K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012)
ChE 124 Chemical Engineering Thermodynamics Laboratory Determination of the Latent Heat of Vaporization of Ethanol Using the Ramsay-Young Set-up Karl Rodney Cerezo, Ruben Salvador Jr., Armin Tajanlangit* Department of Chemical Engineering, University of the Ph ilippines-Diliman, ilippines-Diliman, Quezon City, Philippines
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ABSTRACT
Report History
The experiment aimed to determine the latent heat of vaporization of 96% v/v
Revised
Ethanol, through the use of the Ramsay-Young apparatus. The said setup
Received
allows experimenters to measure the boiling points of ethanol at different
Accepted
pressures. The experimental value, calculated to be 36235.7376 J/mol was then compared to three theoretical models: the Van der Waals, Redlich-Kwong and
Keywords:
the Soave-Redlich-Kwone Equations of state. Results were precise as the range
Latent Heat of Vaporization,
of percent errors was 11.11% to 11.65%. The method has thus proven itself to be a
Ramsay-Young Apparatus,
viable way of determining the latent heat of vaporization, though great caution
Equations of state, Saturation,
must be done in order to produce pr oduce good results.
Pressure
Table of Contents 1. Introduction ..................................................................... ........................................................................................................................................... ..................................................................................... ............... 2 2.
................................................................................................................................... .............................................................. 2 Materials and Equipment .....................................................................
3.
Experimental Design .................................................................. ....................................................................................................................................... ......................................................................... .... 2
4.
................................................................................................................................... .......................................................................... .... 4 Results and Discussion .............................................................
Possible Sources of Error ................................................................... ........................................................................................................................................ ......................................................................... .... 5
5.
................................................................................................................................... ................................................................................................ .......................... 5 Conclusion .............................................................
References .......................................................... ................................................................................................................................ .............................................................................................................. ........................................ 6
*Corresponding Corresponding Author. Tel: +639177591779
Email Address:
[email protected]
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K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012)
be calculated using the different equations of state. 2
1. Introduction The latent heat of vaporization is defined as the amount of heat absorbed when a substance vaporizes from the liquid at constant temperature. No change in temperature occurs; however, a finite amount of heat is transferred into the substance. 1 This experiment aims to determine the latent heat of vaporization of ethanol, through the use of the Ramsay-Young setup. The method involves boiling the substance at different pre-determined pressures, which are maintained as the substance is heated to the boiling point. The heats of vaporizations can then be calculated using the Clapeyron Equation. As shown by Smith, Van Ness and Abott [Introduction to Chemical Engineering Thermodynamics, 7th Ed., Mc Graw-Hill, New York. 2005], the latent heat of vaporization is directly proportional to the slope of the vapor pressure versus temperature curve.
(1)
Arranging the Clausius Equation to another form allows us to acquire the heat of vaporization under the assumptions that the vapor behaves ideally and the heat of vaporization is independent of temperature.
(2)
The slope of the plot of ln P vs 1/T will give a constant value of the latent heat of vaporization of ethanol. However with these assumptions, the heat of vaporization obtained will only be a rough estimate because: (1) The ethanol vapor does not behave as an ideal gas, and (2) the heat of vaporization of ethanol decreases with in increasing temperature and vice versa. With these conditions, the compressibility factor of the vapor should be taken into account. A more general form of the Clapeyron equation can be used, and ΔZ values can
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Smith, J., H.C., V. N., & Abbott, M. (2005). Introduction to Chemical Engineering Thermodynamics, 7th Edition. New York: Mc Graw-Hill.
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2. Materials and Equipment The experiment involves the following equipment and materials: i.
