 0.030341 3.04E-02 7.09E-03 7.79E+09

3 9.46 3.46737E-10 28 0.036194 0.0100714 0.026122 2.62E-02 1.00E-02 7.53E+094.2 9.67 2.13796E-10 29.2 0.034706 0.0135205 0.021186 2.13E-02 1.34E-02 7.40E+095 9.85 1.41254E-10 30 0.033781 0.0156667 0.018114 1.82E-02 1.55E-02 8.31E+09

6.4 10.12 7.58578E-11 31.4 0.032275 0.0191592 0.013116 1.33E-02 1.89E-02 9.30E+097.1 10.22 6.0256E-11 32.1 0.031571 0.0207913 0.01078 1.11E-02 2.05E-02 8.96E+098 10.43 3.71535E-11 33 0.03071 0.0227879 0.007922 8.40E-03 2.23E-02 1.01E+109 10.71 1.94984E-11 34 0.029807 0.0248824 0.004924 5.83E-03 2.40E-02 1.25E+10

10 11.07 8.51138E-12 35 0.028955 0.0268571 0.002098 4.17E-03 2.48E-02 1.97E+10

Table 1: Data of the experiment at 25ᵒC

Vol/mL pH [H+] tvol TA alka [-OH] TH para-1 para-2 Kc0 7.4 3.98107E-08 25 0.040537 0 0.040537 4.05E-02 -4.02E-07 -2.53E+12

0.5 8.16 6.91831E-09 25.5 0.039742 0.0018431 0.037899 3.79E-02 1.84E-03 2.98E+091 8.31 4.89779E-09 26 0.038978 0.0036154 0.035363 3.54E-02 3.61E-03 2.00E+09

1.5 8.42 3.80189E-09 26.5 0.038243 0.0053208 0.032922 3.29E-02 5.32E-03 1.63E+092 8.62 2.39883E-09 27 0.037534 0.006963 0.030571 3.06E-02 6.96E-03 1.83E+093 9.01 9.77237E-10 28 0.036194 0.0100714 0.026122 2.61E-02 1.01E-02 2.66E+094 9.19 6.45654E-10 29 0.034946 0.0129655 0.02198 2.20E-02 1.29E-02 2.63E+095 9.33 4.67735E-10 30 0.033781 0.0156667 0.018114 1.82E-02 1.56E-02 2.48E+096 9.52 3.01995E-10 31 0.032691 0.0181935 0.014498 1.46E-02 1.81E-02 2.66E+097 9.66 2.18776E-10 32 0.03167 0.0205625 0.011107 1.12E-02 2.05E-02 2.50E+098 9.74 1.8197E-10 33 0.03071 0.0227879 0.007922 8.02E-03 2.27E-02 1.94E+099 9.84 1.44544E-10 34 0.029807 0.0248824 0.004924 5.05E-03 2.48E-02 1.41E+09

10 10.16 6.91831E-11 35 0.028955 0.0268571 0.002098 2.35E-03 2.66E-02 1.28E+09

Table 2: Data of the experiment at 43.5 ᵒC

Nur Syazwana Sharim

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123.220 Experiment 7

temperature, °C average Kc standard deviation25 8.07E+09 76988075

43.5 2.59E+09 88959032.91

Table 3: average and standard deviation values at each temperature

Based on Table 3 above, the average Kc value at 25 °C is higher compared to that at 43.5 ᵒC. This indicates that more dipolar ion (zwitterion) was produced at 25 °C compared when at 43.5°C.

The titration curve for both temperatures were produced and presented in Graph 1 and Graph 2. Both graphs gave increasing linear trend lines indicating that as the volume of NaOH (base) increases, the pH of glycine increases (becoming more alkaline).

0 2 4 6 8 10 120

2

4

6

8

10

12

NaOH (mL) vs pH at 25ᵒC

pH

volu

me

of N

aOH

Graph 1: Total volume of NaOH (mL) against pH at 25ᵒC

Nur Syazwana Sharim

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123.220 Experiment 7

0 2 4 6 8 10 120

2

4

6

8

10

12

NaOH (mL) vs pH at 43.5 ᵒC

pH

Volu

me

of N

aOH

(mL)

Graph 2: Total volume of NaOH (mL) against pH at 43.5ᵒC

Calculations

Nur Syazwana Sharim

11137660

123.220 Experiment 7

Exercises

1. Literature value of ∆ H °=43.62kJ /mol (R. M. Izatt, 1992 retrieved on 27th September 2012)Actual value calculated:

2.

Conclusion

Nur Syazwana Sharim

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123.220 Experiment 7

As a conclusion, protonation of glycine depends on the temperature and thus, affecting the enthalpy of its protonation.