12.2.4 Molecular Structure and Boiling Point Flashcards Preview

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Flashcards in 12.2.4 Molecular Structure and Boiling Point Deck (12)
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1
Q

Molecular Structure and Boiling Point

A
  • A substance boils when the vapor pressure exceeds atmospheric pressure.
  • Knowing the intermolecular forces helps predict the boiling point.
  • The Clausius-Clapeyron equation relates vapor pressure to the temperature of a substance.
2
Q

note

A
  • A substance boils when the vapor pressure exceeds
    atmospheric pressure.
  • At a lower atmospheric pressure, for example at higher altitudes, the temperature at which boiling occurs is lower than at sea level (one atmosphere).
  • Knowing the intermolecular forces helps predict the boiling point.
  • The graph shows vapor pressure (y axis) against temperature for four substances. Compare boiling point at one atmosphere by comparing the intersection points of the horizontal line with each compound’s curve. Of the compounds, water is the most polar molecule and has the highest boiling point. Methane, a nonpolar molecule, has a very low boiling point.
  • The Clausius-Clapeyron equation relates vapor pressure to the temperature of a substance.
  • The equation can be used to answer questions such as what the expected boiling point of water is at some altitude other than sea level.
  • The equation uses a logarithmic scale to make the relationship a straight line. Vapor pressure depends on the heat of vaporization of a substance ( H vap ), which
    determines the slope of the relationship. A higher heat of vaporization means the vapor pressure changes more rapidly with a change in temperature than for a substance with a lower heat of vaporization.
3
Q

Look at these graphs of vapor pressure versus temperature for three different substances. All three substances have about the same molecular weight. Which choice correctly shows the substances arranged in order of strength of intermolecular forces, beginning with the weakest?

A

Substance D, Substance E, Substance W

4
Q

A certain liquid has a high vapor pressure. Which of the following statements are most likely to be true of this liquid?

A

The liquid would have a low surface tension

5
Q

How can the Clausius-Clapeyron equation be used to determine the heat of vaporization of a substance? The equation is: ln Pvap = -Hvap / R (1/T) + C

A

Plot a graph of ln (Pvap ) versus 1 / T and find the slope.

6
Q

When does boiling occur?

A

When vapor pressure is high enough to lift the atmosphere

7
Q

According to this graph, which substance has the highest heat of vaporization?

A

Substance E

8
Q

Here is one form of the Clausius-Clapeyron equation,

ln Pvap = -Hvap / R (1/T) + C

The equation is written in slope-intercept form. What is the slope in the standard slope-intercept equation equivalent to in this form of the Clausius-Clapeyron equation?

A

-Hvap / R

9
Q

The atmospheric pressure in Leadville, Colorado is about 0.7 atm. Which of the following statements is / are true?

F. The boiling point of water in Leadville is lower than 100°C.
G. The equilibrium vapor pressure of boiling water in Leadville is 0.7 atm.
H. The heat of vaporization of water in Leadville, Colorado is much lower.

A

Only F and G are true

10
Q

Water, H2O, has somewhat unusual properties because water molecules are primarily held together by which kind of intermolecular forces?

A

Hydrogen bonding

11
Q

Is it possible to raise the temperature of a liquid above the boiling point?

A

Yes, because boiling requires sites of nucleation

12
Q

Which statement best describes the conditions of equilibrium between water and vapor at equilibrium vapor pressure?

A

There is a constant exchange of molecules between the gas and liquid phase.

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