Chemistry Lab-Enthalpy of Vaporization of Water Discussion and Analysis

 

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Lani Chung Professor Pasternack Chemistry Lab (Thursday 2:30) Enthalpy of Vaporization of Water Discussion and Analysis 13 November 2013 Experiment #6: Enthalpy of Vaporization of Water  Discussion: When the temperature is raised in the system, which in this case consists of the beaker that is heated using the Bunsen burner, energy is added into the system in the form of heat. Since the system absorbs heat in this situation, the phase change that occurs as a result is endothermic. e ndothermic. The endothermic nature of the phase p hase change causes the system to have sufficient amounts of energy to break the bonds between the H2O molecules in order to carry out the following reaction of water vaporization: ( )   () It is important to note that higher temperatures correspond with greater amounts of heat energy, energ y, and greater quantities of energy will allow for more molecular bonds to be broken. This process of creating gaseous H2O in the form of water vapor as a result of raising the temperature of the system is consistent with the observations made in the lab. This is because of the relationship that exists between temperature and volume in the system that is considered in the experiment. There is a direct proportionality propo rtionality between temperature and volume — meaning meaning that volume increases when temperature increases —   because greater temperatures cause more bonds to be broken between water molecules and also prompts gases to expand as heat energy causes gas molecules to grow farther apart thought the moles are kept consistent. The consistently increasing volume of the bubble in the graduated cylinder as the temperature was raised until it reached about 80 — which which was noted in the observations — helps helps to prove the direct  proportionality between volume and temperature of gases mentioned previously.

 

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The relationship between vapor pressure and temperature is defined by the equation

 ( (vap) 

 

 + C. This equation, much like the standard linear function of mx+b, generates

a straight line that plots  as a function of 1/T. The slope of this function is defined by -

 

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while the y-intercept is defined by C. The slope of the function in this situation depends on the sign of the  as R is a positive constant. To determine the sign of , the reaction must be determined to either be endothermic or exothermic since  is negative when a reaction is exothermic, and positive when a reaction is endothermic. In the particular system that is being discussed in the context of this experiment, ex periment, the reaction is determined to be en endothermic dothermic because heat is added into the system, s ystem, not released. Therefore, the  is a positive value. According to the function defined above, a positive  will yield a negative slope, which was obtained in the graph generated with the observed  and 1/T values. In the bubble that is generated in the graduated cylinder, the pressure constantly remains at room pressure though the volume of the bubble increases because the pressure is equivalent to that of the room’s air pressing air pressing down on the water surface of the beaker. Therefore, no matter what the temperature might be in the graduated cylinder, the equation        holds true. Although the partial pressures exerted by the air and the water may fluctuate with the temperature, their sum should remain the value of the atmospheric (room) pressure. This  property is consistent with the results obtained in the lab, as accurate values of   were determined by subtracting   (which was obtained using the ideal gas law equation of     

) from  . By using water pressure at each temperature as well w ell as the function plotting

 and 1/T, the enthalpy of vaporization was able to be determined.

 

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 Error Analysis: The standard deviation that was obtained of the th e slope (-4456) was determined to be

78.1, the standard deviation of the y-intercept (11.90) was determined to be , and the standard deviation that was obtained of   was determined to be . The percent deviation for the experimental value compared compa red to the literature value of   was calculated as 8.979%. The standard deviation of   is about 7.23 times greater than that of the percent deviation. As A s for instrumental uncertainty, the 10ml graduated cylinder that was used for the volume measurements of the bubble had an accuracy of 0.1ml. Therefore the final volume measurement at 4.9 of 3.91ml had about a 3% uncertainty value. In addition,  because the graduated cylinder was read in an inverted position, the shape of the meniscus is reversed. Therefore it is necessary to correct for such error by b y subtracting 0.2 ml from all volume readings. There did not seem to be b e instances of temperature dependence in regards to experimental uncertainty as the uncertainty holds true for all volumetric readings. Another instrumental uncertainty is in the yellow digital thermometer, which was used to take the temperatures of the water. The thermometer has a 0.1 uncertainty, meaning that the final temperature measurement of the chilled water (4.9 ) had an uncertainty value of about 2%. Experimental uncertainty can be found in the volume readings that were taken in the inverted graduated cylinder. When reading the th e volume, it is important that the graduated ccylinder ylinder is on a level surface so that the measurements are precise and not skewed due to slight tilts in orientation. However, in the experiment, it is difficult to maintain leveled orientation of the graduated cylinder because of the unbalanced shape of the graduated cylinder’s opening. In addition, the water constantly causes the graduated cylinder to shift in orientation, which adds add s to the uncertainty of the volume measurements taken.

 

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Another instance in experimental uncertainty stems from the inability to truly know when the heat is equally distributed throughout the water, and when the water in the graduated cylinder reaches an equal temperature to the water heated in the beaker. Although it is true that the  procedure mentions a need for continually stirring the water while the beaker is heated over the Bunsen burner in order to maintain even temperatures, without waiting for the water to completely equilibrate in terms of heat, the temperature readings might be inaccurate since the readings would not necessarily represent the temperature of the liquid as a whole or o r the temperature inside of the graduated cylinder. c ylinder. Therefore in order to account for this error, multiple temperature readings can be taken with the thermometer in different positions in the  beaker in order to determine whether an equal distribution of heat has occurred throughout the liquid. If the measured temperatures are consistent, the slight uncertainty of temperature can be eliminated. Another underlying assumption that is made in this experiment has to do with the ideal gas law calculations that were involved in determining det ermining the partial pressure of the air, which was later used to determine the partial pressure of the water. In the calculations involving the ideal gas law, it was assumed that the air in the graduated cylinder was an ideal gas and it was also assumed that the room temperature and pressure remained constant. co nstant. Though the error might be small, it can be corrected for by b y applying the Van der Waals equation of state.