Download a) octane, a chain of 8 C atoms: C8H18 b) benzene, a ring of 6 C

12/12/2011
Which liquid should have the highest vapour
pressure?
Wax consists of long‐chain hydrocarbons (C and H atoms). Which liquid will form droplets on wax, rather than spreading out in a film?
a) ethanol, CH3CH2OH
b) dichloroethane, ClCH2CH2Cl
c) water, H2O
d) carbon disulfide, CS2
a) octane, a chain of 8 C atoms: C8H18
b) benzene, a ring of 6 C atoms: C6H6
c) water, H2O
d) carbon disulfide, CS2
1
Water has the strongest intermolecular forces: the others are all non‐polar compounds. The cohesive forces within water droplets (hydrogen bonding) are stronger than the adhesive forces between water and wax, so water doesn’t form a film.
2
1 mL H2O in a 1 L evacuated flask at 20°C results in 17.2 mg of water vapour. If the flask were instead filled with 1 atm N2
gas and 1 mL H2O, what would be the result?
VP = 80 torr
VP = 23 torr
VP = 370 torr
High VP requires weak attractive forces, so that molecules can easily escape to the gas phase.
Water and ethanol have hydrogen bonding, and dichloroethane is polar. CS2 is non‐polar, with only dispersion forces, so its liquid is in equilibrium with a higher pressure of vapour. (Data at 25°C.)
Which liquid should have the highest boiling point?
a) ethanol, CH3CH2OH
VP 59 torr, bp 78°C
b) dichloroethane, ClCH2CH2Cl VP 80 torr, bp 84°C
c) water, H2O
VP 23 torr, bp 100°C
d) carbon disulfide, CS2
VP 370 torr, bp 46°C
a) less water vapour, because the flask is already filled with 1 atm of N2
b) more water vapour, because collisions with N2
molecules help vaporize the water molecules
c) less water vapour, because the vapour pressure of water has changed
d) the N2 has no effect on the amount of water vapour
High bp requires low VP and strong attractive forces, so that more temperature is required to cause the molecules to escape to the gas phase.
Water has the strongest IM forces, so water has the lowest VP and highest bp. (VP data at 25°C)
Dalton’s Law of Partial Pressures shows that each gas in a mixture behaves as if alone. The N2 has no effect on the water gas particles, or on the water equilibrium: the vapour
pressure and amount of water vapour are unchanged.
3
VP = 59 torr
4
1
12/12/2011
Benzene (C6H6) has a normal boiling point of 80.1°C. If the atmospheric pressure is changed to 120 torr, what will the new boiling point be?
If you make tea in Machu Picchu, it won’t taste as good as tea made in Hong Kong. Why not?
a) Peruvian tea is an herbal tea made from coca leaves, not from the leaves of the tea plant.
b) Hong Kong is at sea level, while Machu Picchu is 2430 m above sea level.
c) You’ll have to pick out all the llama hairs.
d) the Chinese know more about tea than the Peruvians.
5
• Higher elevation = lower atmospheric pressure.
• Lower pressure = lower vapour pressure required to boil water = lower boiling temperature (about 92°C).
• Lower temperature of boiling water = fewer flavorful compounds extracted from tea leaves = weak tea a) Still 80.1°C, because the vapour pressure doesn’t depend on external pressure.
b) Less than 80.1°C, because a lower temperature is needed for the vapour pressure to reach 120 torr
c) There will be no boiling point, because 120 torr is above the critical pressure.
d) More than 80.1°C, because a higher vapour
pressure is now required.
6
Consider the phase diagram of ammonia. Suppose NH3
at 1 atm pressure is cooled 350K to 220K. What physical change(s) occur?
Consider the phase diagram of ammonia. Which statement is false?
a) Liquid ammonia does not exist at 40 torr pressure.
b) At 5 atm and –80°C, ammonia is a solid.
c) At 120°C, gaseous ammonia will condense if pressurized
d) At 20°C, the vapour pressure of ammonia is less than 1 atm.
e) these are all true statements
7
• VP of NH3 is 1 atm at –33°C, and VP ↑ as T ↑.
• At 20°C, NH3 condenses at pressure > 1 atm. External P ↓, so VP needed to boil ↓, so lower temperature required: compounds always boil at lower T under reduced pressure!
a)
b)
c)
d)
There are no changes
It changes from liquid to solid
It changes from gas to liquid
It changes from gas to liquid to solid
e) It is impossible to answer without more information
8
Cooling from 350K (+77°C) to 220K (–53°C) at 1 atm
crosses the l ' g line: it condenses at –33°C.
2
12/12/2011
Consider the phase diagram of ammonia. Suppose NH3
at 1 atm pressure and 25°C is pressurized. What physical change(s) occur?
Amino acids are categorized as hydrophilic or hydrophobic, depending on whether their side chains interact strongly with water. Which amino acid is hydrophilic?
a) There are no changes
b) It changes from liquid to solid
c) It changes from gas to liquid at some pressure
d) It changes from gas to solid at some pressure
e) It is impossible to answer without more information
a) histidine
11
d) vitamin E,
α‐tocopherol
Vitamin C has many OH groups and O LPs capable of forming hydrogen bonds to water. The others are almost entirely non‐polar C‐C and C‐H bonds.
O
NH
O
HN
Histidine has an N‐H bond and an N LP in its imidazole
ring: it can form H‐bonds to water. The other sidechains all contain non‐polar groups, and so do not interact strongly with water.
If ethanol (C2H5OH) and isopropanol (C3H7OH) are mixed in equal proportions, the vapour above the solution will
Line structures do not show C or H atoms explicitly: C atoms occur where lines join, and enough H atoms are assumed present to give each C four bonds. c) vitamin D3,
cholecalciferol
CH3S
d) valine
O
HN
Vitamins are categorized as fat‐soluble or water‐soluble. Which vitamin is water‐soluble?
b) vitamin C, ascorbic acid
O
O
NH
c) phenylalanine
10
a) vitamin A, retinol
b) methionine
O
N
Pressurizing at 25°C from 1 atm will condense the gas: but at what pressure?
9
H
N
a) be mostly ethanol vapour
b) be mostly isopropanol vapour
c) be equal proportions of each, just as in the liquid solution
d) contain neither, because these materials are not volatile
At 20°C, VP C2H5OH = 46.6 torr, VP C3H7OH = 22.1 torr: larger molecule has larger dispersion forces.
Raoult’s Law: PA = XAP°A, so with equal proportions in the liquid, the component with the higher VP will have greater partial pressure in the vapour.
12
3
12/12/2011
Compared to a dilute aqueous solution of ascorbic acid, a more concentrated solution
a) will evaporate more quickly
b) will evaporate more slowly
c) will evaporate at the same rate, because the solvent evaporates, not the solute d) will not evaporate, because ascorbic acid is a non‐volatile solid
ΔP = XsoluteP°solvent
VP of a solution ↓ as solute concentration ↑.
VP ↓ means less solvent vapour and less evaporation.
13
4