Lab: Evaporation and Intermolecular Attractions
2) Explain the differences in the difference in temperature of these substances as they evaporated. Explain your results in terms of intermolecular forces.
The five substances we tested were: Methanol, Ethanol, n-Butanol, Glycerin, and Water. All of these substances have their strongest intermolecular force being hydrogen bonding. Of the five, though, Methanol changed the most in terms of temperature, going down a total of 14 degrees Celsius. We believe that this major change of temperature is due to the high volatility this substance has. This is caused by the fact that the molecule has only one potential hydrogen bond to form with another molecule. The one site where potential hydrogen bonds can form is relatively low if compared to other the other substances. The next largest drop in temperature was Ethanol which 6.3 degrees Celsius. Ethanol, similarly to methanol, only has one site for a possible hydrogen bond to form with a similar molecule. The reason why there is a 7.7 degrees difference between the two is due to the increase of the molar mass. Ethanol has a molar mass of 45.04 grams and methanol has a molar mass of 32.04 grams. The molar mass of ethanol means that the molecules themselves are heavier, and therefore require more energy to change its state of matter, leading to less molecules evaporating. The change in temperature for water was -4.3 degrees Celsius. Water, having two sites for potential hydrogen bonds to form with other molecules definitely has something to do with the 2 degrees difference in change when comparing water to ethanol. The next substance is n-Butanol, with a change of -2.0 degrees Celsius. n-Butanol, having only one site for possible hydrogen bonding, would commonly be thought to have a higher change in temperature, but this is not the case. My partner and I believe it is due to the huge molar mass when compared to water; n-Butanol with 74.04 grams and water with 18.01 grams. Finally, the last substance tested was glycerin. The change in temperature recorded was not what was expected. The change was +1.6 degrees Celsius. After this result, my group can upon the conclusion that since glycerin has three sites for possible hydrogen bonding and has a substantial molar mass of 92, that the cooler temp. inside the beaker and the warmer temp. in the lab over came the evaporation cooling effect. This was most likely due to the lack of said evaporation.
3) Explain the difference in evaporation of any two compounds that have similar molar masses. Explain your results in terms of intermolecular forces.
Methanol and ethanol have the two closest molar masses in the experiment with a difference of 13 grams, but there difference in temperature was among the largest being 7.7 degrees. Even though both substances only have one site for hydrogen bonding to take place, the extra carbon atom and two hydrogen atoms where enough to alter the results. This is most likely due to the idea of London Dispersion Forces. The extra carbon and hydrogen atoms bring electrons with them as well. The more electrons in a molecule, the more of a possibility that a slight change in distribution could occur. The change in distribution could have effect on the overall polarity of the molecule and this has a ripple effect throughout the substance. The increase in the chance for an intermolecular force to occur causes the evaporation rate to decrease because the substance with more intermolecular force requires more energy to change its state of mater.
4) Explain how the number of -OH groups in the substances tested affects the ability of the tested compounds to evaporate. Explain your results in terms of intermolecular forces.
The more -OH groups in the substance, the less it will evaporate. This is due to the fact that hydrogen bonds are difficult to break. The more -OH groups there are, the more potential hydrogen bonds there are for that substance. With these two things in mind, one can come to the conclusion that the more -OH groups there are, the more hydrogen bonds can form, which means that there has to be more energy to change its state of matter, meaning it is less likely to evaporate.
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