These types f experiment simply provide us other possible ways in which such types of liquid can used and at what pressure levels. This experiment is one which will provide us with numerical as well as graphical solutions.
Background theory:
This experiment consists of two governing equation which will derived and used throughout the report therefore the background theory will built on the two equations. We were provided with two equations in this experiment and the one is as follows
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---- Specific enthalpy of evaporation of the liquid
The derivation of the first governing equation:
The equation above provides the rate at which the vapour of liquid pressure of liquid changes with respect to temperature.
The equation of Clausius-Clapeyron is integrated to give the equation below assuming that Vg >> Vf.
The derivation of the second equation is done by integrating the first equation:
The derivation of the second equation is as follows:
Equipment used in the experiment:
- Pressure vessel – Contains the water which is being heated
- Safety valve – Is used to control water and gas
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Mercury thermometer – used to measure the temperature in 0C
- Pressure gauge – used to measure pressure
- Bunsen burner – heat the water in the vessel to increase the pressure to calculate the relation
- Vacuum chamber - is a rigid enclosure from which air and other gases are removed by a vacuum pump
- Vacuum pump - removes gas molecules
- Tap – Which was used to get water
Method: For pressure above atmospheric:
The boiler, a small steel pressure vessel, contained water when the experiment was about start. With the valve (on top of the boiler) open, raise the water to boiling point as quickly as possible. When boiling commences, reduce the heating rate so as not to lose too much water and continue to blow off steam in order to expel air while the thermometer attains a steady reading.
The pressure levels were at 480 kPa set when the experiment. By regulating the heating rate, hold the pressure steady until the thermometer reading becomes steady (about 3 minutes). Record the temperature and pressure.
Remove the Bunsen burners and allow the gauge pressure to fall to around 420 kPa. Again, hold this pressure steady until the thermometer reading becomes steady and record
temperature at different pressure levels. Repeat at intervals of about 60 kPa down to a gauge pressure of around 60 kPa. Turn off the Bunsen burners.
The second experiment: Pressure below atmospheric pressure
We had to ensure that the beaker is filled to about 10 mm from the top with water. Then open the vacuum release valve and the vale connecting the pump and vacuum chamber. Set the variac to zero; switch on the mains supply to the variac. Set the variac potential to maximum (50V). As soon as boiling commences, reduce the variac potential so that the water boils fairly gently (about 30V). Record the temperature when the thermometer attains a steady reading (about 2 minutes).
Close the vacuum release valve and partially open the water valve. When a vacuum of about80 mmHg (difference between mercury levels) is reached, partially close the valve connecting the pump and vacuum chamber and adjust continually so as to hold the pressure constant while the thermometer attains a steady reading.
Record the mercury levels and thermometer reading. Open up the valve and repeat the procedure at a vacuum of about 160mmHg. Obtain further readings at intervals of about 80 mmHg up to the maximum vacuum which can be achieved or until one of the columns leaves the scale.
Raw results including calculations:
To calculate atmospheric pressure constant, use interpolation. At 19 ˚C the barometer read 746 mmHg, use pressure chart to calculate the atmospheric pressure constant.
(740-760) / (740-746) = (2.4-2.46) / (2.4-x)
3.33 = (-0.06) / (2.4-x)
8-3.33x = 0.06
x = 2.442
Atmospheric pressure:
Atmospheric pressure is P -
= 746 – 2.442
= 743.55 mmHg
For changing it to Pascals, we multiply it by 133.3 Pa
=743.55*133.3 = 99 115.215 Pa
= 99.115 kPa
Therefore absolute pressure for the first experiment is:
Absolute pressure = atmospheric pressure + gauge pressure
Example of calculation:
AP = atmospheric pressure + gauge pressure
AP = 99115.215 Pa + 480 000 Pa
AP = 579115.215 Pa = 579.1152 kPa
Therefore absolute pressure (AP) is 579.1152 kPa. Follow the same process for all the Gauge pressure (kPa).
Apply the same process but this instead of adding the atmospheric pressure to gauge pressure, subtract the two types of pressure to calculate the absolute pressure below atmospheric pressure.
First experiment:
Second Experiment: Reading below atmospheric pressure
Discussion:
First experiment:
The graph simply shows that the rate of fall in pressure was much greater than the rate at which temperature fell at. It’s evident when you look at the number from the results table, what it shows is that pressure fell by 420000 Pa, whilst there was only 48 K change in temperature. This means that for every Kelvin the pressure dropped by 8750 Pa, which the fact that the rate of change in pressure was much greater than the rate of change in temperature.
The first experiment carried number errors, the main being that no-one could actually see the gauge pressure so everything was read and recorded at either below or above that actual point because no-one able to see it at the same level.
For the second the experiment the valve should’ve been controlled by the same person to keep the process consistent but during experiment everyone had a go making the degree consistency much lower. The result was gathered after a prolonged session in the lab due to everyone’s inability to change the levels of mercury.
Actually both of the experiment should be repeated so that we could have at least gathered result that did not carry so much errors. We fairly good result that satisfy the line graph but as always it could have much better.
The result would’ve been far more accurate if the measuring equipment were accurate and precise. There were no repetitions of the experiment to minimize the levels of uncertainty in this experiment.
Conclusion:
To summarise:
- The natural log of the liquid rises much slower than its vapour pressure thus producing a curved graph of vapour against pressure.
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The value of is always positive
- A lot of factors contribute to process out the graph, factors such as temperature, pressure and even insulation of the systems
References:
- Thermodynamic Book
- Thermodynamics Lecture notes
