dissociation equilibrium.pdf

7/27/2019 Dissociation equilibrium.pdf

1/547PHYWE Systeme GmbH & Co. KG D-37070 Gttingen Laboratory Experiments Chemistry

Dissociation equilibrium 03.09

LEC 03 Chemical equilibrium

Principle and tasks

Carboxylic acids are potential elec-trolytes, which exist in a weaklydissociated condition in aqueoussolutions. The location of the disso-ciation equilibrium is quantitatively

described by the Ka or pKa value,which can be determined with po-tentiometric measurements.

Neutralisation curve of formic acid.

True and potential electrolytes

Strong and weak acids

Law of mass action

Henderson-Hasselbalchequation

Dissociation constant and

pKavalue

Substituent effects

Potentiometry

Set automatic titration with Cobra3 Chem-Unit 43040.88 1

Storage flask for pH-electrodes 18481.20 1

Immersion probe NiCr-Ni, Teflon, 300C 13615.05 1Motor piston burette, 50 ml 36499.93 1

Rubber stopper, d= 18/14 mm, 1 hole 39254.01 1

Magnetic stirrer, mini 47334.93 1

Magnetic stirrer bar, l= 15 mm 46299.01 1

Glass beaker, 50 ml, tall 36001.00 2

Glass beaker, 150 ml, tall 36003.00 1

Precision balance CPA 623S (620 g/0.001 g),set with software 49224.88 1

Volumetric flask, 100 ml 36548.00 6

Volumetric pipette, 5 ml 36577.00 6

Pipettor 36592.00 1

Pipette dish 36589.00 1

Pasteur pipettes 36590.00 1

Rubber bulbs 39275.03 1

Microspoon 33393.00 1

Wash bottle, 500 ml 33931.00 1

Buffer solution, pH 4.62, 1000 ml 30280.70 1

Buffer solution, pH 9.00, 1000 ml 30289.70 1

Formic acid acid 98...100%, 250 ml 30021.25 1

Acetic acid 99100%, pure, 1l 31301.70 1

Monochloracetic acid, 100 g 30060.10 1

Propionic acid, 500 ml 31753.50 1

N-butyric acid, 100 ml 30047.10 1

Lactic acid, 100 ml 30264.10 1

Caustic soda solution, 0.1 M, 1000 ml 48328.70 1Water, distilled, 5 l 31246.81 1

PC, Windows XP or higher

What you need:

What you can learn about

Dissociation equilibrium P3030940


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Related concepts

True and potential electrolytes, strong and weak acids, law ofmass action, Henderson-Hasselbalch equation, dissociationconstant and pKa value, substituent effects, potentiometry.

Principle

Carboxylic acids are potential electrolytes which exist in a weak-ly dissociated condition in aqueous solutions. The location of thedissociation equilibrium is quantitatively described by the Ka orpKa value which can be determined with potentiometric mea-surements.

Tasks

Measure the alteration of the pH value during a titration ofapproximately 0.1 molar aqueous solutions of formic acid, aceticacid, monochloroacetic acid, propionic acid, butyric acid andlactic acid with a 0.1 molar sodium hydroxide solution at con-stant temperature using the Cobra3 system. From the neutrali-

sation curves read the pKa values of the acids and comparethem.

Equipment

Set automatic titration with Cobra3 Chem-Unit 43040.88 1Storage flask for pH electrodes,

filled with 3.0 M KCl solution 18481.20 1

Glass beaker, 50 ml, tall 36001.00 2Glass beaker, 150 ml, tall 36003.00 1Set of precision balance Sartorius CPA623S

and measure software 49224.88 1

Volumetric flask, 100 ml 36548.00 6Volumetric pipette, 5 ml 36577.00 6Pipettor 36592.00 1Pipette dish 36589.00 1Pasteur pipettes 36590.00 1Rubber bulbs 39275.03 1Microspoon 33393.00 1Wash bottle, 500 ml 33931.00 1Buffer solution, pH 4.62, 1000 ml 30280.70 1Buffer solution, pH 9.00, 1000 ml 30289.70 1Formic acid acid 98...100%, 250 ml 30021.25 1Acetic acid 99...100%, pure, 1000 ml 31301.70 1Monochloracetic acid, 100 g 30060.10 1Propionic acid, 500 ml 31753.50 1

