Surface Tension Report+Theory PDF

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Surface Tension Report+Theory Quim4102. Prof. Nairmen Mina [2019]...

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Department of Chemistry University of Puerto Rico Mayagüez Campus Physical Chemistry Lab II QUIM 4102-061

Experiment 6 Surface Tension

ACF/ JS Dr. Nairmen Mina Date of experiment: October 18, 2019 Date due: November 1, 2019

I-

Introduction A. Objective Use the capillary rise method to study the changes in surface tension of acetone as a function of temperature, using water as a calibration standard. In addition, observe how the surface tension changes when using other organic compounds such as alcohols: Methanol, n-propanol and ethanol at standard conditions (25 ⁰C and 1 atm pressure). B. Theory The molecules of a liquid attract each other, hence the liquid is "cohesive." When there is a surface, molecules that are just below the surface feel forces diagonally, horizontally, and down, but not up, because there are no molecules above the surface. The result is that the molecules found on the surface are attracted into the liquid. The surface tension measures the internal forces that must be overcome in order to expand the surface area of a liquid. The energy needed to create a new surface area, moving the molecules of the liquid mass to its surface, is what is called surface tension. The greater the surface tension, the greater the energy needed to transform the inner molecules of the liquid to surface molecules. Surface tension is caused by the effects of intermolecular forces that exist in the interface. The surface tension depends on the nature of the liquid, the surrounding environment and the temperature. Liquids whose molecules have strong intermolecular attractive forces will have high surface tension. The molecule on the surface supports the action of a resulting force directed towards the interior of the liquid, this situation repeated throughout the entire surface of the liquid produces the contraction of the total surface of the liquid as if it were an elastic membrane. In Figure 1, the internal molecule is uniformly surrounded by other molecules, so it is attracted equally in all directions, the sum of attractive and repulsive forces being equal to zero. The molecule exposed to the atmosphere (found on the surface) has a nonzero result, meaning it is towards the sine of the liquid; because the number of molecules per unit volume is greater in the liquid than in gas, uncompensated forces result.

2

Figure 1. Representation of the interior (right) and exterior molecules (left) and how they attract to other molecules. To increase the surface area it is necessary to perform work to overcome or bring the molecules inside the liquid to the surface, overcoming the forces on the surface. This is always accompanied by a decrease in Free Energy. A typical model for demonstrating surface tension is that of a box with a sliding lid. The interface voltage between the lid and the liquid is assumed as zero. Discovering a new surface, dA, the lid has to move from one point to another. The mechanical work necessary to do this is proportional to the uncovered surface and can be equal to an increase in the Gibbs free energy of the system. The proportionality constant, γ, is the surface tension and is defined for systems of a component (pure) such as: 𝛿𝐺

1

1

( ) 𝑇,𝑃,𝑛 = 𝛾 = 2 𝛥ℎ 𝛿𝐴

𝑎𝑣𝑒.

𝑟𝑔𝜌 + 6 𝑟²𝑔𝜌

(1)

Where, 𝛾:is the surface tension (dyne/cm) 𝑔:is the gravity constant (980.665 cm/s²) 𝜌:is the density at temperature T (g/cm³) 𝑟:is the Capillary Radius (cm) 𝛥ℎ𝑎𝑣𝑒. : is the difference between the height of the liquid in the capillary tube ,ℎ2 ,and the height of the liquid in the test tube,ℎ1 . The surface tension of any liquid can be experimentally calculated by using a capillary and calibrating it with a substance (water usually), which its properties are known at a specific temperature, to obtain the radius of the capillary and therefore, calculate the tension superficial for the other substances with equation 1. The second term of the equation can be neglected since the radius of the capillary is so small that when it is squared the change is practically negligible. 3

II-

Methodology A. Materials: a. Test tube with rubber stopper b. Capillary tube c. d. e. f.

Thermometer Bulb 10 mL pipette Solutions: 1. Methanol [𝐶𝐻3 𝑂𝐻]

2. n-propanol [𝐶3 𝐻8 𝑂] 3. Water [𝐻2 𝑂] 4. Ethanol [𝐶2 𝐻5 𝑂H] 5. Acetone [𝐶3 𝐻6 𝑂] B. Equipment: a. Test tube with rubber stopper b. Capillary tube c. Thermometer d. Bulb e. 10 mL pipette C. Procedure: 1. Wash and dry a capillary tube and a test tube. 2. Add 10.00 mL of distilled water to the test tube without wetting the walls and place the capillary tube inside it using the rubber cork to hold it. 3. Place the test tube in a bath with a thermostat (variable temperature bath) and set the temperature at 25 ° C (use a thermometer). 4. Apply disturbance to the system by causing the meniscus to fall according to the increase or decrease in the pressure applied to the capillary. 5. The meniscus in the capillary must always return in the same place, but if the capillary happens it is dirty and must be washed. 6. Using the cathetometer (instrument to measure height differences very accurately) determine the height at which the liquid rises through the capillary, measuring the height of the water meniscus in the test tube and the height of the meniscus in the capillary tube. (see Figure 2). 7. For the same temperature repeat step 4 and 5 in triplicate. 8. Repeat steps 1 through 6 using other substances at 25 ° C: methanol, ethanol, and n-propanol. 4

9. To acetone perform the same procedure (steps 1 to 6) at different temperatures, taking triplicate height readings at: 25 ° C, 30° C, 35 ° C, 40 ° C, 45 ° C, 50 ° C and 55 ° C. 10. At the end of the experiment, turn off the variable temperature bath and discard the solutions in the waste area. 11. Clean the capillary and the test tube.

