ngac (ttn793) H11: Solids and Liquids mccord (51600) 1

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1 ngac (ttn793) H11: Solids and Liquids mccord (51600) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page find all choices before answering points What would be the most significant type of intermolecular forces in a liquid sample of fluoroform (CHF 3 )? 1. ionic 2. hydrogen bonding 3. dispersion 4. dipole-dipole correct 5. covalent London forces, dispersion forces, van der Waals or induced dipoles all describe the same intermolecular force. London forces are induced, short-lived, and very weak. Molecules and atoms can experience London forces because they have electron clouds. London forces result from the distortion of the electron cloud of an atom or molecule by the presence of nearby atoms or molecules. Permanent dipole-dipole interactions are stronger than London forces and occur between polar covalent molecules due to charge separation. H-bonds are a special case of very strong dipole-dipole interactions. They only occur when H is bonded to small, highly electronegative atoms F, O or N only. Ion-ion interactions are the strongest due to extreme charge separation and occur between ionic molecules. They can be thought of as both inter- and intramolecular bonding. CHF 3 is a polar molecule that does not contain H bonds; therefore, dipole-dipole forces will be the most significant type of intermolecular forces present points Forces between particles (atoms, molecules, or ions) of a substance are called 1. None of these 2. armed forces. 3. intramolecular forces. 4. intermolecular forces. correct Bonds within molecules or formula units are called intramolecular forces. Bonds between particles are called intermolecular forces points What is the predominant intermolecular force between IBr molecules in liquid IBr? 1. ionic forces 2. hydrogen bonds 3. dispersion forces correct 4. covalent bonds 5. dipole forces You might have thought that dipole forces was right. The fact is that the dipole between iodine and bromine is incredibly weak -tooweaktoaccountforthefactthatthis compound is a liquid. Another FACT is that dispersion forces are the dominant forces here due to the SIZE of these atoms and hence molecule. Just know that not ALL dipole forces are bigger than any set of dispersion forces. Size matters, and dispersion forces win out here points Which of the following structures represents apossiblehydrogenbond? 1. Cl H Cl 2. C H O 3. F H F correct

2 ngac (ttn793) H11: Solids and Liquids mccord (51600) 2 4. Br H Br H-bonds are a special case of very strong dipole-dipole interactions. They only occur when H is bonded to small, highly electronegative atoms F, O or N only points Identify the kinds of intermolecular forces that might arise between molecules of CH 3 OH. 1. dipole-dipole 2. London forces, dipole-dipole 3. London forces, dipole-dipole, hydrogen bonding correct 4. None of these 5. London forces 6. hydrogen bonding H H C H O Hispolar. 006 (part 1 of 2) 10.0 points Calculate the ratio of the potential energies for the interaction of a water molecule with a Ca 2+ ion (of radius 100 pm) and with an In 3+ ion (of radius 72 pm) None of these correct r Ca =100pm The energy E p z µ r 2 E Ca E In = z Ca r 2 In z In r 2 Ca = r In =72pm, so the ratio is 2 (72 pm)2 3 (100 pm) 2 = (part 2 of 2) 10.0 points Which ion will attract a water molecule more strongly? 1. In 3+ correct 2. Equal attraction 3. Ca 2+ The attraction of the Ca 2+ ion will be less than that of the In 3+ ion because it has both alargerradiusandalowercharge points If the interaction between two species is proportional to 1 r 3,whichofthefollowingislikely involved? 1. Na + and H 2 O 2. ions in an ionic solid 3. bromine molecules in the liquid phase 4. chloromethane molecules in the liquid phase 5. chloromethane molecules in the solid phase correct 1 r 3 E p for dipole-dipole interactions points Which of the following can form intermolecular hydrogen bonds?

3 ngac (ttn793) H11: Solids and Liquids mccord (51600) 3 1. PH 3 2. (CH 3 ) 2 NH correct 3. H 2 CO 4. CH 3 COCH 3 Only molecules with H attached to the electronegative atoms N, O, and F can hydrogen bond. Of the molecules given only (CH 3 ) 2 NH 2 has the H directly bonded to N, so it can undergo hydrogen bonding points Which of the following cations is likely to be hydrated in compounds? 1. Rb + 2. NH K + 4. Cs + 5. Li + correct Li + is the smallest points Which of the following compounds is UN- LIKELY to be commonly available and thus not found in the laboratory? 1. KCN 2H 2 O correct 2. La(NO 3 ) 3 6HO 3. CuSO 4 5H 2 O 4. Na 2 CO 3 10H 2 O 5. BaCl 2 2H 2 O K + is only a +1 cation AND it is too large to substantially attract waters of hydration. +2 and +3 cations will generally form hydrates. Na + will also attract water due to its smaller size points Adropofliquidtendstohaveaspherical shape due to the property of 1. capillary action. 2. vapor pressure. 3. close packing. 4. surface tension. correct 5. viscosity. Molecules on the surface of a liquid are influenced by intermolecular attractions towards the interior; these attractions pull the surface layer toward the center. The most stable situation is one in which the surface area is minimal. For a given volume, a sphere has the least possible surface area points Surface tension describes 1. the inward forces that must be overcome in order to expand the surface area of a liquid. correct 2. adhesive forces between molecules. 3. the forces of attraction between surface molecules of a solvent and the solute molecules. 4. the forces of attraction between the surface of a liquid and the air above it. 5. the resistance to flow of a liquid. 6. capillary action. Molecules in the interior of a liquid interact with molecules all around them, whereas

