Chemical Bonding sec. 7

Chemical Bonding Unit 2

Section 7:

● Characteristics of Phase Diagrams

● Triple Point

Phase Diagrams

Piston designed to maintain exact pressures during temperature and phase changes.

Cylinder contains the substance being investigated (there is no air or empty space)

Regulated Heat Source

Phase diagrams describe the states of matter ( solid, liquid, or gas ) exhibited by a substance under

different conditions of pressure and temperature

inside this apparatus.

Phase diagrams do not describe the states of matter that exist under normal atmospheric conditions, nor

do they describe the conditions that would exist in

an open system.

Phase diagrams only describe the states of

matter that exist within a closed cylinder under very specific conditions of pressure and

Phase Diagram of H

O

P

re

ss

ur

e

(a

tm

)

Temperature (℃)

0.01 100

1 218

.006

All phase diagrams are plots of Temperature vs. Pressure.

The pressure is the pressure inside the cylinder.

As our cylinder is all H₂O, with no air, it is the pressure that all existing phases of water are under.

The temperature is the temperature inside the cylinder.

The phase diagrams that we will be looking at consist of three lines

Phase Diagram of H

O

Temperature (℃)

0.01 100

1 218

.006

374

H₂O_(l) H₂O_(g)

Gas and liquid coexist in the cylinder when on the red line. (the water is boiling).

All water in the cylinder is gaseous when below the red line.

G

L

All water in the cylinder is liquid when above the red line.

The Vapor Pressure Curve

At any point on this line, liquid and gas exist in

equilibrium with one another (inside the apparatus we looked at two slides earlier).

This could be considered the “boiling point

curve” as water only boils on the points that fall on this line.

Liquid and gas only co-exist within the cylinder on this line.

Relating Phase Diagrams to Open Systems

In an open system water will boil at every point

along this line.

At 1 atm water boils at 100oC, and the vapor pressure coming off of the surface of the liquid is 1 atm.

If we travel up a mountain, the pressure (y-axis) drops and so does the boiling temperature (x-axis), as the water will boil when the vapor pressure equals the reduced atmosphere pressure.

However, water does evaporate at 25oC, and an

equilibrium between liquid and solid can be set up

in a closed system at 25OC.

Why does this graph state that only liquid water

can exist at 1 atm and 25oC?

The vapor pressure coming off the surface of liquid of

If we took the water vapor that comes off the liquid water at 25oC, which has a pressure of 0.0313 atm, and increases the pressure to 1 atm, all of that water vapor would turn into water.

This is why no water will turn to vapor within the cylinder, unless the vapor pressure equals the pressure within the cylinder.

At the point the water boils.

Phase Diagrams of H

O

Temperature (℃)

0.01 ₁₀₀

1 218

.006

374

H₂O_(l) H₂O_(g)

G

L

All water in the cylinder is liquid between the blue and red lines.

H₂O_(s) H₂O_(l) Solid and liquid coexist in

the cylinder when on the blue line

S

Only ice exists in the cylinder to the left of the blue line

Solids and liquids co-exist in equilibrium with one another within the cylinder at any point along the blue line.

As you know from experience, ice and water can coexist in equilibrium with one another in a closed system at 0oC.

This point falls on the blue line.

Increasing the temperature from any point on the blue line will give you only liquid in the cylinder.

Phase Diagrams of H

O

Temperature (℃)

0.01 ₁₀₀

1 218

.006

374

H₂O_(l) H₂O_(g)

G

L

Negative Slope

● _{If you increase the}

pressure, the substance will liquefy.

H₂O_(s) H₂O_(l)

S

Increasing the pressure from any point on the blue line will give you only liquid.

This is because solid water is less dense than liquid

water.

Ice floats on water, as it is less dense.

When you increase the pressure of ice on it attempts to reduce that pressure by increasing its density.

When you decrease its density it turns into water. As you can see from the graph, you can have liquid water at temperatures below 0oC if the pressure is above 1 atm.

Glaciers and Pressure

At the bottom of a glacier you will have a little bit of water that flows at about -2.0oC.