Figure 2. Measurements of the heights using the cathetometer

III- Data Table 1. Measures of heights of different compounds at 25 C.

Compound

Methanol

n-propanol

Run

ℎ1 [cm]

ℎ2 [cm]

1

4.52

5.93

2

4.56

5.92

3

4.55

5.91

1

3.79

6.32

2

3.88

6.35

3

3.85

6.34

ℎ1,𝑎𝑣𝑒 [cm]

ℎ2,𝑎𝑣𝑒. [cm]

𝛥ℎ𝑎𝑣𝑒. [cm]

4.54

5.92

1.38

3.87

6.34

2.46

5

Water

Acetone

Ethanol

1

2.13

6.27

2

2.20

6.45

3

2.21

6.49

1

2.91

4.63

2

2.92

4.58

3

2.90

4.61

1

4.23

5.94

2

4.25

5.96

3

4.26

5.95

2.18

6.40

4.22

2.91

4.61

1.69

4.25

5.95

1.70

Table 2. Properties of water used for the calibration of the capillary at 25 C [Ref. NIST] Property:

Symbol:

Units

Value

Surface Tension

𝛾

dyne/cm

72.0

Gravity Constant

g

cm/s²

980.665

Density

𝜌

g/cm³

0.9989

Capillary Radius

r

cm

0.0348

Table 3. Measures of the experimental surface tension for different compounds at 25 C. [Ref. NIST] Compound

𝜌[g/cm³]

𝛾𝑒𝑥𝑝. [dyne/cm]

𝛾𝑙𝑖𝑡. [dyne/cm]

%EA

Methanol

0.7877

18.51

22.08

16.2

6

n-propanol

0.8035

33.78

23.70

42.5

Water

0.9989

72.00

71.99

0.01

Acetone

1.697

22.73

22.73

0.1

Ethanol

1.703

22.79

21.80

4.5

Table 4. Measure of the heights of Acetone at different temperatures. Temperature [C]

25

30

Run

ℎ1 [cm]

ℎ2 [cm]

1

2.91

4.63

2

2.92

4.58

3

2.90

4.61

1

2.63

4.32

2

2.64

4.37

3

2.66

4.35

ℎ1,𝑎𝑣𝑒 [cm]

ℎ2,𝑎𝑣𝑒. [cm]

𝛥ℎ𝑎𝑣𝑒. [cm]

2.91

4.61

1.69

2.64

4.65

1.70

7

35

40

45

50

55

1

2.46

4.09

2

2.44

4.12

3

2.54

4.19

1

2.52

4.16

2

2.59

4.12

3

2.51

4.14

1

1.96

3.58

2

1.97

3.55

3

1.96

3.52

1

1.91

3.48

2

1.89

3.45

3

1.85

3.41

1

1.57

3.16

2

1.60

3.20

3

1.59

3.16

2.48

4.13

1.65

2.54

4.14

1.60

1.96

3.55

1.59

1.88

3.45

1.56

1.59

3.17

1.59

Table 5. Measures of the experimental surface tension for Acetone at different temperatures. [Ref. DDB] Temperature [C]

𝜌[g/cm³]

𝛾𝑒𝑥𝑝. [dyne/cm]

𝛾𝑙𝑖𝑡. [dyne/cm]

%EA

25

0.7858

22.75

22.73

0.11

8

30

0.7802

22.68

22.11

2.58

35

0.7746

21.86

21.48

1.76

40

0.7689

21.00

20.86

0.65

45

0.7632

20.67

20.24

2.11

50

0.7573

20.21

19.62

2.99

55

0.7514

20.35

19.01

7.04

9

Graph 1. Surface Tension in function of the temperature of Acetone for experimental and reference data. [Ref. NIST and DDB]

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IV- Discussion Surface tension is the energy, or work, required to increase the surface area of a liquid due to intermolecular forces; another way to look at it is the tendency of a fluid surface to shrink into the minimum surface area possible. There are two types of molecules when talking about surface tension, the molecules on the exterior and the molecules on the interior. The interior molecules are attracted to each and every molecule around it, the exterior molecules are attracted to the other exterior molecules and those below them (refer to figure 1). In the case of water, being a polar molecule, the hydrogen end (positive) and the oxygen end (negative) makes the molecules “stick” together stronger. The energy required to break these hydrogen bonds is high, resulting in a high surface tension of 72.00 dyne/cm at standard pressure (1 atm) and temperature (25 °C). The literature value for the surface tension of water is 71.99 dyne/cm; the percent error is 0.01% and can be due to sight error in interpretation of the cathetometer.

