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Enthalpy Of Vaporization Of Water Biology Essay

The purpose of this experiment is to measure the vapor pressure of water in order to calculate the enthalpy of vaporization of water. An air bubble will be created and its volume will be measured over a range of temperatures. From this data, nair, Pwater, and ln(Pwater) will be calculated and these values can then be used to determine the enthalpy of vaporization of water.

In this experiment, many chemistry concepts were used that enabled the recorded data to be used to calculate the enthalpy of vaporization of water, which is defined as the amount of energy required to break the intermolecular forces between water molecules. An air bubble was created by inverting a graduated cylinder inside a large, water-filled beaker. The air bubble's volume was measured at a variety of different temperatures, which were measured using a digital thermometer, in order to determine the quantity of molecules that evaporated into water vapor as the molecules overcame the intermolecular forces. As the temperatures increased, the bubble's volume increased as well due to larger amounts of water vapor. While the temperatures changed, the amount of moles of water vapor fluctuated while the moles of air remain constant throughout the experiment. After the data was recorded, the temperature of the bubble was decreased to 5.0 degrees Celsius since at such a low temperature, water vapor is negligible. Recording the volume of the air bubble at the low temperature allowed one to determine the amount of moles of air in the graduated cylinder by using the ideal gas law Pair= ((nair)(RT))/V. In this equation, nair is the amount of moles of air while R is the ideal gas constant, T is the temperature in Kelvin, and the volume is the corrected volume of the air bubble at the low temperature in liters. It was also important to be aware that the total pressure of the air bubble was equal to that of the system's surroundings since the air in the room exerted pressure onto the water in the beaker. Hence, the total pressure of the system was found to be 743.7 mm of Hg using the barometer in the lab which measured the room temperature. Once Pair and Patm were calculated, the equation Pwater= Pair + Patm was used to determine the partial pressure of the water. The equation ln(Pvap)= - (ΔH°vap)/RT + C, in which C= ΔH°vap/373.15R, then provided a direct connection between the partial pressure of water and temperature. Multiplying the negative slope of plot Ln(Pwater) Vs. 1/Temperature by the gas constant - 8.315 J/mol K yields the enthalpy of vaporization of water which in this experiment was found to be 43.2 kJ/mol. The literature value, or recognized standard value, of the enthalpy of evaporation of water is actually 40.66 kJ/mol. The slight percent deviation of 6.2% signifies that overall, the experiment lacked large errors or miscalculations and can be considered successful.

Error and Uncertainty Analysis

As mentioned earlier, the acknowledged literature value of the enthalpy of vaporization of water is 40.66kJ/mol. From the data of this experiment, the enthalpy of vaporization of water was calculated to be 43.2 +/- 0.1 kJ/mol. The standard deviation of 0.1 in 43.2 kJ/mol provides an uncertainty of .24%. Overall, the calculated value deviates from the literature value by 6.2% and there are multiple reasons for this uncertainty. Firstly, the literature value is of a room with pressure of exactly 1.00 atm and the room in which the experiment was carried out had a pressure of 0.9786 atm. This difference in room pressure allows the calculated values for nair to have a smaller value which is then used to find Pair and will decrease the amount of Pwater in the equation Pwater= Pair + Patm. It then leads to an increased enthalpy of vaporization of water after the values of natural log of Pwater are taken and plotted. Also, the graduated cylinder used in the experiment had an uncertainty of +/- .01 ml and although the accuracy was improved by subtracting 0.2 ml, the graduated cylinder readings cannot completely be dismissed as a source of error, especially since the bubbles and water vapor surrounding the graduated cylinder made its markings difficult to distinguish. It is possible this error could have created both increased and decreased Pair and Pwater values depending if the markings had been over- or under-estimated which lead to deviations in the values of natural log of Pwater. Furthermore, it was assumed during the experiment that at 5 degrees Celsius the water contained a negligible amount of water vapor. Yet, the small amount of water vapor present did exert vapor pressure but the error of dismissing it created a larger ΔHvap since it increased the value of each Pair and decreased each Pwater value.


I have completed this lab independently and have not looked at completed work from other students from this year or from previous years in 030.105.

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