Intermolecular Forces of Various. Substances Lab Report. Chemistry. Period 3. Crater School of BIS. January 14, Joshua L.

Transcription

1 Intermolecular Forces of Various Substances Lab Report Chemistry Period 3 Crater School of BIS January 14, 2016 Joshua L. Idiart

2 Introduction: In this experiment, six substances were tested in various ways to determine the strength of the intermolecular forces between them. In order to determine this strength difference, one must first understand what intermolecular forces are and what they do. Intermolecular forces, or IMF s, are the attractive forces between molecules. They are not to be mistaken with intramolecular forces, which are the forces between atoms inside the molecule itself. IMF s are the forces that keep multiple molecules together. There are different IMF s for different atoms, which vary in their levels of strength. There are two major types of IMF s: Van Der Waal s forces, and Hydrogen Bonds. Van Der Waal s forces are further subcategorized as London Dispersion and Dipole-Dipole. London Dispersion IMF is the weakest of all the IMF s, because they occur in molecules that have nonpolar bonds. If a bond is non-polar, that means that the electrons between the two covalently bonded atoms are shared equally. A bond is determined to be non-polar if the electronegativity difference between the two atoms in question is 0.4 or less. If the difference is greater than 0.4, then the bond is polar, meaning that the electrons between the two atoms aren t shared equally. Polar bonds have a partial charge, so they have a stronger IMF: Dipole-Dipole. If a molecule is partially charged, it is more easily able to stick to other molecules with partial charges, so the IMF must be stronger. The next strongest IMF is the Hydrogen Bond. This bonding between molecules occurs when Hydrogen atoms in one molecule are extremely attracted and almost bond to an Oxygen, Nitrogen, or Fluorine atom of another molecule. Hydrogen bonds are stronger than Dipole-

3 Dipole IMF s because they occur when bonds with Hydrogens inside molecules are extremely polar, meaning that the electronegativity difference is very large. Ionic molecules are a completely different story. Ionic bonds have a full charge, so they stick to other ionic molecules perfectly. There is no sharing of the electrons, so there is no unequal distribution. They don t have intermolecular force because they are their own. Because of all this, ionic bonds have a stronger force between them then any intermolecular force, so in other words, they are the strongest on the scale of strength. The purpose of this experiment was to test six substances, see which has the strongest IMF, and to see what that means. The liquids that were tested were Hexane (C 4 H 14 ), Water (H 2 O), and Vegetable Oil (C 54 H 100 O 7 ). Hexane contains only non-polar bonds (between Carbon and Hydrogen), meaning it had London Dispersion IMF. Vegetable oil contains non-polar bonds (between Carbon and Hydrogen), but it also contains polar bonds (between Carbon and Oxygen), meaning it has Dipole-Dipole IMF. Water contains extremely polar bonds between its Oxygen and Hydrogens. This causes it to have Hydrogen Bonds between one molecule of it, and another, with the partial negative charge of the O in one, to the partial positive charge of the H in another. Theoretically, because water has the strongest IMF, it should stick to itself more than vegetable oil, which should more than hexane. As for the solids, Bees Wax (C 15 H 30 COOC 31 H 61 ), Sugar (C 12 H 22 O 11 ), and Salt (NaCl) were tested. Bees wax contains long strands of non-polar bonds (between the Carbons and Hydrogens), and some polar bonds (between the Carbons and Oxygens), making it have Dipole- Dipole IMF. Sugar contains some non-polar and polar bonds as well, but it has polar bonds with the Hydrogen, making it have Hydrogen Bonding IMF. Salt is an ionically bonded molecule, so

