Enthalpy of Water and Vaporization

Vapor Pressure and the Enthalpy of Vaporization of Water Lab Partners: Abstract: The report depicts an experiment to study the relationship between pressure and temperature by comparing the effect of adding water into a closed container. There were two goals of this experiment. The first goal was to observe based off the collected data how pressure relates to temperature when undergoing a phase change; the second goal was to calculate the enthalpy of vaporization of water in order to see how much heat energy was needed to change one mole of liquid into one mole of vapor. The report presents two experiments (1) controlled experiment, in that no water was added to the fixed volume and (2) variable experiment, in testing the effect of adding water into a fixed volume. The collected data from the controlled experiment followed an expected trend predicted by the ideal gas law, and the collected data from the variable experiment followed an exponential trend predicted by the properties of water such as higher intermolecular forces will exert more pressure. Also, the ΔHvap of water calculated based off the variable experiment was 32.6 kJ/mol. The calculated ΔHvap of water from the experiment was different than the true value of ΔHvap of water ( ΔHvap of water= 40.7 kJ/mol) from the Chemistry: Structure and Properties by Nivaldo J. Tro, 2nd edition. II. Results a. Air pressure vs. Temperature Graph (for trial with the empty flask) Figure 1 below shows Air Pressure vs. Temperature plots of a linear Pressure (torr) 750 f(x) = 1.59 x + 682.46 R² = 1 740 730 regression of air pressure 720 710 of the empty flask 700 690 depicting the relationship 680 670 5 10 15 20 25 30 35 40 45 between air pressure and temperature. Temperature (°C) Figure 1. Plots of a linear regression of air pressures measured at different temperatures. b. Plot of lnPvap vs. 1/T (K) for trial with water in your flask Figure 2 below shows lnPvapor vs. 1/T the Clausius-Clapeyron 3.9 ln (Pvapor) 3.7 3.5 f(x) = − 3920.85 x + 16.49 R² = 0.94 3.3 3.1 2.9 2.7 2.5 0.0032 0.0033 0.0033 0.0034 0.0034 1/T (K) 0.0035 0.0035 0.0036 plots of the vapor pressure of water in the flask depicting the relationship between pressure and temperature. Figure 2. Plots of a linear regression of vapor pressures of water measured at different temperatures. c. Calculation of the enthalpy of vaporization of water Linear regression line: y= mx+b Clausius-Clapeyron equation: lnPvap= Linear regression line: y= − ΔHvap (1/T) + ln β R Clausius-Clapeyron equation: lnPvap= mx+b ln β lnPvap − ΔHvap R x 1/T b ln β How to calculate the enthalpy of vaporization of water y m Step 1: Slope = m = − ΔHvap R ¿ R) Step 2: ΔHvap = - (m Step 3: ΔHvap = - (-3920.8K ¿ (8.314J/mol ¿ K)) − ΔHvap (1/T) + R 4 Step 4: ΔHvap = 3.26 * Step 5: ΔHvap = 32.6 kJ/mol 10 J/mol d. Value of the enthalpy of vaporization of water from your textbook ΔHvap for water = 40.7 kJ/mol (Chemistry: Structure and Properties by Nivaldo J. Tro, 2nd edition) III. Discussion The objective of the lab experiment was to see the effect of pressure with temperature change. There were two goals of the experiment. The first goal was to observe based off the measurements how pressure relates to temperature in a fixed volume; the second goal was to calculate the enthalpy of vaporization of water. The enthalpy of vaporization is the heat energy required to turn one mole of a liquid into one mole of a gas. The enthalpy of vaporization is important to measure because it will help us determine (1) the quantitative amount of heat required to vaporize one mole of a liquid to a gas, (2) the relative strength of the intermolecular forces, which will help us determine what bonds need to be completely overcome to change the phase from a liquid to a gas (3) the temperature of the boiling point of the liquid, where the liquid’s vapor pressure equals the external pressure. The first experiment conducted was a controlled experiment where the flask (closed container) had no water. The observed behavior from the controlled experiment showed the expected trend predicted by the ideal gas law (PV=nRT, where P is pressure, V is volume, n is moles, R is the ideal gas constant, T is temperature). As seen from Figure 1, the trendline is an upward linear line with a positive slope. The linear regression line is y=1.5911x + 682.46 where y is air pressure (torr) and x is temperature (°C). The slope ( ∆y ) is 1.5911 and is calculated ∆x from the change in y divided by the change in x. The slope indicates that as x (temperature) increases by 1, y (pressure) increases by approximately 1.6. The second experiment conducted was a variable experiment where water was added to the flask (closed container). The observed behavior from the variable experiment showed an exponential trend between vapor pressure and temperature. When one unit of temperature increased, vapor pressure increased by almost triple the amount of temperature. Since the collected data did not represent a straight linear line, the Clausius-Clapeyron equation was used to expresses the relationship between vapor pressure and temperature. The Clausius-Clapeyron helps determine the enthalpy of vaporization as seen above in the calculation of the enthalpy of vaporization of water. The enthalpy of vaporization calculated from the variable experiment was 32.6 kJ/mol while the textbook value of the enthalpy for water was 40.7 kJ/mol. Using the Percent Error equation ( experimentalvalue−truevalue ×100%) truevalue , the enthalpy calculated from the variable experiment showed there was approximately a -19.9% error based off the textbook value and the experimental value. Probable reasons for error are (1) experimental error (2) the room (the surroundings) having a different pressure (3) potential air leaks when the rubber septum was punctured (4) inaccurate temperature results caused by placing ice cubes near the thermometer. IV. Conclusion The purpose of this lab was to compare the independent variable of temperature and how adding or having no water affected the dependent variable of pressure and observing the effect of water in a controlled variable, 50-mL flask. In order to make scientific conclusions on the effects of adding water, two experiments were conducted. The first experiment was a controlled experiment where no water was added to a fixed volume (50-mL flask). The controlled experiment showcased a positive linear trendline predicted by the ideal gas law. As temperature increases, the kinetic energy increases between molecules. The increase in kinetic energy triggers more molecules hitting the wall within the container which increases the amount of pressure. The relationship between temperature and pressure is predicted by Gay-Lussac's law, which states that at a constant volume, the pressure of an ideal gas is directly proportional to its temperature. The second experiment was a variable experiment where water was added to the fixed volume. The pressure recorded was significantly higher when water was added to the flask compared to having no water added to the flask. The enthalpy of vaporization is the heat necessary to change one mole of a liquid into one mole of vapor at a constant temperature. The enthalpy of vaporization is directly proportional to intermolecular forces because the strength of intermolecular forces determines how strong are the attractive forces between particles. If an intermolecular force is strong, then the enthalpy of vaporization is high and vice versa. Water has one of the strongest intermolecular forces due to the bonding between the oxygen and hydrogen atoms and the molecular bent shape of the water molecule. Hydrogen bonding is prevalent in water molecules, so it is harder to break apart water molecules due to the strong intermolecular forces. Since intermolecular forces and the enthalpy of vaporization are directly proportional, the strong intermolecular force of water results in a higher enthalpy of vaporization. It is also important to note, when the phase changes from a liquid to a gas, the temperature does not change, but the movement between molecules increases till all liquid molecules vaporize to gas molecules. The high ∆Hvap for water is important because the high energy needed to overcome the intermolecular forces of water allows organisms the ability to resist temperature changes that would cause internal damage.