Faculty
Lee Kong Chian Faculty of Engineering and Science
Department:
Department of Mechanical and Materials Engineering
Unit Code and Name
UEME 2252 Engineering Thermodynamics II
Experiment No.:
Experiment 2
Title of Experiment:
Experiment 2 : Ideal Gas Experiment
Laboratory Room No. and Name:
KB 731 Thermofluid Lab
Experiment Duration (hour):
3 hours
Number of Student per Group
5 students
Title
Ideal gas experiment.
Objectives
To verify the ideal gas law: pV=mRT………. ( Equation 1 ) Where p = pressure; V = volume; m = mass; R = gas constant; and T = temperature.
Equipment and Materials Item Description
*Item category
Quantity estimation (e.g. per set/group of student)
E
1
2 stages vacuum pump
E
1
Air compressor
E
1
Apparatus
consists
of
two
interconnected rigid containers isolated by a valve
Item Category: SP
Sample or specimen
C
Consumable
CH
Chemical
W
Labware, glassware, tool and components
E
Equipment
S
Software
Experimental setup
#Interconnected vessel
#Recorder
# Vacuum pump
#Valve
# Air Compressor
Procedure 1.)
The experimental set up was studied.
2.)
The location of all the valves and how they are connected to the compressed air supply and the vacuum pump were studied.
3.)
Vessel A was filled with air until the pressure reached 0.40 bar.
4.)
The air in vessel B was evacuated until the pressure reached -0.95 bar.
5.)
The temperatures in vessel A and B were recorded.
6.)
The interconnected valve was opened to allow the air expand to fill both containers.
7.)
The pressures and temperatures in vessel A and B were recorded.
8.)
The steps above were repeated for pressure 0.2 bar, 0 bar, -0.2 bar, -0.4 bar, -0.6 bar and -0.8 bar.
9.)
The results were tabulated and graph was plotted.
Results
Pressure, volumes, temperatures and masses in Vessel A before mixing
Pressures, volumes, temperatures and masses in Vessel B before mixing
Pressures, volumes, temperatures and masses in Vessel A after mixing
Pressures, volumes, temperatures and masses in Vessel B after mixing
Graphing data
Graph of Ideal Gas Law
0.025
0.020
B 2 m0.015 + A 2 m = 0.010 2 m
Series1
0.005
0.000 0.000
0.005
0.010
0.015
m1 = m1A + m1B
0.020
0.025
Sample calculations
Before mixing ( Vessel A ) Gas constant of air ( R )
= 0.287 kJ/kg.K
Volume in vessel A ( V 1A )
= 0.012 m3
Temperature in vessel A ( T1A ) = 28.1 + 273.0 = 301.1 K Pressure in vessel A ( P1A )
= P1g + Patm = 0.4 + 1.0 = 1.4 bar
Mass of air in vessel A ( m1A ) =
P V 1 A 1 A RT 1 A
1.4 x10 0.012 5
=
287 x301.1
= 0.019441 kg Before mixing ( Vessel B ) Volume in vessel B ( V1B )
= 0.007 m3
Temperature in vessel B ( T1B ) = 37.9 + 273.0 = 310.9 K
Pressure in vessel B ( P 1B )
= P1g + Patm = -0.95 + 1.0 = 0.05 bar
Mass of air in vessel B ( m1B ) =
=
P V 1 B 1 B RT 1 B
0.05 10 0.007 x
5
287 x310.9
= 0.00039 kg After mixing ( Vessel A ) Gas constant of air ( R )
= 0.287 kJ/kg.K
Volume in vessel A ( V 2A )
= 0.012 m3
Temperature in vessel A ( T2A ) = 27 + 273 = 300 K Pressure in vessel A ( P2A )
= P2g + Patm = -0.122 + 1.0 = 0.878 bar
Mass of air in vessel A ( m2A ) =
P 2 AV 2 A RT 2 A
0.878 x10 0.012 5
=
287 x300
= 0.012237 kg Before mixing ( Vessel B )
Volume in vessel B ( V2B )
= 0.007 m3
Temperature in vessel B ( T2B ) = 40.2 + 273.0 = 313.2 K Pressure in vessel B ( P 2B )
= P2g + Patm = -0.117 + 1.0 = 0.883 bar
Mass of air in vessel B ( m2B ) =
=
P 2 BV 2 B RT 2 B
0.883 10 0.007 x
5
287 x313.2
= 0.00688 kg
Total masses of air in vessel A and B
M1 = M1A + M1B
M2 = M2A + M2B
= 0.019441 + 0.00039
= 0.012237 + 0.00688
= 0.019831 kg
= 0.019117 kg
Ideal gas law
M1 = M2 = 0.019831 kg
Discussion
An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no inter -molecular attractive forces. One can visualize it as a collection of perfectly hard spheres which collide but which
otherwise do not interact with each other. In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature. An ideal gas can be characterized by three state variables: absolute pressure (P), volume (V), and absolute temperature (T). The relationship between them may be deduced from kinetic theory and is called the ideal gas law : PV =mRT. The ideal gas law can be derived from the kinetic theory of gases and relies on the assumptions that the gas consists of a large number of molecules, which are in random motion and obey Newton's laws of motion. Next, the volume of the molecules is negligibly small compared to the volume occupied by the gas and there have no forces act on the molecules except during elastic collisions of negligible duration. Besides, its assume that all collisions between gas molecules are elastic and all motion is frictionless (no energy is lost in collisions or in motion). Lastly, the temperature of the gas is proportional to the average kinetic energy of the molecules. The relationship between temperature and pressure is known as Gay-Lussac's law. It states that if the volume of a container is held constant as the pressure of a gas increases, the temperature inside the container will also increase. As with the other gas laws, this one can be represented in the form of an equation:
