CHM170L Exp3 Surface Tension

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Surface tension

Transcript of CHM170L Exp3 Surface Tension


CHM170L Physical Chemistry 1 Laboratory 4th Quarter SY 2009-2010

Surface Tension of LiquidsNieva, Aileen D.1, Arceo, Mary Anne V., Cuales, Jelline C., Kim, Sung Min, Ngan, Emil Joseph T., Rivera, Jainie Lynne B.21Professor, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Student (s), CHM170L/A41, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology

ABSTRACTIn this experiment, the objectives are to measure the surface tension of a pure liquid and of an aqueous solution, to determine the effect of bulk solute concentration on the surface tension of an aqueous solution and to evaluate graphically the parameters of the Gibbs isotherm. Surface tensionis a property of the surface of aliquidcaused bycohesionof like molecules, which is responsible for many of the behaviors of liquids. Since the molecules on the surface of the liquid are not surrounded by like molecules on all sides, they are more attractedto their neighbors on the surface. This is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid. The experiment uses the static method: the tensiometer-ring method. It was initialized by doing the zero adjustment and calculating the correction on the tensiometer. From there the surface tension of the liquids: water and different concentrations (0.1- 0.8) of n-butanol were measured. Results showed that addition of more n-butanol lowered the surface tension of the solution. A generalization was then made upon performing the experiment; the addition of the solute causes a decrease in the surface tension of the solvent because the solute concentrates slightly in the neighborhood of the surface.

INTRODUCTIONThe surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Liquids that have strong intermolecular forces also have high surface tensions. It is also the cohesive forces between liquid molecules are responsible for the phenomenon. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface film which makes it more difficult to move an object through the surface than to move it when it is completely submersed. The surface tension of a liquid mixture is not a simple function of the surface tensions of the pure liquids. Also, the composition of the bulk phase and the composition at the vapor-liquid interface are not always the same. At the interface, there is migration of the species having the lowest surface tension, or free energy per unit area, at the temperature of the system. This migration at the interface results in a liquid-phase rich in the component with the highest surface tension and a vapor phase rich in the component with the lowest surface tension.The more volatile component is adsorbed from the mixture and extends about one atomic diameter into the gas phase beyond the other species.It has been found out that the surface tensions of solutions are in general different from those of the corresponding pure solvents. It has also been found out that solutes whose addition results in a decrease in surface tension tend to concentrate slightly in the neighborhood of the surface (positive surface concentration) while those whose addition results in an increase in surface (negative surface concentration). Equilibrium is reached when the tendency for the free energy decrease due to lowering surface tension is balanced by an opposing tendency for free energy increase due to non-uniformity of solute concentration near the surface. The dependency of the surface tension of a given ideal solution on the concentration and temperature was first shown by Willard Gibbs thus the following equation is named Gibbs Isotherm. (1)

(2)where is surface tension (dynes/cm), C is the bulk concentration (moles/cm2). One of the devices used in measuring the surface tension of a liquid is the use of Du Nouy tensiometer. Du Noy Ring Tensiometer, is the type of tensiometer uses a platinum ring which is submersed in a liquid. As the ring is pulled out of the liquid, the tension required is precisely measured in order to determine the surface tension of the liquid. This method requires that the platinum ring be nearly perfect; even a small blemish or scratch can greatly alter the accuracy of the results. A correction for buoyancy must be made. This method is considered obsolete and is no longer widely used.METHODOLOGYEXPERIMENTAL METHOD

Equipment and Glassware

Du Nuoy tensiometer, 50 mL glass-stopperd Erlenmeyer flasks, 5 mL volumetric pipets, flasks, stirrerExperimental Procedure

Adjust the tensiometer adjustment to zero. Measure the surface tension of distilled water at least three times and get the mean reading (m). Record the liquid temperature. Obtain the literature value of the surface tension of water at the temperature recorded. Calculate the correction for the tensiometer. Do the measurement with pure water at least three times. Repeat the measurement with n-butanol solutions for eight different concentrations.

