Department of Chemical Science

UDEC 2194 CHEMISTRY LABORATORY III

Faculty : Faculty of Science

Course : Bachelor of Science (Hons) Chemistry

Year : Year 2 Trimester 3

Session : Jan 2012

Component A : Thermodynamic & Electrochemistry

EXPERIMENT No. : 4

Title : Determination of the Conductances of Strong & Weak Electrolytes

Objective of experiment:

-To measure the conductances of potassium chloride, hydrochloric acid, sodium chloride and sodium acetate.

-To determine the disscociation constant of acetic acid.

Lecturer : Dr. Sim Yoke Leng

Name : Hue Bit Kie 0906785

Group members: : Low Hui Yee 0907510

Kho Jia Wen 0907014

Kong Bee Ying 0903723

Ng Yu Jen 1004559

Marks

Title:

Determination of the Conductances of Strong & Weak Electrolytes

Objective:

a) To measure the conductances of potassium chloride, hydrochloric acid, sodium chloride and sodium acetate.

b) To determine the disscociation constant of acetic acid.c) To study the changes in molar conductivities of strong electrolyte and weak

electrolyte on dilution.

Introduction:

Electrolyte solution can conduct electricity because it forms mobile ions when dissolved in water or other solvents.

The acid-dissociation equilibrium constant, Ka determines types of electrolytes: Strong electrolytes and weak electrolytes. The former ionize essentially completely in solvent and the latter ionize partially only. The migration of these mobile ions under electric field enables the electrolyte solution conduct electric current. According to Ohm ’s Law,

E = IR

Where E is the potential difference, I is the current and R is the resistance. The electrical resistance of any solution is inversely proportional to its area of cross section, A.

R α 1A

Reciprocal of resistance of the solutions is called conductance (L). It is a measure of tendency of a material to allow the flow of current through it.

L (ohm-1 or 1R

Conductivity or specific conductance (X) in cm is the conductance offered by a material of one centimeter long and its area of cross section 1cm2. Cell constant (k) is the ratio of distance between the electrode, d to the cross-sectional area, A of the electrodes.

X cmd

ARkL

Where A and d is the area and distance between the electrodes of the measurement cell,.

The conductance of electrolytic solution containing one mole of electrolyte is defined as molar conductivity (

cm2 mol-1) = XC

= kLC

There are a number of factors controlling the conductance of electrolyte: nature of electrolyte, temperature, nature of solvent, concentration and ionic size and mobility. Nature of electrolyte is referred to degree of dissociation of electrolyte. Generally, conductance is proportional to temperature due to increase in extent of ionization. Thirdly, conductance decreases if the viscosity of solvent is high due to increase in difficulty of movement of ions. Besides, specific conductance (X) increases when the solution is more concentrated because the number of ions per unit volume increases. Lastly, since a larger ion diffuses slower hence conductivity of larger ion is lower. Example:

Limiting equivalent conductivity (0) is the point at which the equivalent conductivity reaches a maximum value at certain dilution and does not change upon further dilution. The molar conductivity of weak electrolyte increases steeper than strong

electrolyte when the solutions become dilute. From Onsager law, Kohlrausch’s law and

Ostwald dilution law, the 0 strong electrolyte and weak electrolyte can be calculated.

Materials:

0.2000M potassium chloride solution, 0.1000M hydrochloric acid, 0.1000M sodium chloride solution, 0.1000M acetic acid, 0.1000M sodium acetate solution.

Glassware/Apparatus:

Conductivity meter, 100mL volumetric flask, pipette

Experimental Procedure:

1. Determination of cell constant

The conductance (l) of 0.2000M potassium chloride solution is measured. ↓

The specific conductance (X) of this solution is 2.768x10-3 ohm-1cm-1.↓

The cell constant (k) is determined.

