ABSTRACTS
The vapor pressure of water refers to the pressure exerted by water vapor molecules when they
are in equilibrium with liquid water at a specific temperature and the enthalpy of vaporization, also
known of vaporization , is the amount of heat energy required to convert one mole of liquid water
into vapor at a constant temperature and pressure. The experiment conducted aimed at determining
the vapor pressure and estimating the enthalpy of vaporization of water. During the experiment
800mL beaker with filled distilled water to just below the lip. Then 6.5ml of water was put in a
measuring cylinder. The mouth of the cylinder was closed and inverted into the beaker of water.
The water was heated to 80oC and allowed to cool. The volume of gas in the cylinder was recorded
at every 5oC to 30oC then an ice was put in the water to drop the temperature to 5oC and the volume
of the gas was recorded. From the recorded temperature and the volumes, the values of 𝐼𝑛𝑝 𝑎𝑛𝑑
were obtained and a graph of 𝐼𝑛𝑝 𝑎𝑛𝑑
1
𝑇
1
𝑇
was plotted. The value of the enthalpy was calculated
using the Clausius Clapeyron Equation. The enthalpy was calculated to be 8.7kJ.
1
INTRODUCTION
In thermodynamics and chemical engineering, vapor pressure of water refers to the pressure
exerted by water vapor at thermodynamics equilibrium with its liquid phase at a specific
temperature. It represent the tendency of molecules to escape from the liquid and enters the gas
phase. This process is called phase transition. The phase transition is the spontaneous conversion
of one phase into another phase, occurs at a characteristic transition temperature, Ttrs, for a given
pressure (Atkins, P. W. (2006). At this point the vapor pressure is called saturated vapor pressure.
As the temperature increases, the average kinetic energy of water molecules also increases. When
the kinetic energy is increasing, more molecules escapes the water surface and the saturated vapor
pressure is correspondingly higher.
The enthalpy of vaporization, ∆𝐻𝑣𝑎𝑝 of water is the energy required to evaporate one molecule of
water at constant temperature and pressure. It is a measure of the strength of intermolecular
forces in a pure water substance. The relationship between the temperature of a water and its
vapor pressure is not a straight line. The Clausius Clapeyron Equation is a mathematical
relationship relating the vapor pressure of a liquid to the temperature of the liquid. When vapor
pressure is plotted against temperature, a nonlinear trend is observed,
𝐼𝑛𝑝 = −
∆𝐻𝑣𝑎𝑝
𝑅𝑇
+𝐶
Where Hvap is the enthalpy of vaporization, R is the universal gas constant (8.314 J mol-1k-1),
T is the Kelvin temperature, and C is a constant based on the liquid. Equation 1 is an equation in
the straight line form y=mx+b. The slope of this plot is equal to −
∆𝐻𝑣𝑎𝑝
𝑅
. Using this relationship
the value of the enthalpy of vaporization can be determined. Thus, this experiment allows to
calculate the enthalpy of vaporization of water using the Clausius-Clapeyron equation by obtaining
the vapor pressure of water over a range of temperatures.
2
METHODOLODY
An 800mL beaker was filled with water just below to the lip. About 6.5ml of water was poured
into a 10mL graduated measuring cylinder. The top of the tube was covered by the figure and
inverted. While inverted the cylinder was put into the beaker. The beaker was put on a wire gauze
on a tripod stand. The cylinder was clamped alongside a thermometer. The water was heated to
80°C by the Bunsen burner. The heat was removed and water was allowed to cool. During cooling
the volume of water was recorded at every 5°C till the temperature drops to 30°C from 80°C.
During cooling half of the water was removed and replaced by an ice till the temperature drops to
5°C and the volume of trapped air in the tube was recorded.
3
RESULTS AND DISCUSSIONS
The table bellows shows the temperature and the volume reading obtained during the experiment
TEMPERATURE OF THE WATER DURING COOLING
(oC)
VOLUME OF THE AIR IN THE TUBE
(mL)
80
6.8
75
6.7
70
6.5
65
6.3
60
6.1
55
5.9
50
5.7
45
5.5
40
5.3
35
5.1
30
4.9
5
4.0
Assuming that the atmospheric pressure is 760mmHg. The low temperature volume can be
taken as air in the cylinder at that temperature and pressure since vapor pressure of water is
negligible at 5°C.
