Intermolecular Forces and Colligative Properties
Abstract
Intermolecular forces and colligative properties are both important factors when considering properties of compounds. Activities were performed to investigate these properties. Melting points and solubility were used in conjunction with knowledge of polarity to determine the role intermolecular forces play. Freezing point depression was then calculated for four different substances to find out what factors play a role in colligative properties. The data and calculations suggest that as the strength of intermolecular forces increases, so does melting point. The activity also reveals that substances with similar intermolecular forces dissolve in each other. The colligative property data shows that the main factors affecting them are van’t Hoff factors, or the amount of particles present is solution, and molality. Intermolecular forces play a role in colligative properties, as they determine what substances can dissolve in what and a substances initial properties. Introduction
These activities investigate intermolecular forces and various colligative properties. Intermolecular forces are the forces between different molecules, as opposed to the bonds in the molecule, like covalent bonds. The types of intermolecular forces are London dispersion forces (LDFs), dipole-dipole, hydrogen bonding, and ion-dipole. Hydrogen bonding can be seen in our own DNA, as hydrogen bonds hold the two strands of DNA together. Individual hydrogen bonds, like all intermolecular forces, are very weak, but when many are combined they can be strong, as is the case in DNA. The strength of intermolecular forces can also be seen in states of matter. Solids have the strongest intermolecular forces, with gases having the weakest and liquids in between. Colligative properties are properties that depend on the number or particles, not the identity of the particles. These properties are vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. Each of these properties describes what happens when you add particles to a pure substance with the exception of osmotic pressure, which will increase with more molecules present. Freezing point depression is something that is common in everyday life, like the use of salt to prevent ice. The addition of salt stops ice formation because the freezing point of a salt water solution will be lower than the freezing point of pure water. The structures that the hydrogen bonds form in ice are disrupted by the Na+ and Cl-ions from the dissociation of the salt, making it harder for the water to freeze.
Materials and Methods
The intermolecular forces activity began by drawing the best Lewis structure for each of carbon dioxide (CO2), water (H2O), propane (C3H8),methanol (CH3OH), ammonia (NH3) and oxygen(O2). The electron and molecular geometry were determined from the Lewis structures, as well as determining whether the molecule contains polar bonds and if it is a polar molecule. Then the molar mass, melting point and dipole moment of each molecule was looked up. The molecules were then ranked by melting point and the different types of intermolecular forces each of them contained was determined. The next part involved making predictions about whether potassium permanganate would be soluble in water, ethanol, and hexane and whether ethanol and hexane are miscible in water and if hexane is miscible in ethanol. An experiment performing these actions was watched to see if the predictions made were correct. The last part of this activity was done by attaching fourpieces of filter paper to thermometer probes using rubber bands. The thermometer probes were inserted into three different liquids, water, ethanol, and acetone, and the temperature was recorded for each after it stabilized. The thermometer probes were removed and placed on the lab bench. Temperatures were recorded every 30 seconds. The rate of cooling for of each liquid was calculated after all the data was recorded. The colligative properties activity was performed by thinking about how a student could keep a jar of solution outside all winter without it ever freezing, assuming the temperature never drops below -10 °C. A table was created in excel to show the molar mass and the predicted van’t Hoff factor for each of C12H22O11, NaCl, CaSO4, and FeCl3. A second table was created to calculate the molality and change in temperature of the freezing point for each substance, starting at 0.25 grams added and increasing by 0.25g each time. These calculations were done assuming the solution of water was 100 g and the freezing point depression constant, Kf, was 1.86 °C/m. The substances were added in 0.25 g increments until one of two things happened: the change in temperature reached 10°C or 100 g of the substance had been added. The second part of this activity involved answering question regarding each of vapor pressure lowering, freezing point depression, boiling point elevation and osmotic pressure. Results and Discussion The results of the intermolecular activity show that as polarity increases, strength of intermolecular forces increases, and the melting points will be higher. The reason for this is that the stronger intermolecular forces are, the stronger the molecules will be held together. More heat will be needed to break these forces. The one exception in the results is CO2, which has a high melting point despite only having LDFs, the weakest intermolecular forces. This can be explained by the unusual properties of CO2. At STP, CO2will not melt, but sublimate. The melting point is based on a higher pressure, and as pressure increases the melting point will also increase. The results from the solubility and miscibility part of the activity can be summed up as “like dissolves like”. Potassium permanganate is an ionic compound with ion-dipole intermolecular forces. It will dissociate into K+ and MnO4-. Water and ethanol are both polar, and thus will interact with those ions and be soluble. Hexane in non-polar so it will not be soluble. Ethanol is miscible in water because they are both polar, but hexane is not miscible in either water or ethanol because it is non-polar and the other two are polar. The identity of the three liquid that were tested were able to be identified using the collected data. Liquids 1 and 2 are water and ethanol because they evaporated slowest due to their hydrogen bonds. Liquid 3 is ethanol, which does not contain hydrogen bonding, meaning it will evaporate faster. The main takeaway from the colligative properties activity is the affects will be stronger with more particles present. The smallest change in temperature was seen with C12H22O11, a molecular compound. The other three are all ionic compounds that dissociate in water, and in those we see a much greater change in temperature. The molality matters as well. The least amount of substance needed to change the freezing point by 10° was NaCl, because it will have more moles than CaSO4 and FeCl3 when the number of grams of each is the same. Molarity is mol/kg, so more moles will lead to a higher molality. The calculations of the questions proved these points to be true as well. A higher van’t Hoff factor, meaning more particles present in solution, will have a strong affect on the colligative property.
Conclusions
There are some correlations between intermolecular forces and colligative properties. The “like dissolves like” expression applies to both. A polar substance will dissolve in another polar substance, and that will create a larger van’t Hoff factor, which will affect colligative properties. Intermolecular forces also play an important role in the initial freezing, boiling etc. points, which are important to know when doing colligative property calculations. Intermolecular forces play a vital role in life, as is seen in DNA. Hydrogen bonds are one of the stronger intermolecular forces, and those hydrogen bonds allow DNA, and us, to exist. The data from the colligative properties explain why salt is used to prevent ice from forming. Salt is not just an abundant substance, but a small amount of it will decrease the freezing point of water by a relatively large amount. Both intermolecular forces and colligative properties play a role in our daily lives and it is important to have a good understanding of them.
References
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