4 ngac (ttn793) H11: Solids and Liquids mccord (51600) 4 molecules at the surface of a liquid can only be affected by those beneath the surface layer. This phenomenon leads to a net inward force of attraction on the surface molecules, contracting the surface and making the liquid behave as though it had a skin. Surface tension is a measure of the inward forces that must be overcome to expand the surface area of a liquid points Predict which of ethanol (CH 3 CH 2 OH) or dimethyl ether (CH 3 OCH 3 )hasthegreater viscosity. Assume that they are both at the same temperature and in their liquid form. 1. ethanol correct 2. Equal viscosities 3. dimethyl ether Ethanol should have a greater viscosity than dimethyl ether because ethanol can hydrogen bond while dimethyl ether cannot points Viscosity usually increases with increasing temperature. 1. False correct 2. True Viscosity = measure of a liquid s resistance to flow. Generally as liquids get warmer they flow more readily (think of molasses or honey), so viscosity decreases as temperature increases points Which would you expect to be most viscous? 1. C 4 H 8 at 30 C 2. C 4 H 8 at 50 C 3. C 8 H 18 at 30 C correct 4. C 8 H 18 at 50 C points As a liquid is heated, its vapor pressure 1. increases. correct 2. disappears. 3. decreases. 4. does not change. 5. may increase or decrease, depending on the liquid. As temperature (kinetic energy) increases, rate of evaporization increases and rate of condensation decreases; therefore, vapor pressure will increase with increasing temperature points The vapor pressure of all liquids 1. is the same at their freezing points. 2. is the same at 100 C. 3. increases with volume of liquid present. 4. increases with temperature. correct 5. decreases with the increasing volume of the container. As temperature (kinetic energy) increases, rate of evaporization increases and rate of condensation decreases; therefore, vapor pressure will increase with increasing temperature points Consider two empty containers A and B whose volumes are 10 ml and 20 ml, respectively. You now put 1 ml of liquid H 2 Ointo each container and adjust the temperature of each container to 20 C. The vapor pressure

5 ngac (ttn793) H11: Solids and Liquids mccord (51600) 5 of H 2 OincontainerBisfoundtobe17torr. The vapor pressure of H 2 OincontainerAis which of the following? torr 2. between 17 and 34 torr torr correct 4. greater than 34 torr torr Vapor pressures are a function of temperature, and are independent of the volume of the container. The volume of the empty space in the container will not affect the vapor pressure of the liquid. In this example, each container has the same temperature, so the vapor pressure of the water will be the same in each container points Why is the heat of vaporization of water much larger than the heat of melting (fusion)? 1. Melting only requires a partial disruption of the hydrogen bonding network of water whereas vaporization requires complete dissociation of the hydrogen bonds. correct 2. Vaporization is a very exothermic process while melting is endothermic. 3. The bonds holding liquid molecules together are stronger than those holding together solid water molecules (ice). 4. Liquid water molecules are held close together by hydrogen bonds, but solid water molecules (ice) are held close together by covalent bonds. 5. The statement is not true; the heat of melting is actually greater than the heat of vaporization points Based on the general concepts that govern intermolecular attractions, which of the following orderings of boiling points of the fluorocarbons 1) CF 4 2) F 3 C (CF 2 ) 4 CF 3 3) F 3 C (CF 2 ) 2 CF 3 is correct when going from highest to lowest? 1. BP 1,BP 2,BP 3 2. BP 3,BP 2,BP 1 3. BP 2,BP 1,BP 3 4. BP 1,BP 3,BP 2 5. BP 2,BP 3,BP 1 correct All 3 molecules are non-polar, so their relative boiling points will be governed by their relative weights; small molecules tend to have low boiling points and large molecules high boiling points points Tetrabromomethane has a higher boiling point than tetrachloromethane. 1. True correct 2. False In general, the stronger the intermolecular forces, the higher the boiling point of the substance. Both molecules are non-polar so only exhibit dispersion forces, but as Br is larger than Cl, CBr 4 is larger than CCl 4,so CBr 4 has stronger dispersion forces points Which of KBr or CH 3 Br is likely to have the higher normal boiling point? 1. KBr correct

6 ngac (ttn793) H11: Solids and Liquids mccord (51600) 6 2. CH 3 Br 3. They are same KBr; it is an ionic compound as opposed to the molecular compound CH 3 Br (1435 Cfor KBr vs. 3.6 CforCH 3 Br) points Which of the following would you expect to boil at the lowest temperature? 1. C 8 H KF 3. PCl 3 4. C 3 H 6 5. CH 4 correct points The forces of attraction between a liquid and asurfacearecalled 1. adhesive forces. correct 2. cohesive forces. 3. dispersion forces. 4. molecular forces. 5. intramolecular forces. Forces between molecules within a liquid are called cohesive forces. Forces of attraction between a liquid and a surface are called adhesive forces.