How is boiling point related to pressure?
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Transcript of How is boiling point related to pressure?
How is boiling point related to pressure?
…it’s more than you think!
• If you reduce the pressure on a liquid, its boiling point decreases
• BP H20=100oC
at 760mmHg (1.00 atm, sea level)
• If you reduce the pressure on a liquid, its boiling point decreases
• BP H20=100oC
at 760mmHg (1.00 atm, sea level)
• BP H20=95.1oC
at 635mmHg (.836 atm,Denver)
• If you reduce the pressure on a liquid, its boiling point decreases
• BP C2H50H=78.4oC
at 760mmHg (1.00 atm, sea level)
• If you reduce the pressure on a liquid, its boiling point decreases
• BP C2H50H=78.4oC
at 760mmHg (1.00 atm, sea level)
• BP C2H50H=63.5oC
at 400mmHg (.526 atm,way high)
Consider a phase diagram
• Pop Quiz—What substance?
• Line 1-5 represents increasing the pressure at 0oC
• Line 6-9 represents warming at 1.00 atm.
• The slope of the left hand line shows that the substance is water (that and the normal melting point)
• The item of interest today is the slope of the right hand line—
How does the boiling point respond to pressure?
• Compare
• Compare
• Yes, I know
the MP and BP
are different.
Look at the slopes
• Water has a steeper liquid/gas line
• Carbon dioxide has a shallower liquid/gas line
• Water has a steeper liquid/gas line
--its boiling point changes (↔) only a little with changes in pressure (↨)
• Carbon dioxide has a shallower liquid/gas line
• --its boiling point changes more with changes in pressure
Why?
Why?
• Because water has a large heat of vaporization
The Clausius-Clayperon equation
• --relates heat of vaporization to changes in vapor pressure at different temperatures
• --If a small change in T makes a big change in vapor pressure, the substance is easy to boil (low Hvap)
• Ln P = -Hvap/R (1/T) + C
• P is the pressure (any units)
• ln P is the natural log of P
• Hvap is the heat of vaporization in J/mol
• R is the ideal gas constant 8.31 J/mol k
• T is the absolute temperature
Please notice:
• Don’t use P
• Don’t use T
Please notice:
• Don’t use P, use ln P
• Don’t use T, use 1/T
Please notice:
• Don’t use P, use ln P
• Don’t use T, use 1/T
• This is a linear relationship
• The slope of the line is -Hvap/R
Clausius-Clayperon graph
(c)
Ln P
1/T
Slope=Hvap/R
• There are two ways this equation is used
• --to find the heat of vaporization of a substance
• You will need two sets of (T,VP) data• Convert to (1/T,lnP), graph, find the slope, and
solve OR• Calculate by difference
• There are two ways this equation is used
• --to find the normal boiling point• You will need one set of (T,VP) data, and Hvap
• Convert to (1/T,lnP), graph, calculate the slope, and extend the line to ln(1atm) OR
• Calculate C and use with ln(1atm) to find 1/T, solve for T
What is the heat of vaporization?
• Acetic Acid has the following vapor pressures: 10 mmHg at 17.5oC
40 mmHg at 43.0oC
• Ammonia has the following vapor pressures: 100 mmHg at -68.4oC
760 mmHg at -33.6oC
What is the normal boiling point?
• The vapor pressure of Hg is 100 mmHg at 1784oC. Its heat of vaporization is 259 kJ/mol
• VP of N2=10mmHg at -219oC,
Hvap=5.58 kJ/mol