2. Measurement of conductance

From the solution of acetic acid provided, conductivity water solution of 0.0500, 0.0250, 0.0125, 0.00625, 0.00312, 0.00156 and 0.00078M are prepared by

successive dilution.↓

The conductances of these solutions are measured.↓

The steps are repeated with hydrochloric acid, sodium chloride and sodium acetate.

↓The conductance of the water used is measured.

Result:

1. Conductances (L) of 0.2000M KCl =2.47 x10-2

XCkL

C

k = XL

Given that specific conductance, X of KCl = 2.768x10-3 ohm-1cm-1

k = 2.768 x 10−3 ohm−1 cm−1

2.47 x10−2 ohm−1

=0.112cm-1

2. The conductance measured is the total conductance of electrolyte and distilled water. Conductances (L) of distilled water is 3.06 x10-6 After deduction, the conductance of electrolyte is as followed:

Table 1: Conductances of ElectrolytesConductances, L ()

C (mol dm-3) CH3COOH HCl NaCl CH3COONa0.1000 527.94 x 10-6 38666.94 x 10-6 11156.94 x 10-6 7086.94 x 10-6

0.0500 373.94 x 10-6 19956.94 x 10-6 5816.94 x 10-6 3726.94 x 10-6

0.0250 257.94 x 10-6 10126.94 x 10-6 3076.94 x 10-6 2036.94 x 10-6

0.0125 179.34 x 10-6 5080.94 x 10-6 1475.94 x 10-6 1024.94 x 10-6

0.00625 131.34 x 10-6 2537.94 x 10-6 792.94 x 10-6 512.94 x 10-6

0.00312 88.44 x 10-6 1252.94 x 10-6 408.94 x 10-6 255.94 x 10-6

0.00156 56.14 x 10-6 632.84 x 10-6 175.64 x 10-6 130.04 x 10-6

0.00078 36.84 x 10-6 517.34 x 10-6 35.94 x 10-6 77.64 x 10-6

3.Table 2: Specific Conductances of Electrolyte

Specific Conductances, X (cm)C (mol dm-3) CH3COOH HCl NaCl CH3COONa0.1000 5.913 x 10-5 4.331 x 10-3 1.250 x 10-3 7.937 x 10-4

0.0500 4.188 x 10-5 2.235 x 10-3 6.515 x 10-4 4.174 x 10-4

0.0250 2.889 x 10-5 1.134 x 10-3 3.446 x 10-4 2.281 x 10-4

0.0125 2.009 x 10-5 5.691 x 10-4 1.653 x 10-4 1.148 x 10-4

0.00625 1.471 x 10-5 2.842 x 10-4 8.881 x 10-5 5.745 x 10-5

0.00312 9.905 x 10-6 1.403 x 10-4 4.580 x 10-5 2.867 x 10-5

0.00156 6.288 x 10-6 7.088 x 10-5 1.967 x 10-5 1.456 x 10-5

0.00078 4.126 x 10-6 5.794 x 10-5 4.025 x 10-6 8.696 x 10-6

Specific conductance, X = kL

Example:

a) For 0.1000M of CH3COOH, X = (0.112cm-1)( 527.94 x 10-65.913 x 10-5cm-1

b) For 0.0050M of HCl, X = (0.112cm-1)( 19956.94 x 10-62.235 x 10-3cm-1

4.

C (mol cm-3) C1/2 (mol cm-3)1/2

Molar Conductivity, cm2 mol-1)CH3COOH HCl NaCl CH3COONa

1.000 x10-4 0.0100 0.5913 43.31 12.50 7.9370.500 x10-4 0.0071 0.8376 44.70 13.03 8.3480.250 x10-4 0.0050 1.1556 45.36 13.78 9.1240.125 x10-4 0.0035 1.6072 45.53 13.22 9.1840.0625 x10-4 0.0025 2.3536 45.47 14.21 9.1920.0312 x10-4 0.0018 3.1747 44.97 14.68 9.1890.0156 x10-4 0.0012 4.0308 45.44 12.61 9.3340.0078 x10-4 0.0009 5.2900 74.28 5.16 11.149

Molar Conductivity, cm2 mol-1)

=XC

Example:

a) For 25.00 mol cm-3 NaCl, 3.446 x10−4 cm−1

0.2500 x10−4 mol cm−313.78 cm2 mol-1

b) For 12.50 mol cm-3 NaCl, 1.148 x 10−4 ohm−1 cm−1

0.1250 10−4 mol cm−3 9.184 cm2 mol-1

5.

