AP Chemistry Unit Review
Unit 3: Intermolecular Forces and Properties
How it applies to our lives
The difference in intermolecular forces is the reason why oil and water don’t mix. Water is polar and has a strong intermolecular force because of the hydrogen bonds between water molecules. Oil is nonpolar and only has London dispersion force holding the molecules together. The water molecules are holding so tightly together that they exclude the oil molecules, and the oil molecules also don’t want to break away from each other. In addition, oil is nonpolar so it doesn’t have a charge to attract water molecules.
Key Vocabulary
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Metallic Solids
Dipole-Dipole
Distillation
Beer-Lambert Law
Maxwell-Boltzmann Distributions
Ideal Gas Law
Main Takeaways
London Dispersion Force increases as the size of molecules increases because larger molecules have more polarizable electron clouds. The electron clouds are more polarizable because the molecules have more electrons, so the magnitude of the temporary dipole is stronger. They are the weakest bond forces and as well as happen in every molecule. They are only temporary not permanent, bigger molecules and atoms have more electrons therefore they have stronger dispersion forces.
Gases are more ideal at low pressure and high temperatures. Gases naturally don’t have a fixed shape and they can generally move freely about. Gases have low density as they are much further apart. They can also be compressed as there is a lot of empty space between the particles that can help the gases squeeze into smaller volumes. They also often collide with each other within the walls of the container that then creates pressure.
When all the variables stay constant, if volume increases, pressure decreases and also vice versa. When pressure stays constant however temperature increases and volume also increases. But if the volume stays constant, temperature increases and pressure also increases. And if temperature and pressure stays constant, increasing the number of particles can also increase the volume.
At the same temperature, lighter molecules have a greater proportion of molecules with higher energy. Even with the same temperature molecules still move, with the same kinetic energy but not all as some might move a little slower or faster than others but most of the time is at the same Average kinetic energy. Temperature mostly affects motion of the molecules first then if its temperature change is large enough it can and will change the state of the substance.
Substances with similar polarity and intermolecular forces have greater affinity. Some polar substances mix well with other polar substances and no polar substances usually mix well with other nonpolar substances. Molecules with similar forces attract more easily as they stay mixed together much more easily. While on that note nonpolar substances and polar substances don’t tend to mix together.
Common Misconceptions
hile people might think volume is directly proportional to pressure, this is not true. Volume is inversely proportional to pressure. So, if the total volume of a gas mixture is doubled, the partial pressure of each gas does not stay the same or double with the volume. What actually happens is that if the total volume of a gas mixture is doubled, the partial pressure of each gas is halved. For example, a 1 L container is originally filled with 1 atm of oxygen gas and 1 atm of nitrogen gas. The partial pressure of each gas is 1 atm. When the gases are transferred to a 2 L container, the new partial pressure of oxygen becomes 0.5 atm. According to Boyle’s Law, P1V1=P2V2. P1 = 1atm V1 = 1L P2 = ? V2 = 2L (1atm)(1L)=(? atm)(2L) ½ = ? atm —> 0.5 atm When the volume of the container is doubled from 1L to 2L, the partial pressure of oxygen is halved from 1 atm to 0.5 atm.
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