Viscosity of Liquids: Methanol and Water

Transcription

1 of Liquids: Methanol and Water Colin McGuire January 4 and February, 01 March, 01 1

2 1 Abstract This experiment was done in order to determine the viscosity of mixtures of methanol and water of concentrations 0, 40, 60, 80, and methanol by volume. An Otswald viscometer and a stop watch were used to record the time of flow for the reference fluid, water, and each mixture. This time was then used in conjunction with the density of our reference liquid and mixture to in order to calculate the viscosity of the mixture using equation 1. The viscosity of each solution was used to find the fluidity of the solution in equation. The fluidity and viscosity of the ideal solutions were calculated. The viscosities of the methanol solutions were g 1.36, 1.64, 1.55, 1.19, and centipoisse ( cms ) for the solutions of 0, 40, 60, 80, and respectively. Procedure This procedure was performed from a handout that is labeled Transport Properties and Chemical Kinetics. Experiment 17: of Liquids, Part I: Low viscosities. The procedure we used was slightly different in that we only used the methanol and water section and didnt use the toluene and p-xylene solution. A constant temperature bath was brought to 5 degrees Celsius and an Otswald viscometer was inserted. Water was added and pipetted up past the start line. The time is took for the solution to flow from the top band to the bottom band was recorded. Water was the reference sample and runs were conducted until the time of flow agreed within 0. seconds. When switching samples the viscometer was washed with soap and water and dried with acetone and suction. This same procedure was followed for each sample. 3 Data Collected The time of flow data was collected for water moving through the viscometer, which is our reference liquid, and is presented in table A. The time of flow data for each methanol mixture, table B was used with the water average time in equation 1 to calculate the viscostiy. For all of the tables the averages were calculated using the three values that agreed within 0. seconds. The densities used were those at 5 degrees Celsius. Within this equation is the viscometer constant, k, which is all of the data for the reference liquid. This value 4 ml is 1.48e cms. It was obtained from running water. With this value equation 1 is reduced to equation 3. The viscosities were used to find the fluidities using equation 4. The mole fractions of methanol and water and the viscosities of pure methanol and water were used to calculate the ideal viscosities of the solutions using equation 5. Then the mole fractions and viscosities of pure water and methanol were used to calculate

3 the ideal viscosities of the solutions. The mole fractions were calculated based on the volume of methanol in 10 ml of solution with water. The density of methanol and water at 5 degrees Celsius was used. These are presented in table C. Water Average Table A: of Flights for water Average Average Average Average Average Table B: of Flights for methanol and water mixtures Table C Measured χ χh O Ideal Ideal

4 3.1 Calculations η = ( η r ρ r t r )ρt (1) F = 1 η () η = kρt (3) Where k is the viscometer calibration constant, 1.48e 4 F = χ A F A + χ B F B (4) η = χ A ηa 1 + χ (5) B ηb 4 Discussion This experiment was successful in measuring the viscosities of pure liquids. This is known by the percent error of the pure substances. The percent error of the calculation of pure methanol is 8.56 %. The calculations of ideal fluidity were not accurate compared to the measured values. Tables D and E show the percent errors for these calculations. These large percent errors show that these solutions do not behave ideally. In this case the behavior of these solutions is not ideal making these equations not applicable. The effect of mixing methanol and water is actually making the resulting solution more viscous than either of the two liquids separately. This means that the intermolecular forces of the liquids are strongly interacting with each other. This prevents sliding of the molecules past each other and results in a more viscous fluid. Because the fluidity is very far off using the ideal case approximation it is valid to conclude that solutions of methanol and water are not at all ideal. If the calculated and ideal fluidities were close their ratio would be close to 1. In this case the ratio of the fluidities are much less than from 1 as shown in Table F. This experiment was not very successful in measuring the viscosities of mixtures as seen in Table G. The percent differences of these values is incredibly high indicating that this is not a good way to measure the viscosity of a solution. 4

5 χ χh O Ideal (%) Table D: in Ideal calculation of viscosities. χ χh O Ideal (%) Table E: in Ideal calculation of fluidities. (cp) CRC (cp) % Table G: Measured viscosities versus the CRC handbook value and percentages of that value. 5