Ramsay-Young Setup Vacuum Pump Manometer Flasks Rubber tubing Iron Clamps ii. Thermocouple iii. Hot plate/Bunsen burner iv. 1000 ml, 500 ml, 50 ml beakers v. 50 ml EtOH vi. Oil (for oil bath) and ice (for ice bath) vii. Boiling chips
3. Experimental Design The Ramsay-Young apparatus (Figure 1) present in the laboratory is composed of a heated receiving flask (flask A) connected to a cooled collecting flask (flask B). The pressure of the entire system, sealed tight, is set with the use of a vacuum pump. From a container with a stopcock, the sample is allowed to drip into flask A where it is allowed to vaporize. The temperature where this vaporization occurs is monitored by a thermocouple whose sensor wires are inserted into flask A. The resulting vapors rise into flask B where the low temperature due to the surrounding ice bath returns the sample to its liquid form. This procedure is done over a series of
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Leonardo, M.C., Madlambayan, K., Peralta S. Determination of
the Latent Heat of Vaporization of Ethanol Using the RamsayYoung Set-up Experiment Proposal. 2011
K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012)
pressures, yielding a corresponding temperature for each pressure value (Garland & Shoemaker, 2003). The experiment started with the preheating of the hotplate under flask A. This was followed by the evacuation of the system to the highest pressure allowable. This facilitated easier pressure variation as in order to change the system pressure, the manometer valve simply had to be released. There was no need to start the pump again and again. This provided a set of descending pressure values during the experiment. After the evacuation, the sample was allowed to drip into flask A. Ideally, the sample should drip at a constant rate into flask A. if this ideal condition is met, the system will attain a constant temperature for a certain pressure even as the sample continues to drip. This is reflective of the fact that the latent heat of vaporization entails no temperature change. However, during the experiment, it was very difficult to keep the sample dripping at a constant rate. This inconsistent rise and fall in drop rate caused temperature to rise steadily. This was circumvented by adjustments in the actual procedure. Pressure remained set for a run while heating was continuously done without letting the sample drip. This caused the temperature to rise continuously. When the temperature had somehow stabilized (it changed slowly or only by fractions of degrees), a drop of sample was released into flask A. This suddenly brought system temperature down as the sample absorbed heat from the system. The reading to which temperature dropped was taken as the temperature reading at that pressure. Since vapor was visibly seen to have risen instantly from flask A, it was justifiably assumed that the sample absorbed its latent heat of vaporization from the system. This was done for a series of pressures,
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starting from the highest possible pressure. For this run of the Latent Heat of Vaporization experiment, the assigned pressures (in inches Hg) are 2, 4, 6, and 8. The height difference of the surfaces of mercury in the two columns determines the pressure reading of the manometer. It should be noted that the manometer is not at its zero inches Hg mark. Detach the flasks from the set-up to clean and dry them properly. Procure 80 mL of EtOH and put it, together with some of the boiling chips, inside the first Florence flask. Put enough oil and the rest of the boiling chips in the 500-ml beaker and place it on top of the hotplate. Properly immerse the first Florence flask into the oil. Make sure that the oil would not overflow but is just enough so that there is contact between the oil and the smaller Florence flask. Fill a 1000-mL beaker with ice and immerse the second flask. After preparing the two Florence flasks, set-up the Ramsay-Young experiment. Make sure that the tubes are connected correctly, and the set-up is tightly sealed and no air escapes from the tube connections. Plug the vacuum pump into the transformer, and then plug the transformer and the hot plate in to the power outlet. The metal knob on the left side of the vacuum pump should be locked counter clockwise, and the knob located at the iron stand open (not locked clockwise) at the start of the experiment. Since all the necessary preparations have been made, the Ramsay-Young set-up is now ready to be operated. Turn on the transformer. Turn the vacuum pump on and slowly turn the valve of the vacuum pump until the reading of the manometer is 8.5 in Hg. Turn the pump off and close the knob located on the iron stand (by turning it clockwise). The manometer reading should be stable at this point. Slowly and simultaneously release the metal knob of the vacuum pump and the one at the iron stand until the height difference in the manometer reaches 8 in Hg. Close the knobs once the desired pressure is attained. Turn the thermocouple on and start heating the first flask by turning on and adjusting the heating temperature of the hotplate. Wait until the ethanol boils and the thermocouple reading becomes stable. Record the temperature
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K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012)
and the operating pressure. Turn off the hotplate. Repeat the experiment for operating pressures of 6, 4, and 2 in Hg. When the experiment is complete, switch the transformer and hotplate off, remove the plugs, detach all the connecting tubes, dispose the Ethanol in its proper waste bottle, return the oil in its container, and wash all the glassware used in the experiment.
Determination of ΔHvap of Ethanol 11.8
11.75 ) t
a s
11.7
P ( n 11.65 l
11.6
11.55 0 .0 02 86 0 .0 02 87 0 .0 02 88 0 .0 02 89
4. Results and Discussion During the experiment, the boiling temperature is recorded when the reading settles in order to satisfy equilibrium condition between the ambient pressure and the vapor pressure of liquid. The table below shows the boiling temperature for each pressure.