The vapor pressure of water in the cylinder is:
𝑃𝐻2𝑂 = 𝑃𝑎𝑡𝑚 − 𝑃𝐻2𝑂
4
Temperature(K)
Volume (cm3)
𝑃𝑎𝑖𝑟 (mmHg)
𝑃𝐻2𝑂 (mmHg)
𝐼𝑛𝑃
353
6.8
583.1
287.0
5.66
1
𝑇
2.832×10-3
348
6.7
583.4
286.6
5.65
2.874×10-3
343
6.5
592.7
277.3
5.62
2.915×10-3
338
6.3
602.6
267.4
5.59
2.959×10-3
333
6.1
613.2
256.8
5.54
3.000×10-3
328
5.9
624.4
245.6
5.50
3.049×10-3
323
5.7
636.5
233.5
5.45
3.096×10-3
318
5.5
649.4
220.6
5.39
3.144×10-3
313
5.3
663.3
206.7
5.33
3.195×10-3
308
5.1
678.3
191.7
5.26
3.247×10-3
303
4.9
694.6
175.4
5.17
3.300×10-3
278
4.0
780.6
89.4
4.49
3.597×10-3
The graph of
𝐼𝑛𝑝 =
𝐴
𝑇
+𝐵
5,8
5,7
5,6
Inp
5,5
5,4
5,3
y = -1054,6x + 8,694
5,2
5,1
0,0028
0,0029
0,003
0,0031
0,0032
0,0033
0,0034
I/T
During heating the volume of the vapour of water collected in the empty space of the measuring
cylinder started to increase till the thermodynamic equilibrium is achieved. This is because during
5
heating the temperature of the water molecules increases thus increases the kinetic energy of
the molecules thereby escaping from the water surface. During cooling the vapor pressure
decreases following the phenomena inverse relationship with the temperature. The temperature
was cooled to 5°C. At this temperature the vapor pressure of water is negligible and can be
neglected. Pair is taken to be equal to the barometer pressure. Given the number of the moles of air
trapped in the cylinder, the 𝑃𝑎𝑖𝑟 is calculated using the ideal gas equation because the volume
vapour pressure is assumed to behave ideally (𝑃𝑎𝑖𝑟 =
𝑛𝑎𝑖𝑟 𝑅𝑇
𝑉
). At temperature above 5°C, the total
pressure in the cylinder (Pcylinder) is the sum of water vapor (Pw) and that of the trapped air (Pair)
according to the Dalton’s
𝑃𝑤 + 𝑃𝑎𝑖𝑟 = 𝑃𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 …………………………………………………………… 1
The total pressure in the cylinder, 𝑃𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 was calculated from the measured barometer
pressure and the difference in water levels in the beaker and in the cylinder.
𝑃𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 = 𝑃𝑎𝑡𝑚𝑜𝑠𝑝ℎ𝑒𝑟𝑒 + ℎ(𝑚𝑚𝐻2 𝑂) ×
1.0𝑚𝑚𝐻𝑔
13.6𝑚𝑚𝐻2 𝑂
……………………………………… 2
The partial pressure of water at every temperature is calculated using the equation 1 and 2.
The analyzed data of pressure- temperature leads to the plotting of the graph of 𝐼𝑛𝑃𝑤 versus 1/T.
the enthalpy of vaporization of water, ∆𝐻𝑣𝑎𝑝 was calculated from the slope of the linear
produced graph. The graphical method of calculating the enthalpy of vaporization is more
accurate than the method of using two different points because it use multiple of data points.
The more points available, a truer correlation is made as there is a possibility of measurements
errors that goes by unnoticed.
During the experiment an estimation was made that h (gas level) remained constant throughout
all the measurement despites the change of the gas volume level in the cylinder. The fact is that
the level of the gas (h) was changing due to the change of the volume as the pressure was
changing and the expectation is that the value of (h) probably should have been measure for each
6
temperature. They were some error associated with the experiment. For example, human errors
caused by human factor which were due to incorrect readings of the temperature, improper and
poor set up of the apparatus mostly in inverting the measuring cylinder and calculations
mistakes.
7
CONCLUSION
The purpose of the experiment was to estimate the enthalpy of vaporization of water vapor
pressure which was successful met. The enthalpy of vaporization was calculated using the slope
of the linearly graph of 𝐼𝑛𝑝 𝑣𝑒𝑟𝑠𝑢𝑠
1
𝑇
. The thermodynamic phenomena is that when the water
is been heated, the molecules increases in kinetic energy and escape from water surface to gas
phase and during cooling the kinetic energy decreases thus increasing the intermolecular force
among the particles and condenses back to liquid. Through the process the data of pressure and
temperature was obtained and analyzed to obtain the enthalpy of vaporization. Some of the
associated errors during the experiment can be reduced by being accurate in taking readings.
8
REFERENCE
Atkins, P. W. (2006), Physical Chemistry, 8th Ed. Oxford University Press: Oxford
CHE 411 & 421, 2013/2014 Physical Chemistry Practical’s Laboratory Manual. Chemistry
Department, Chancellor College, Zomba.
J.M. Smith, Hendrick Van Ness, Michael Abbott, Mark Swihart(2018) Introduction to Chemical
Engineering Thermodynamics. McGraw-Hill Education (2018)
Thomas Engel, phillip Reid, (2006) “physical chemistry” Pearson Education INC,,New York 2006
9
APPENDIX
𝑃𝑉
760×0.004
nwater =𝑅𝑇 , = 62.4×278 =1.8×10−4mole
1.8×10−4 ×62.4×278
Pair at 278 k=
𝛥𝑝
0.004
=780mmHg
5.33−5.62
Slope=𝛥𝑇=0.00144−0.00295= - 1,543
∆𝐻
Since, slope= - 𝑅 ,
∆𝐻 =1,543K×8.314J/k.mol=12.8KJ/mol
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