Figure 3. Visual representation of how water molecules stick together. Methanol was also evaluated at standard pressure (1 atm) and temperature (25 °C) and gave a surface tension of 18.51 dyne/cm, the literature value for surface tension being 22.08 dyne/cm resulting in a percent error of 16.2% (refer to table 3). This percent error can be due to a misreading of the cathetometer or from the capillary being dirty. Since the capillary tube is very small, it is difficult to clean properly. Methanol is a polar molecule that exhibits dipole interactions and has hydrogen bonding (refer to figure 4).

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Figure 4. Visual Representation of how methanol molecules interact In a dipole molecule, there are attraction arrangements and repulsion arrangements (refer to figure 5). In all free liquids, the molecules will always be passing by each other, resulting in some repulsion and some attractive forces. This causes the surface tension to decrease because at times the molecules aren’t attracting each other as they should be. This occurs in all the alcohols studied in this experiment.

Figure 5. Visual representation of how a dipole molecule has attraction and repulsion forces. N-propanol was also evaluated at standard pressure (1 atm) and temperature (25 °C) and gave a surface tension of 33.78 dyne/cm, the literature value for surface tension being 23.70 dyne/cm resulting in a percent error of 42.5% (refer to table 3). This percent error can be due to a misreading of the cathetometer or from the capillary being dirty. Since the capillary tube is very small, it is difficult to clean properly. N-propanol has hydrogen bonding as its primary intermolecular force, but since n-propanol is a very viscous alcohol, the surface tension is lower. Because there is only one oxygen molecule in n-propanol (refer to figure 6), the amount of hydrogen bonds that form is minimal, therefore, although hydrogen bonds are the strongest, npropanol doesn’t form many.

Figure 6. Molecule n-propanol Ethanol was also evaluated at standard pressure (1 atm) and temperature (25 °C) and gave a surface tension of 22.79 dyne/cm, the literature value for surface tension being 21.80 dyne/cm resulting in a percent error of 4.5% (refer to table 3). This percent error can be due to a misreading of the cathetometer or from the capillary being dirty. Since the capillary tube is very small, it is 12

difficult to clean properly. Ethanol has hydrogen bonding as its primary intermolecular force, but since ethanol is a viscous alcohol, the surface tension is lower. Because there is only one oxygen molecule in ethanol (refer to figure 7), the amount of hydrogen bonds that form is minimal, therefore, although hydrogen bonds are the strongest, ethanol doesn’t form many; n-propanol and ethanol are very similar in terms of surface tension.

Figure 7. Ethanol intermolecular interactions Acetone was evaluated at various temperatures and at the standard pressure of 1 atm, to see how temperature affects the surface tension. At 25 °C, the surface tension was 22.75 dyne/cm, the literature value was 22.73 dyne/cm, resulting in a percent error of 0.11%. The main intermolecular force in acetone is dipole-dipole interaction, but due to dipole repulsion forces present on occasion, the surface tension isn’t as high. By raising the temperature, the intermolecular forces will be disturbed, and the surface tension decreases. In general, surface tension decreases when temperature increases because cohesive forces decrease with an increase of molecular thermal activity. The influence of the surrounding environment is due to the adhesive action liquid molecules have on the interface. This trend can be observed in table 5.

Figure 8. Acetone intermolecular interaction 13

V- Conclusion At 25 °C water has the highest surface tension because of the hydrogen bonds at 72 dyne/cm; compared to the literature value of 71.99 dyne/cm. Ethanol has a surface tension of 22.79 dyne/cm; compared to the literature value of 21.80 dyne/cm. Methanol has a surface tension of 18.51 dyne/cm; compared to the literature value of 22.08 dyne/cm. N-propanol has a surface tension of 33.78 dyne/cm; compared to the literature value of 23.70 dyne/cm, this percent error was significant at 42.5%. Acetone at 25 °C had a surface tension value of 22.75 dyne/cm; compared to the literature value of 22.73 dyne/cm. With increasing temperature, the acetone surface tension decreased (refer to table 5), at the highest temperature 55°C the surface tension reached 20.35 dyne/cm.

VI- References Levine, I. N. (1978). Physical Chemistry (6th ed.). Auckland: McGraw-Hill Ball, D. and Baer, T. (2015). Physical chemistry. 11th ed. Singapore: Cengage Learning Asia. https://employees.csbsju.edu/cschaller/Reactivity/photochem/PCabsorbance.htm (accessed Oct 25, 2019). Tro, N. J.; Fridgen, T. D.; Shaw, L.; Boikess, R. S. Chemistry: a molecular approach; Pearson Canada: Toronto, 2014.

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