P 1 T 1
P 2 T 2
. From this
experiment, it found out that as the pressure of the gas increases in vessel A and B, the temperatures will also increases in most case. Besides, the volume in vessel B is smaller compare to vessel A, in this experiment we could analyze that when volume is smaller the temperature is higher. First, temperature is a measure of the speed and frequency of the collisions of the gas molecules with their surroundings. As the vessel gets smaller, they have a smaller distance to travel before they collide with the walls,
and thus the time between collisions gets increasingly smaller. In a given amount of time the particles hit the walls more, which results in a greater amount of pressure and lead to an increase in temperature. Bases on ideal gas law, the total masses of air before mixing and after mixing is the same. However in this experiment there have a slightly different between the value of M1 and M2. The total masses of air after mixing (M 2) following a trend that it is always lower then the total masses before mixing (M1). This may due to some leakage of gases when the two connecting valve were opened. Besides, the mass of air in vessel B is always lower than vessel A. Based on ideal gas law equation: PV=MRT, when the volume of vessel is lower and temperature is higher, this will causes a decrease in the masses of air and this is the main reason why the mass of air in vessel B is lower compare to vessel A. In this experiment, there were many possible causes of error. First of all, a faulty vacuum pump could’ve let the air escape and so the pressure sensor would read a smaller value. Next, error increases as volume decreases because when compressing the gas with higher pressure, the density increases and the gas behave less like an ideal gas. As the volume decreases, the movement of gas restricted and become less randomly move, thus its break the assumption of ideal gas law which state that gases are made up of molecules which are in constant random motion in straight lines. Another sources of error was reading the pressure and temperature which due to uncertainties of the equipment, it take times and difficult to make the values of pressure and temperature to stabilize. A few precaution steps should be taken in order to get a better result. Firstly, adjust the control valve slowly to prevent any over leakage of gases. Next, record
down the values of pressure and temperature after it stabilize. Furthermore, a high-tech equipment should be use to replace the faulty equipment like vacuum pump which have problem in leakage of gases. There are some improvement can be make to the equipment. This experiment can be done with or without a computer connected, however, for quicker tests with easier recording of results, TecQuipment can supply the optional Versatile Data Acquisition System (VDAS). This gives accurate real-time data capture, monitoring and display, calculation and charting of all the important readings on a computer. To improve the accuracy of data, few equipment can be connected inside the apparatus. For example, a thermocouple use to measure the temperature of the heater surface for the controller. While, two thermocouples measure the temperature of the air in the vessel. A pressure transducer measures the pressure of the heated air in the vessel. A digital display shows the absolute pressure, both temperatures and their average value.
Conclusion
In conclusion, the ideal gas law was the main equation used in the calculations for this experiment. Since both trials came out with almost identical calculations, it is believed that the experiment went as planned and no errors were made. Under the present experimental conditions, air behaves like an ideal gas. At fixed volume and fixed moles, measurements of pressure vs. temperature exhibited a linear relationship, consistent with the equation of state for an ideal gas. However, as the pressure increases, the gas behave less like an ideal gas because the movement of gas restricted and become less randomly move. Lastly, the largest source of uncertainty came from a faulty vacuum pump which cause the leakage of gases.
References 1.)
Palmer,
WP
(1991), "Philately, Science Teaching and the History of
Science" (PDF), Lab Talk , 35 (1): 30–31 2.)
Barnett, Martin K. (Aug 1941), "A brief history of thermometry", Journal of
Chemical Education, 18 3.)
Tippens, Paul E. (2007). Physics, 7th ed. McGraw -Hill. 386–387.
4.)
Cooper, Crystal (Feb. 11, 2010). "Gay -Lussac's Law". Bright Hub Engineering.
Retrieved from http://www.brighthubengineering.com/hvac/26213-gay-lussacs-law/
