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Honors Chemistry Intermolecular Forces, Phase Changes and Heating Curves

Honors Chemistry Intermolecular Forces, Phase Changes and Heating Curves. Forces between molecules determine things like boiling points and freezing points. They happen between molecules , and there are three major types. Intermolecular Forces. #1. London Dispersion forces

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Honors Chemistry Intermolecular Forces, Phase Changes and Heating Curves

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  1. Honors ChemistryIntermolecular Forces,Phase Changes and Heating Curves

  2. Forces between molecules determine things like boiling points and freezing points. • They happen between molecules, and there are three major types. Intermolecular Forces

  3. #1. London Dispersion forces weakest of all, caused by motion of e- LDF’s increase as the number of e-increases (or, as molecular mass increases) Large molecules have lots of LDFs. halogens start as gases; bromine is liquid; iodine is solid – all in Group 7A Dominant intermolecular force in nonpolar molecules. Intermolecular Forces

  4. Occurs when polar molecules are attracted to each other. • 2. Dipole interaction happens in water • positive region of one molecule attracts the negative region of another molecule. #2. Dipole interactions

  5. d+d- d+d- H F H F • Occur when polar molecules are attracted to each other. • Slightly stronger than dispersion forces. • Opposites attract, but not completely hooked like in ionic solids. #2. Dipole interactions

  6. d+d- d+d- d+d- d+d- d+d- d+d- d+d- #2. Dipole Interactions d+d-

  7. …is the attractive force caused by hydrogen bonded to N, O, F • N, O, F are very electronegative, so this is a very strong dipole. • And, the hydrogen shares with the lone pair in the molecule next to it. • This is the strongest of the intermolecular forces. #3. Hydrogen bonding

  8. d- d+ O d+ H d+ d- H H O H d+ Hydrogen Bonding(Shown in water) This hydrogen is bonded covalently to: 1) the highly negative oxygen, and 2) a nearby unshared pair.

  9. H O O H H O H H H H O H H H H O O O H H H Hydrogen bonding allows H2O to be a liquid at room conditions.

  10. London dispersion forces are the weakest • A little stronger are dipole interactions • The strongest is hydrogen bonding • All of these are weaker than ionic bonds Order of Intermolecular attraction strengths

  11. There are six major phase changes: Phase Changes

  12. Starting with solid ice at -25 ºC (Point A) Energy Changes with Changes of State

  13. Adding heat will raise the temperature to Point B. Energy Changes with Changes of State

  14. At Point B, the ice starts to change into liquid water. • The heat added doesn’t raise the temperature, but separates the molecules apart. • The total heat needed to do this is called the heat of fusion of H2O. • Energy overcomes intermolecular forces. Energy Changes with Changes of State

  15. At Point C, the ice has completely melted (have added the entire heat of fusion) • Heat added at this point raises the temperature of water to Point D. Energy Changes with Changes of State

  16. At Point D, the liquid is ready to boil. • Again, the temperature does not rise as you add heat, the heat added separates the molecules into a gas. • This heat is the heat of vaporization. Energy Changes with Changes of State

  17. Once you have completely boiled, you can increase the temperature of the gas. Energy Changes with Changes of State

  18. At any temperature, the liquid and the vapor reach equilibrium – the rate that the vapor comes out is the same that the vapor goes back in. Vapor Pressure

  19. The boiling point of a liquid is where vapor pressure = atmospheric pressure. • The normal boiling point is where the vapor pressure = 1 atm. • At this point, the atmosphere can not hold the liquid together. Vapor Pressure and Boiling Point

  20. A phase diagram is a plot of pressure vs. temperature that shows where phase changes take place. Phase Diagrams

  21. There are regions for solids, liquids, and gases, as well as lines on which those phases are in equilibrium. (both phases exist at once)

  22. Traveling from one region to the other results in a phase change. For example, to go from a liquid to a gas, one can either increase the temperature or decrease the pressure.

  23. At the triple point, T, all three phases are in equilibrium.

  24. Past the critical point, C, the liquid and gas phases are indistinguishable from each other, and create a supercritical fluid.

  25. The phase diagram of water is unique because of the different slope of the solid-liquid line, and the very high critical point. • These are due to the strong van der Waals forces between water molecules.

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