CHEM 151- Fall 2013- EXAM 3 KEY 1- What are the primary intermolecular forces that ethylene glycol exhibits? Dispersion, Dipole-Dipole and H-bonding 2- Propylene glycol, glycerol and methanol are also antifreeze compounds. Which of the following would you predict to be the correct ranking melting points for propylene glycol (PG) , methanol (CH3OH), glycerol and ethylene glycol (EG)? glycerol > EG > PG > methanol 3- Which of the following Newman projects would you predict represents the lowest potential energy configuration for propylene glycol? CH3 is larger than OH! 4- Both ethylene glycol and propylene glycol mix very well with water and there is no noticeable change in the mixing in cold vs. warm water. Which of the following PEC diagrams best represents the mixing of these antifreeze components in water? (M-Mixed; UM- Unmixed) 5- In Minnesota, the roads are often “salted” with soluble ionic compounds to reduce ice formation. Which of the following ionic compounds do you think would be a poor choice for this application? MgO 6- Which do you think would have the lowest melting point? CaCl2 7- Which of the following submicroscopic representation best illustrates CaCl2 dissolved in water? (water molecules are not shown in each) Ethylene Glycol Propylene Glycol GlycerolChemical deicers are soluble compounds added to water and sprayed onto aircraft to prevent ice buildup on wings. Some of these compounds have an added benefit in that they release heat when mixed with water which further enhances their deicing properties. The PEC diagrams for 4 compounds are shown. 8- Which compound would you predict to be the best choice for a deicing agent that also releases heat when mixed with water? Ca(HCOO)2 Scientists have figured out clever ways to use the freezing point depression effect to indirectly measure the molar mass of compounds. For example, lauric acid (shown below) can be melted with an unknown substance, allowed to cool, then frozen again. The freezing point change of the mixture is then used to estimate the molar mass of the unknown substance. Very cool! 9- How many hydrogen bonding interactions are likely to form when two lauric acid molecules interact? 2 10- The introduction of a double bond into the hydrocarbon chain alters the melting point of lauric acid. Of the 4 compounds shown, which would you predict to have the highest melting point? 11- Research has shown that when metals atoms are clustered in small nanoparticles that they exhibit melting points lower than the bulk metal. Imagine you required very low melting point paramagnetic nanoparticles for a research project. Which pure metal would you select for the nanoparticles? Au 12- Metal oxide nanoparticles exhibit this same property in that they exhibit lower melting points than the bulk metal oxides. Which of the following metal oxide nanoparticles would you predict to contain diamagnetic metal ions? TiO213- Room temperature ionic liquids have been shown to lower the freezing point of water when mixed. Imagine you were investigating this effect with the three ionic liquids shown. (6) For simple ionic compounds, the freezing point depression effect increases for the following compounds: LiCl < NaCl < KCl. Based on this trend, which of the above ionic liquids would you predict to exhibit the largest freezing point depression in water? Assume an equal amount of each fully dissolved. Justify your thinking. You would predict the trioctyl because according to the trend, as the size of the cation increases, so does the effect. (6) The “tributyl” compound is the most soluble in water? Provide an explanation for why it is more soluble than the other compounds. You could make an energetic or configurational argument. The energetic argument would be that the smaller tributyl ion has stronger attractive forces for water so the increased solubility is driven by more of a lowering of potential energy. In a configurational argument, you could say that the tributyl ion is not as large so it is less disruptive to the H-bonding network and number of available configurations of water than the other two compounds. This could also lead to it being more soluble. Of course, it could also be a combination of both of these. (6) Based on your answers to the last two questions, which compound would be the most efficient freezing point depression substance (the largest effect based on the smallest amount)? Justify your thinking. Well, if you wanted to balance the effect with the amount, then you might choose the trihexyl as it will be more soluble than the trioctyl but have a stronger freezing point depression effect than the tributyl. But, if the effect differences were larger than the solubility differences, then you might just pick the trioctyl OR if the solubility differences were greater you might go with the tributyl. A reasonable argument could be made for each…key word being REASONABLE. (6) Imagine you wanted to reduce viscosity of the “trioctyl” compound. How could you structurally modify the cation to accomplish this? Justify your thinking You could shorten the chain or possibly introduce branching if the dispersion forces were playing a role. If the ion-ion forces were the dominant factor, then making the chains longer would work. Specifying your thinking here is important in justifying your decision.14- It is thought that antifreeze proteins lower the freezing point of water by binding small ice crystals preventing crystal growth. The figures below illustrate two different models proposed to explain the favorable and selective binding that some antifreeze proteins have for very small ice crystals. Figure A shows illustrates one model for how the protein binds ice crystals and figure B shows an alternative model. In both the H-bonding between the protein and ice crystals are shown with (-----). - (7) Discuss how the overall potential energy of the model A and model B are different. Make sure to clearly justify your thinking and predict which model would be of lower overall potential energy. Model B should be lower in overall potential energy. There are more H-bonding interactions between the protein and ice crystal and the water molecules appear to be closer as they crowd around the region. - (7) Discuss how the number of available configurations are different