4 it doesn t share electrons in any way, it gives or takes them. That means it stocks together the best out of the three solids. Equipment and Materials: Hexane (C 4 H 14 ) Bunsen Burner Ring Stand Water (H 2 O) Clay Triangle 3 Evaporating Dishes Vegetable Oil (C 54 H 100 O 7 ) Scupula Wire Gauze Bees Wax (C 15 H 30 COOC 31 H 61 ) 3 Pennies Eye Dropper Sugar (C 12 H 22 O 11 ) Matches Beaker Tongs Salt (NaCl) Stop Watch Fume Hood Procedures: 1) Under a fume hood, hexane was dropped with an eye dropper onto a penny, head s up, until it spilled over, and the number of drops was recorded. This was repeated twice more, with the penny being cleaned and dried off between each trial. The experiment was then repeated with water instead of hexane, on a new penny, not under a fume hood. After three trials of water were tested, three trials of vegetable oil were tested, using the same method as the water. Each time the liquids spilled over the sides of the penny, the number of drops were recorded in a data table. 2) A pea-sized amount of bees wax was placed into an evaporation dish, and placed on a small piece of wire gauze above a lit Bunsen burner. The amount of time it took for the wax to completely melt to the point where no solid was visible was recorded for three trials, with the dish being cooled and washed out between each trial. After the bees wax, three trials of sugar were tested in the same fashion, but in a new evaporating dish. Salt was then tested thrice, with a time cap of three minutes (180 seconds).

5 Amount of Time (sec) Amount of Drops Data: The Amount of Drops of Various Liquids on a Penny Trial Number Hexane Vegetable Oil Water The Amount of Time Taken to Melt Various Solids Trial Number Wax Salt Sugar

6 Conclusion: It can be concluded that from this lab, that the London Dispersion and Dipole-Dipole intermolecular forces were the weakest of all the IMF s. Hydrogen Bonds are the next strongest and Ionic Bonds are by far the strongest. This was affirmed from the data because water, which has Hydrogen Bonding IMF, could fit the most drops of itself on a penny s surface - an average of 35 - meaning it could stick to itself the best because its IMF was the strongest. As for hexane and vegetable oil, the data that was collected was somewhat faulty. The vegetable oil should have fit more drops on the penny than the hexane, because the oil had London Dispersion, a weaker IMF, than hexane, which has Dipole-Dipole, the stronger of the two. Unfortunately, due to error, the data was somewhat flawed, but it was still concluded that Hydrogen Bond IMF s are stronger than Dipole-Dipole and London Dispersion IMF s. For the solids, the data was much clearer and straight-forward. The theory that ionic bonds are stronger than all IMF s and that Hydrogen Bonds are again stronger than Dipole- Dipole was proved correct. Salt, which is an ionically bonded molecule took more time than was even allowed to melt, the melting point was so high, because ionic molecules have such a strong intermolecular force. Sugar was the next highest boiling point because it had polar bonds involving Hydrogen, making it have Hydrogen Bonding IMF. The last solid that was tested was beeswax. Beeswax had the lowest boiling point because it had mostly non-polar bonds, but a few polar bonds, making it have Dipole-Dipole IMF, the weakest of the three solids. Intermolecular force affects boiling point, because the stronger the IMF, the more tightly-packed the molecules are together, the more energy - heat - is required to break those molecules apart to melt them. So in short, the stronger the IMF is, the higher the melting point is.

7 One possible source of error came from the eyedroppers being different sizes. At every station, different eyedroppers were used, which may have resulted in different sized drops. This possible error could ve been more precise if the same eyedropper was used for each liquid each time, or just using a new one each trial for each liquid. Another possible source of error were the actual pennies themselves. Between each trial they were washed and dried off, but some substances could easily have been left on them that could very well have affected the results. This could be eliminated in future experiments by more thoroughly washing and drying the pennies, or just using different pennies each time like the eyedropper situation. To further test IMF s to support data from this experiment, one could test the boiling point of different liquids. Intermolecular forces affect the boiling point of substances because, like the melting point, one is loosening those intermolecular bonds. In the case of the new, hypothetical lab, the boiling point is just taking it another step further than the melting point from this lab. A subject could take three liquids; each with different IMF s, put them over heat, and time them to see when each of them boils. They could then see if the results support or disprove the IMF theory that the stronger the IMF, the higher the boiling point.