Intermolecular Forces

Bishop State Community College

Why do substances exist as gases or liquids or solids at room temperature?

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The answer is the forces of attraction between particles determines whether a substance will be a solid, liquid or gas AT room temperature

If temperature changes, the kinetic energy of the particles changes and so can the phase of matter.

The phase diagram to the right shows the dependence of phase on temperature and pressure. All phases (solid, liquid and gas) are affected by temperature and phase changes can occur as temperature is increased or decreased. Every substance has a characterisitc phase diagram due to intermolecular forces. For example, water boils at 100 C but ammonia boils at -33.34 C.


Pressure affects gases much more than in solids or liquids. Gases are compressible and, therefore, the pressure on a gas can be increased. Pressure of gases comes from the collision of the gas particles with any surface (including the earth's surface!) so the more of them you have, the more pressure. The faster the gas particles are moving (higher temperatures), the more often they will collide with a surface, and therefore, the higher the pressure. Because the motion of gas particles is so much greater than in liquids and solids, there are more collisions with the container than in solids or liquids.

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The attractions between molecules are not nearly as strong as the intramolecular "force" such as the covalent bond in the example below.

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These intermolecular attractions are, however, strong enough to control physical properties, such as boiling and melting points, vapor pressures, and viscosities.

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An important exception to intra-vs-inter is in ionic compounds. The interparticle force is the same as the intermolecular force: the ionic bond.

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3 Main IMF's explained below

Factors affecting London Dispersion forces

  • The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane) tend to have stronger dispersion forces than short, fat ones (like neopentane).This is due to the increased surface area in n-pentane.

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Find the molecules with the higher dipole moments in the figure above.

Obviously, molecules that are more polar, have stronger attractions and, therefore, higher boiling points. Why use boiling vs melting??
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How do we explain this diagram?

Account for the differences between the groups (think size, polarity, and geometry). For example, why is group 4 below all of the other groups? Another question is to account for the big deviation for NH3, HF and H2O. The dotted line drawn for the red curve is an interpolation of the red curve-where water would be expected to be.

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Practice

1. True or False:

(a) For molecules with similar molecular weights, the dispersion forces become stronger as the molecules become more polarizable.

(b) For the noble gases the dispersion forces decrease while the boiling points increase as you go down the column in the periodic table.

(c) In terms of the total attractive forces for a given substance dipole-dipole interactions, when present, are always larger than dispersion forces.

(d) All other factors being the same, dispersion forces between linear molecules are greater than dispersion forces between molecules whose shapes are nearly spherical. (#20)


2. Rationalize the difference in boiling points in each paior: (a) HF (20°C) and HCl (-85°C); (b) CHCl3 (61°C) and CHBr3 (150°C); (c) Br2 (59°C) and ICl (97°C). (#26)


3. Identify the type of types of intermolecular forces present in each substance and then select the substance in each pair that has the higher boiling point: (a) propane C3H8 or n-butane C4H10; (b) diethyl ether CH3CH2OCH2CH3 or 1-butanol CH3CH2CH2CH2OH; (c) sulfure dioxide (SO2) or sulfur trioxide (SO3); (d) phosgene Cl2CO or formaldehyde H2CO. (#28)


4. Arrange substances Ga, Ne, and Br2 in order of increasing boiling point. (#12)


5. Which type of intermolecular force accounts for each of these differences: (a) CH3OH boils at 65 C; CH3SH boils at 6 C (b) Xe is a liquid at atmospheric pressure and 120 K, whereas Ar is a gas under the same conditions. (c) Kr, atomic weight 84, boils at 120.9 K, whereas Cl2, molecular weight about 71, boils at 238 K. (d) acetone boils at 56 C, whereas 2-methylpropane boils at -12 C. (#18)

Practice

6. Hydrazine (H2NNH2). hydrogen peroxide (HOOH), and water (H2O) all have exceptionally high surface tensions compared with other substances of comparable molecular weights. (a) Draw the Lewis structures for these three compounds (formulas are given as structural formulas so you know how they are connected). (b) What structural property do these substances have in common, and how might that account for the high surface tensions? (#36)


7. Would you expect the viscosity of n-pentane (CH3CH2CH2CH2CH3) to be larger or smaller than the viscosity of n-hexane (6 C's instead of 5-from Table 11.4)? (b) If you compared their viscosities at 270 K, would you expect the viscosity of neopentane (CH3)4C, to be smaller or larger than n-pentane. (#38)


8. Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) bromine vapor turns to bromine liquid as it is cooled. (b) crystals of iodine disappear from an evaporating dish as they stand in a fume hood. (s) rubbing alcohol in an open container slowly disappears. (d) Molten lava from a volcano turns to solid rock. (#40)


9. Compounds like CCl2F2 are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of CCl2F2 is 289J/g. What mass of this substance must evaporate to freeze 200 g of water initially at 15 C? (The heat of fusion of water is 334 J/g; the specific heat of water is 4.18 J/gC. (#44, #46 is similar)

Practice

10. True or False:

(a) CBr4 is more volatile than CCl4 (b) CBr4 has a higher boiling point than CCl4

(c) CBr4 has a weaker intermolecular forces than CCl4

(d) CBr4 has a higher vapor pressure at the same temperature than CCl4 (#52)


11. Explain the following observations: (a) water evaporates more quickly on a hot dry, day than on a hot, humid day. (b) It takes longer to cook an egg in boiling water at high altitudes than it does at lower altitudes. (#54)


12. Referring to the figure below, describe the phase changes (and the temperatures at which they occur) when CO2 is heated from -80 C to -20 C at (a) a constant pressure of 3 atm, (b) at a constant pressure of 6 atm. (#60)

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What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Blackboard everyday. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!