0 0.002 0.004 0.006 0.008 0.01 0.0120

1

2

3

4

5

6

f(x) = − 445.303974221584 x + 4.16209517884123

Graph 1: L vs C1/2 for CH3COOH

C1/2 (mol cm-3)1/2

Mol

ar C

ondu

ctivi

ty, L

( W-1

cm

2 m

ol-

1 )

For strong electrolyte: C1/2

0 0.002 0.004 0.006 0.008 0.01 0.0120

10

20

30

40

50

60

70

80

f(x) = − 1463.37352360709 x + 54.4885549980947R² = 0.20164858695361

Graph 2: L vs C1/2 for HCl

C1/2 (mol cm-3)1/2

Mol

ar C

ondu

ctivi

ty, L

( W-1

cm

2 m

ol-

1 )

of HCl cm2 mol-1

0 0.002 0.004 0.006 0.008 0.01 0.0120

2

4

6

8

10

12

14

16

f(x) = 249.181953279057 x + 11.4015861184655

Graph 3: L vs C1/2 for NaCl

C1/2 (mol cm-3)1/2

Mol

ar C

ondu

ctivi

ty, L

( W-1

cm

2 m

ol-1

)

of NaCl = 11.40 cm2 mol-1

0 0.002 0.004 0.006 0.008 0.01 0.0120

2

4

6

8

10

12

f(x) = − 234.312854986166 x + 10.1197864674409

Graph 4: L vs C1/2 for CH3COONa

C1/2 (mol cm-3)1/2

Mol

ar C

ondu

ctivi

ty, L

( W-1

cm

2 m

ol-

1)

of CH3COONa = 10.12 cm2 mol-1

6. Kohlrausch’s Law:

0CH3COOH  = λ0H+ + λ0

CH3COO- 

=λ0H+ + λ0

Cl- +λ0Na+ + λ0

CH3COO- - λ0Na+ - λ0

Cl- …

=λ0HCl + λ0

CH3COONa - λ0NaCl

=(54.48+10.12-11.40) cm2 mol-1

= 53.20 cm2 mol-1

7. Determine degree of dissociation of CH3COOH at concentrations of

a) 0.0500M

Degree of dissociation, = 0

= 0.8376 ohm−1 cm2mol−1

53.20 ohm−1 cm2 mol−1 = 0.0157

b) 0.0125M

Degree of dissociation, = 0

= 1.6072 ohm−1 cm2 mol−1

53.20 ohm−1cm2 mol−1 = 0.0302

c) 0.00156M

Degree of dissociation, = 0

= 4.0308 ohm−1 cm2mol−1

53.20 cm2mol−1 = 0.0758

8. Calculate dissociation constant, ka of CH3COOH.

a) 0.0500M

ka = ❑2C

1−¿¿ = 0.01572 x0.0500 M

1−0.0157 = 1.2521X10 -5M

b) 0.0125M

ka = ❑2C

1−¿¿ = 0.03022 x 0.0125 M

1−0.0302 = 1.1756x10-5M

c) 0.00156M

ka = ❑2C

1−¿¿ = 0.07582 x 0.00156 M

1−0.0758 = 0.9698X10 -6M

9. Ostwald dilution law:

1

= 1

0 +

C ❑

ka 0❑2

1/¿ C(mol cm-3)¿1.6912 5.913 x 10-5

1.1939 4.188 x 10-5

0.8654 2.889 x 10-5

0.6222 2.009 x 10-5

0.4249 1.471 x 10-5

0.3150 9.905 x 10-6

0.2481 6.288 x 10-6

0.1890 4.126 x 10-6

10.