N-butyric acid, 100 ml 30047.10 1Lactic acid, 100 ml 30264.10 1Caustic soda solution, 0.1 M, 1000 ml 48328.70 1Water, distilled, 5 l 31246.81 1PC, Windows 95 or higher

PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D-37070 Gttingen P3030940 1

LEC

03.09Dissociation equilibrium

Fig. 1. Experimental set-up.


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Set-up and Procedure

Set up the experiment as shown in Fig. 1.Prepare approximately 0.1 molar solutions of each of the car-boxylic acids which are to be investigated by weighing the

masses of the acids given in Table 1 into 100 ml volumetricflasks and filling them up to the mark with distilled water.

Table 1: The masses of the carboxylic acids R-COOH to beweighed out for 0.1 molar solutions.

Connect the control input for manual key button of the motorpiston burette to the TTL-output of the Cobra3 Chem-Unit withthe cable supplied for this particular purpose. Connect the pHelectrode to the pH-input, and the temperature probe to tem-perature input T1 of the Chem-Unit.Call up the Measure programme in Windows and enter as measuring instrument. Set the measurement parame-ters as shown in Fig. 2. Select as mode in under . Set the display range to1-13 in menu prompt . Set Digital display 1 to , and select, under Diagrams, forDiagram 1, for Diagram 1a, for the dis-play range and . Confirm your entries with.

Now calibrate your pH electrode using two buffers. To do this,enter the appropriate pH value in under themenu prompt , dip the electrode into the buffer andsave with . To calibrate the temperature sensor,

either balance it against a temperature measured with a ther-mometer, or with the level of a temperature probe connected toT2 or T3.After having made all settings, press to reach thefield for the recording of measured values. Arrange the displaysas you want them.Follow the operating instructions supplied with the motor pistonburette to fill it with 0.1 molar sodium hydroxide solution. Set themotor piston burette to , and .Fill 60 ml of distilled water into a 150 ml glass beaker and add amagnetic stirring bar. Pipette 5 ml of the carboxylic acid solutionthat is to be titrated into the beaker. Cut the rubber stopper withcentral hole lengthwise from one side to the hole and fit the tem-

perature probe into it. Fix the dispensing tip of the motor pistonburette, together with the pH electrode and the temperatureprobe, in the electrode holder. Ensure that the sensingdiaphragm of the pH electrode is covered by the solution.Adjust the magnetic stirrer to an intermediate stirring speed,then push to begin the titration. The firstmeasurement (for V = 0.0 ml) is immediately recorded, followingwhich the motor piston burette dispenses the first portion ofsodium hydroxide solution. The titration now proceeds accord-ing to the parameters that have been set. It is automaticallystopped after 10 ml of sodium hydroxide solution have beenadded. On completion of the series of measurements, save thedata with .Thoroughly rinse the beaker and the pH electrode with distilled

water after each titration.Fig. 3 shows the graph as presented by the programme afterhaving stopped the measurement. To have the equivalence pointand the pK displayed, use under the menu prompt .

Theory and Evaluation

Carboxylic acids R-COOH are weak electrolytes, which are onlypartially dissociated in aqueous solutions, i.e.

R-COOH R-COO- + H+

PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D-37070 GttingenP30309402

LEC

03.09 Dissociation equilibrium

Fig. 2: Measurement parameters

Acid R Mass in g

Formic acid H 0.460

Acetic acid CH3 0.601

Monochloracetic acid CH2Cl 0.945

Propionic acid C2H5 0.741

Propionic acid CH(OH)CH3 1.001

n-Butyric acid C3H7 0.881

Fig. 3: Determination of the pKa value of formic acid from theneutralisation curve


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The position of the dissociation equilibrium is quantatively char-acterised by the dissociation or acid constant Ka or the pKavalue, from which it is derived.

