In this lecture worksheet, there were questions where we had to choose which compound would have the highest vaporization and why this was the case. We learned that the reason there is a different enthalpy of vaporization is because of the difference in the intermolecular forces between each compound.
We also discussed physical and chemical changes and what type of reaction classified each category. We found out that a phase change in molecular compounds is a physical change. A chemical change for molecular compounds would be a decomposition reaction. The interesting thing about ionic compounds is that when they become aqueous, the ions break apart. This type of reaction is both a chemical and physical reaction. It is chemical because the ionic bonds are being broken between the ions due to ion-dipole interactions with the water. The reaction is also physical because the ionic compound is going from a solid to a liquid via the water. It can be argued that it is a physical change because if the water is taken out, the ionic compound returns to a solid.
The standard heating curve (seen below) was discussed in detail, including when phase changes were happening, if the change was endothermic or exothermic, and if the substance was gaining energy or losing energy.
From A-> B the substance is a solid, from C->D the substance is a liquid, and from E->F the substance is a gas. The lines that are parallel with the X-axis are where the phase changes are happening. The reason that they are not increasing in temperature at this point is because all of the extra energy is going into breaking apart the intermolecular forces between the molecules so that the compound can change phases. Going up to the right, the reactions are endothermic meaning that heat is being absorbed during the reaction. Going down to the left the reactions are exothermic meaning that heat is being lost during the reaction.
We discussed vapor pressure, which is the pressure of a vapor inside of a closed system. We defined vapor as the gaseous particles of a substance that is a liquid at room temperature. We learned that liquids like water don't need extra heat to evaporate and that these substances are continuously evaporating and condensing. Dynamic equilibrium is when particles are condensing and evaporating at the same rate inside a closed system. The boiling point of a liquid is the temperature where the atmospheric pressure is equal to the vapor pressure. The pressure needed to boil is 760 torr. The reason the boiling point of water is lower than 100 degrees celsius at higher elevations is because there is higher atmospheric pressure in the mountains and therefore it doesn't take as much heat energy to reach the equilibrium between atmospheric pressure and vapor pressure. We also learned that as IMFs increase the boiling point also increases because more energy is needed to break the molecules from these forces.
The last thing that we discussed this week was lattice energy. Lattice energy is the energy required to separate a mole of an solid ionic compound into gas ions. Lattice energy increases with charge, and decreases with size of the ions. Coulombs law can be used to explain this relationship. As lattice energy goes up, the melting point goes up as well because it takes more energy to break apart the ions from a solid lattice than a liquid.
I would give my understanding of the material this week a 9/10. I felt like the discussions that we had about the various IMFs and the properties that are influenced by them like vapor pressure, and boiling point were very helpful. I now feel that with a little more studying I will be very prepared for the test on Tuesday.
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