0 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0.000070

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

f(x) = 27462.4415965955 x + 0.0586435380787295R² = 0.998531274751755

Graph of 1/Λ against CΛ

y-intercept of the graph = 1

0 = 0.058ohm cm−2 mol❑

0 =1

0.058 ohm cm−2mol❑=17.2413 ohm−1cm2 mol−1

Gradient of the graph = 1

ka 0❑2 = 27462 ohm2 mol cm-1

ka = 1

0❑2 x 27462 ohm2 mol cm−1

= 1

17.2413❑2 ohm−1cm2 mol−1 x27462 ohm2 mol cm−1

= 1.2250x10-7 mol cm-3

=1.2250x10-4 mol dm-3

11. Calculate Percentage Error

= ¿ ExperimentalValue−TheoreticalValue∨ ¿TheoreticalValue

¿ x 100%

Acetic acid:

a) ka

According to Atkins’ Physical Chemistry (8thedition), the theoretical value of dissociation constant of acetic acid (ka) is 1.4 x 10-5M.

Theoretical value of ka = 1.4x 10-5MExperimental value of ka = 1.2250x10-4 M

Percentage error = 1.2521 x 10−5 M−(1.4 x 10−5 M )

1.4 x10−5 M x 100% = 10.56%

b)0

From Lab Manual, limiting ionic conductances in water at 298K (mSm2mol-1)

H+ 34.96CH3COO- 4.09

Theoretical value of0 +¿ ¿−¿¿

= 34.96 + 4.09

= 39.05 mS m2 mol-1

=390.5ohm−1 cm2mol−1

Experimental value of 0 ¿53.20 ohm−1 cm2mol−1

Percentage error = ¿53.20−(390.5)¿ohm−1 cm2mol−1

390.5 ohm−1 cm2mol−1 x 100% = 86.38 %

Discussion:

After calibration of conductivity meter, the conductivity meter is ready to be used for this experiment. In the first part of experiment, the cell constant is determined by measure the conductance (L) of 0.2000M potassium chloride solution. Since the specific conductance (X) of KCl is known as 2.768x10-3ohm-1cm-1, the cell constant is calculated by using this equation:

kLC

This cell constant value is used for determination of the conductivity of all solutions.

The electrolytes examined are acetic acid, hydrochloric acid, sodium chloride and sodium acetate. All of them have initial concentration of 0.1M. Different concentrations of the electrolytes are prepared by dilution technique. Next, the conductance of each solution is measured. Actually the result is contributed from the electrolyte and distilled water, to find the conductance of pure electrolyte, conductance of distilled water (3.06 x10-6 is deducted from the result.

Conductance values (L) provide more information than simple ion concentrations. From the result of conductances of each electrolyte, it is realized that HCl has the highest conductance while acetic acid has the lowest value of conductance. This is due to the degree of ionization. HCl is the strongest acid, so it can ionize completely to produce most number of free moving ions. Besides, as the concentrations of electrolytes decrease from 0.1M to 0.00078M, the conductance of each electrolyte shows a decreasing trend. This is because conductance is directly proportional to concentration of ions.

Conductivity (X) is conductance of 1cm3 of solution. In other words, it depends on the number of ions present in unit volume. It is calculated from the conductance in previous part by using this equation:

X = kL

Conductivity indicates the charges and mobility of all ions presents in solution other than their concentrations. From the table of conductivity, it is noticed that, decrease in concentration is always followed by a decrease in conductivity no matter which type of

the electrolyte is. The main reason is concentration of ions per cubic cm reduced when the solution is diluted, hence this lead to decrease in conductivity as well.