(1)

(ai = activity of the substance i. In extremely diluted solutionswith intermolecular or interionic interactions which can beneglected, it is equal to the concentration ci)

pKa = - log Ka (2)

When the formulation (2) and the analogous definition of the pHvalue are taken into consideration and the logarithm is taken, theHenderson-Hasselbalch equation (3) is obtained from (1). Thisnew equation describes the correlation between pH value andthe composition (cR-COOH/ cR-COO-) of buffer systems or the pro-

portion of both forms on the total concentration (c0 = cR-COOH +cR-COO-) of the weak acid for a given acidic strength (pKa).

(3)

During the successively neutralisation of a weak acid, cR-COO-corresponds virtually to the concentration of the salt formed. Incontrast, the equilibrium concentration cR-COOH is identical to theremaining total acid concentration c0.If half of the acid has reacted (half neutralisation), it follows thatcR-COOH = cR-COO- and (3) becomes (3.1).

pKa = pH (3.1)

The pKa value of a weak acid is thus equal to the pH value athalf neutralisation. This can be potentiometrically determined viathe measurement of the cell voltage U between a hydronium-ion-sensitive electrode (glass electrode) and a reference elec-trode (silver chloride electrode), which are available in combina-tion as single-rod glass electrodes (measuring chains).Subsequent to calibration with buffer solutions of known pH, thelinear relationship between pH and U in the measuring sequencein the glass electrode:

U = const. pH + const. (4)

is saved in the Cobra3 Chem-Unit, so that the pH values that

correspond to the measured cell voltages can be immediatelydisplayed.

On completion of the titration, the pKa value can be directlydetermined from the neutralisation curve at half neutralisation bymeans of equation 3.1.At constant temperature and for the same solvent, Ka and pKa

are a function of the nature of the residue (substituent) R.Consequently, compared to R = CH3 , electron-attracting sub-stituents (acceptors) such as R = CH2Cl lead to a facilitated dis-sociation of the proton via a depression of the electron densitywithin the carboxyl group (-I effect) and thus to an elevation ofthe acid constant Ka or a decrease in the pKa value. In contrast,electron-repelling substances (donors) such as R = C2H7 resultin a reduction of the acidic strength via a +I effect.The polar substituent influence can be quantified by empirical-ly determined substituent constants s* which correlate in astatistically significant manner with the determined pK value(Fig. 3). The s* constants, which are interesting in this context,are given in Table 2 together with the pK values of the investi-gated carboxylic acids (taken from the chemical literature) for

T = 298 K.

Table 2: Literature values for the pKa values (T = 298 K) ofselected Carboxylic acids R-COOH and their polar sub-stituent constants.

Data and results

Fig. 2 shows the neutralisation curve for the titration of approxi-mately 0.1 molar formic acid with 0.1 molar NaOH. A value of3.74 is obtained for pKa, and this agrees well with the literaturevalue given in Table 2 for T = 298 K. The analogously determinedpKa values of the other carboxylic acids are shown as a functionof their polar substituent constants in Fig. 3.

pKa pH cRCOOH

cRCOO

Ka aRCOO

aH

aRCOOH

cRCOO

cH

cRCOOH

PHYWE series of publications Laboratory Experiments Chemistry PHYWE SYSTEME GMBH & Co. KG D-37070 Gttingen P3030940 3

LEC

03.09Dissociation equilibrium

R pKa s*

H 3.75 0.490

CH3 4.76 0.000

CH2Cl 2.85 1.050

C2H5 4.86 -0.100

CH(OH)CH3 3.86 0.450

C3H7 4.83 -0.115


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03.09 Dissociation equilibrium

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