For each concentration, perform at least five measurements.

Calculate the surface concentration of n-butanol from the surface tension data graphically.Set-up of Apparatus

RESULTS AND DISCUSSIONMeasured Surface Tension of Water:

Trial 1 85.9 dynes/cm

Trial 2 85.3 dynes/cm

Trial 3 86.7 dynes/cm Concentration, mole / literSurface Tension, dynes / cm











Table 1. Surface Tension of n-butanol Solution of Different ConcentrationsThe first thing to do is to get the Erlenmeyer flask and measure 5 mL of n-butanol and dilute it to the mark with water. Water surface tension was 72.0 dynes/cm at 25C and 67.9 dynes/cm at 50C temperature. The temperature of the water is 32 C, and its surface tension was 98.18 dynes/cm. The surface tensions literature value was computed using interpolation. The instrument used to get the surface tension of the liquids was Du Nuoy tensiometer. It consists of a platinum-iridium ring supported by a stirrup attached to the upper part of the instrument. The ring is placed at the surface of a liquid with air. It is then pulled upward until it breaks free of the liquid and moves into the second liquid or into the air. The force that is just required to break the ring free of the liquid/air interface is proportional to the surface tension. Doing three trials, we obtained 85.967 dynes/cm. This gives us a difference of 12.133 dynes/cm.

The n-butanol solutions surface tension with different concentration was also measured. Different values of surface tension were recorded due to the difference in its concentration. Addition of more n-butanol lowered the surface tension of the solution.

CONCLUSION AND RECOMMENDATIONFrom the experiment performed one can conclude that the surface tension of water is relatively high. In the solution of n-butanol if we are to mix it with water or dilute it, its concentration would be much lesser because from the experiment, as the group dilute the n-butanol with water starting from 0.10 mole/L to approximately 0.45mole/liter. It has been observed that as the n-butanol is more concentrated the more its surface tension to be more lesser because as we all know that the surface tension of water is more higher thus if we are to put more n-butanol therefore less water will be in the content. As a result of that lesser surface tension value will be come up.

As to the percent error, the group thought that perhaps/maybe the error may have occurred not only to the operator of the machine but as well as the machine itself because the value that the machine is reading is not that constant or should the group say not that accurate anymore perhaps it has not been calibrated.

It is recommended to read first the background and procedures of the experiment thoroughly before performing the experiment. Handle the tensiometer carefully.Since the tensiometer in the laboratory room cannot be adjusted to zero, after pushing the reset button, take note of the initial reading. This should be added to the final reading of the surface tension of the liquids. And lastly, proper dilution should be done.REFERENCES1. General Chemistry The Essential Concepts by Raymond Chang2. An Approximate Theory of Interfacial Tension of Multicomponent Systems: Applications Binary Liquid-Vapor Tensions, Winterfield, P.H., Scriven, L.E., and Davis, H.T.3. Physical Chemistry Laboratory Manual, Part 1 (2006) by Alvin R. Caparanga, John Ysrael G. Baluyut and Allan N. Soriano4. The Surface Tension and Density of Binary Hydrocarbon Mixtures: Benzene-n-Hexane and Benzene-n-Dodecane, Schmidt, R.L., Randall, J.C., and Clever, H.L.5. Surface and Interfacial Tensions, Gambill, W.R.6. Interfacial Properties of Mixtures of Molecular Fluids. Comparison between Theory and Experiment: CH3I + CCl4 and CH3CN + CCl4, Teixeira, P.I.C., Almeida, B.S., Telo da Gama, M.M, Rueda, J.A., and Rubio, R.G.APPENDICESAppendix A: Graph (Surface Tension)

Appendix B: Du Nouy ring

1-The ring is above the surface and the force is zeroed.2 - The ring hits the surface and there is a slight positive force due to the adhesive force between ring and surface.3 - The ring must be pushed through the surface (due to the surface tension) which causes a small negative force.4 - The ring breaks through the surface and a small positive force is measure