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 largeheat 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 = -DHvap/R (1/T) + C • P is the pressure (any units) • ln P is the natural log of P • DHvap 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 -DHvap/R

Clausius-Clayperon graph (c) Ln P 1/T Slope=DHvap/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, • DHvap=5.58 kJ/mol

How is boiling point related to pressure?