To compare the solutions easier, molar conductivity are calculated because it different with conductivity, which depend on concentration. Next, molar conductivity is found out by using this formula:

XC

It is the product of conductivity and volume of the solution containing 1 mole of the electrolyte. From the table of molar conductivity, increasing trend is observed when concentration decreases. This is because on dilution, conductivity decreases but decrease in concentration is much greater than the decrease in conductivity, hence molar conductivity still will increases with dilution. Dilution causes the degree of ionization increase, so more and more electrolytes undergo dissociation. When there is no more ions produced on further dilution, a point called limiting equivalent conductivity, is reached. The degree of dissociation () at a given concentration C is given by:

= 0

At high concentration, molar conductivity of strong electrolyte (HCl, NaCl and CH3COONa) is higher than weak electrolyte (acetic acid). However, strong electrolyte does not show significantly change in molar conductivities upon dilution as weak electrolyte. This can be proven by smaller gradient of the three strong electrolyte in Graph 2,3 and 4. This is because strong electrolytes have already completely ionized in the solution at all concentrations. The increase of molar conductivities with dilution is because of decrease in interionic force. Interionic force is force of attraction between ions of the opposite charges. It is stronger when concentration is high. When electrolyte is diluted, the distance between ions is longer and more ions are available to transfer electricity. When concentration of solution approaches zero, the interionic interaction become negligible and molar conductivity reaches the limiting value, 0 at infinite dilution. For weak electrolyte, the variation of is greater because degree of dissociation increase the number of ions.

According to Debye-Huckel theory, the molar conductivity of strong electrolyte is given by:

= o – (A+B o) C1/2

To obtain the limiting molar conductivity of electrolyte, graph of molar conductivity against square root of concentration is plotted. However, the o of acetic acid cannot

obtained in this way because the variation of o with C1/2 is very large and o at infinite dilution cannot obtained by extrapolation. Thus, it can be calculated from Kohlrausch’s law. The molar conductivity at infinite dilution for a given electrolyte can be expressed as the sum of the concentration from its individual ions.

λ0  = λ+0 + λ+

0 

Hence, 0 of CH3COOH (71211 cm2 mol-1) can be determined from 0of the three strong electrolytes. Furthermore, the degree of dissociation of CH3COOH  at 0.0500, 0.00125 and 0.00156 M are 0.0157, 0.0302 and 0.0758 respectively. This proves that degree of dissociation increases on dilution. The dissociation constant (ka) is 9.3755X10 -

6M

By using Ostwald dilution law, the value of 0 and of CH3COOH are determined from the graph.

1

= 1

0 +

C ❑

ka 0❑2

Conclusion:

As concentration of electrolyte decreases, the conductance and conductivity decrease because less ions are available to conduct electricity. However, molar conductivity increases on dilution of electrolyte because more and more electrolyte dissociates. Increase of molar conductivity of strong electrolyte is less when compared with weak electrolyte because it is completely dissociates at all the concentration. By applying Debye-Huckel theory, the limiting molar conductivity of strong electrolyte can be determined by plotting a graph of o against C1/2. Since the percent ionization and the ion mobilities are both changing with concentration, the limiting molar conductivities of acetic acid has to be determined by using Kohlrausch’s law.

Reference:

1. Martin S.S (2005). Chemistry, The Molecular Nature of Matter and Changes, fifth edition, Mc Graw Hill.

2. James E.B (2009). Chemistry, fifth edition, John Wiley & Sons, Inc3. Atkins P.W & J.De Paula (2002), Physical Chemistry, eight edition, Oxford. 4. Electrochemistry, Retrieved on 10 March 2012, from

http://www.docstoc.com/docs/10624136/Electrochemistry5. Conductance, Specific conductance and Molar Conductance, Retrieved on 10

March 2012, from

http://www.askiitians.com/iit-jee-chemistry/physical-chemistry/Electrolytic-Conductance-Molar-Conductance-and-Specific-Conductance.aspx

6. Kohlrausch’s Law, Retrieved on 10 March 2012, from http://www.adichemistry.com/physical/electrochemistry/introduction/electrochemistry.html#ELECTRICAL RESISTANCE & CONDUCTANCE