Table 1. Experimental Data Manometric (inHg) 7.65 5.9 3.9 1.9
Pressure Boiling Point (°C) 68.95 71.25 73.45 75.2
The setup uses a vacuum pump in order to operate at pressure less than 1 atm. In this condition, liquids will boil at lower temperature than the normal boiling point. In order to compute for the experimental heat of vaporization, the logarithm of Psat = Patm –Pmanometric is related to inverse of boiling temperature which is based from the Clapeyron Equation (2). This yields a graph shown below:
0 .0 02 9 0 .0 02 91 0 .0 02 92 0 .0 02 93
1/T (K-1)
Figure 1. Plot derived from Clapeyron Equation Table 2. Equation of the Line
Parameter Slope Intercept r2
Value -4358.4
2.587 0.9863
To calculate for ΔHvap, the slope is multiplied to the negative of ideal gas constant, R=8.314 J/mol-K. The unit of the constant depends on the pressure and temperature units. Thus, the value of heat of vaporization is 36235.7376 J/mol. The value of r2 represents the discrepancy of assuming that the vapour is ideal and heat of vaporization is constant as temperature changes.
In order to express the non-ideality of gas in terms of Z, Equations of State (EOS) are used which involves different parameters which is summarized below. Table 3. Parameter Assignments of Equations of State from (Smith et al., 2005)
α (Tr) σ ε Ω ψ Zc vdW 1 0 0 1/8 27/64 3/8 -1/2 RK Tr 1 0 .08664 .42748 1/3 SRK α SRK 1 0 .08664 .42748 1/3 Where α SRK = [( 1 + (0.480 + 1.54ω - .176 ω2 )(1- Tr1/2 )]2 EOS
Using the models above, there are two possible values of z than can be calculated, one for saturated liquid and another for saturated vapor.
5 K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012) The enthalpy data above are shown with decreasing boiling temperature but the latent heat
The difference between the two z values yields and will be multiplied to RT 2 . The last
multiplier is and can be calculated using the Antoine’s equation of ethanol (where T is in °C and Psat in kPa).
This gives the dependence of enthalpy of vaporization on temperature. Below is the summary of calculated ΔHvap using Van der Waals, RedlichKwong, and Soave-Redlich-Kwong equations.
Table 4.Comparison of EOS with Experimental Enthalpy (J/mol)
VDW ΔHvap %deviation ΔHvap 41014.95 11.65237483 41004.54 40934.53 11.47879188 40923.7 40858.1 11.31321904 40846.78 40797.26 11.18096001 40785.41 SRK ΔHvap 40987.44 40905.84 40828.03 40765.71
Possible Sources of Error
From the equation above, an expression of the differential is produced:
of vaporization should be decreasing steadily with temperature and is zero at the critical point. This discrepancy could be due to small number of data points for only four boiling temperatures are observed. Based from the table above, there is a significant difference when ethanol vapor is considered as non-ideal gas and its enthalpy changes with temperature. Comparing the three EOS, Van der Waals gives the highest enthalpy values while SRK gives the lowest, though their difference is not that significant.
RK %deviation 11.62995 11.45538 11.28864 11.15515
%deviation 11.59307 11.41672 11.2479 11.11222
Boiling chips provide nucleation sites in the liquid so that it boils smoothly without being superheated. If no boiling chips are used, it may cause rapid boiling and may cause the reagent to splatter and be expelled out of the flask. As the hotplate is heated, the operating pressure decreases. This may be due to the loose valves, an opening somewhere in the Ramsay-Young set-up or due to the behavior of the gas itself. Thus it should be ensured that the set-up is air tight and does not leak at any part (such as the tube connections, corks, etc.). This must be remedied by constantly adjusting the pressure since the experiment requires a condition of constant operating pressure. Failure to do so will cause a lower reading for the final pressure (P when Ethanol boils), thus constitutes to a value of Z nearer 1.0. The mercury inside the barometer should be at the zero inches mark. If not, the deviation from the zero mark should be taken into account in getting the difference of the two liquid levels to get the pressure reading.
5. Conclusion The Ramsay-Young method has so far proved to be a viable way of obtaining a substance’s Latent Heat of Vaporization. The calculations from the experiment data show good precision. Throughout the range of data points, the percent error ranges from 11.11% to 11.65% from all three theoretical models.
K.R. Cerezo, R. Salvador Jr., A. Tajanlagit / Chem. Eng. Therm. Lab. (2012)
References Levine, I. (2009). Physical Chemistry, 6th Ed. New York: McGraw-Hill, Inc. Maloney, J. (2008). Perry's Chemical Engineers' Handbook, 8th Ed. New York: Mc Graw-Hill, Inc. Poling, B., Prausnitz, J., & O'Connell, J. (2001). The Properties of Gases and Liquids, 5th Ed. New York: McGraw-Hill, Inc. Smith, J., H.C., V. N., & Abbott, M. (2005). Introduction to Chemical Engineering Thermodynamics, 7th Edition. New York: Mc Graw-Hill.
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