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Hydrogen Bonding in Alcohols: Ethanol and Dimethyl Ether

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Although they both have the same C2H6O chemical formula, dimethyl ether, CH3OCH3, boils at -25C while ethanol, CH3CH2OH boils at 76C. Why?

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Solution Preview

An alcohol is an organic molecule containing an -O-H group.

Any molecule which has a hydrogen atom attached directly to an oxygen or a nitrogen is capable of hydrogen bonding. Such molecules will always have higher boiling points than similarly sized molecules which don't have an -O-H or an -N-H group. The hydrogen bonding makes the molecules "stickier", and more heat is necessary ...

Solution Summary

The boiling points of ethanol and methoxymethane show the dramatic effect that the hydrogen bonding has on the stickiness of the ethanol molecules.

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Assorted State Change Questions

1 Describe the intermolecular force that must be overcome in converting each of the following from a liquid to a gas:

A. Liquid O2
B. Mercury
C. CH3I (methyl iodide)
D. CH3CH2OH (ethanol)

2 Rank the following in order of increasing strength of intermolecular forces in the pure substances. Which do you think might exist as gasses at 25"C and 1 atm: Ne, CH2, CO, and CCl4?

3 Explain why the boiling point of H2S is lower than that of water?

4 Which of the following compounds would be expected to form intermolecular hydrogen bonds in the liquid state?

A. CH3OCH3 (dimethyl either)
B. CH4
C. HF
D. CH3CO2H (acetic acid)
E. Br2
F. CH3OH (methanol)

5 The enthalpy of vaporization of liquid mercury is 59.11 kj/mol. What quantity of heat is required to vaporize 0.500 mL of mercury at 357'C, its normal boiling point? (The density of mercury is 13.6 g/mL.)

6 Assume you seal 0.1 g of diethyl ether in an evacuated 100.-mL flask. There are no molecules of any other gas in the flask. If the flask is held at 30'C, what is the approximate gas pressure in the flask? If the flask is place in an ice bath, does additional liquid ether evaporate of does some ether condense to a liquid?

7 Rank the following substances in order of increasing strength of intermolecular forces:
A. Ar
B. CH3Oh
C. Co2
D. Cao

8. If you place 1.0 of ethanol in a room that is 3.0 m long, 2.5 wide, and 2.5 high, does all the alcohol evaporate? If some liquid remains, how much remains? The vapor pressure of ethanol at 25'C is 59 mm Hg, and the density of the liquid at this temp is 0.785 g/cm3.

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