Under these high pressures, the ice reduces its density and becomes water that exists at temperatures below 0oC.

Pulling a wire Through an Ice Cube

This is also why you can pull a wire through an ice cube. The pressure causes the ice to melt even though it is colder than 0oC.

After the wire passes, the pressure is reduced ( to 1 arm again ), and the water then refreezes, as it is less than 0OC

Density and the Solid Liquid Line

For most substances, density decreases as they solidify.

Such substances would have solid/liquid equilibrium lines with positive slopes.

Phase Diagrams of H

O

Temperature (℃)

0.01 100

1 218

.006

374

H₂O_(l) H₂O_(g)

G

L

Solid and gas coexist in the cylinder when on the green line.

H₂O_(s) H₂O_(l)

S

Only

ice Only water _{vapor (gas)}

The green line is the sublimation curve.

It represents the equilibrium between solid and

gas.

In the cylinder discussed earlier, water would

co-exist as a liquid and a gas at any point on this line.

To the left of this line, there is only ice in the

cylinder.

Liquids cannot exist along this line.

We have only ice and gas.

Freeze Drying Foods

Foods can be freeze-dried at temperatures below

0.01

C and pressures below 0.006 atm.

Under these conditions, the water in the food

sublimes.

Phase Diagrams of H

O

Temperature (℃)

0.01 100

1 218

.006

374

H₂O_(l) H₂O_(g)

G

L

Triple Point

H₂O_(s) H₂O_(l)

S

The triple point is the only place where solid, liquid,

and gas can all exist in equilibrium with one

another in the cylinder.

For water, the triple point occurs at 0.006 atm and

0.0098

C.

Triple Point

Solid

Liquid

Gas

Conditions of pressure and temperature where a substance exists in a state of equilibrium between its solid, liquid, and gaseous phases.

The triple point is the only condition of pressure and temperature where a substance can exist as a solid, liquid, and gas.

The three phases maintain a state of equilibrium with one another.

This occurs because both ice and liquid water have identical vapor pressures (0.06 atm for water) at this point.

Phase Diagrams of H

O

Temperature (℃)

0.01 100

1 218

.006

374

H₂O_(l) H₂O_(g)

G

L

Critical Point

H₂O_(s) H₂O_(l)

S

Beyond the critical point it becomes impossible to distinguish between liquids and gases.

For water, this occurs when the pressure is greater than 218 atm and the temperature is greater than 374o_C.

As you continue to heat a substance in a closed

container the concentration of gas increases as more gas evaporates.

This causes the density (particles/unit volume) of the gas to increase.

As the liquid in the container absorbs heat its density decreases.

Critical Point

At temperatures beyond the critical

temperature,

T

,

and pressures beyond the

critical pressure,

P

,

the entire system

becomes a supercritical fluid.

Supercritical Fluid

Too dense to be a gas

At temperatures above 374

C you cannot get

H

O vapor to liquefy, even if you increased

the pressure to 400 atm or higher.

At pressures above 218 atm, you cannot get

liquid water to vaporize.

Phase Diagram of CO

Temperature (℃) -57

5.1 73

1

31.1

CO_2(l) CO_2(g)

G

L

CO_2(s) CO_2(l)

S

CO_2(s) CO_2(g) -78.5

Positive Slope

● _{If you increase}

The solid/liquid equilibrium line has a

positive slope for CO

.

This means that increasing the pressure of

a liquid at low temperatures, will give you a

solid.

Most substances behave this way.

Sublimation

Dry ice is solid carbon dioxide.

At 1 atm, CO

sublimes from solid straight to

gas.

The vapor coming off the solid is -78.5

C.

Liquid CO

can never exist at 1 atm.

Ex)Describe the phase changes that occur if CO₂ is kept at -52o_C while the pressure increases from 1 atm to 50 atm.

P re ss ur e (a tm )

Temperature (℃) -57

5.1 73

1

31.1

CO_2(l) CO_2(g)

G

L

CO_2(s) CO_2(l)

S

CO_2(s) CO_2(g) -78.5

Step 5

Step 4

Step 3

Step 2

Step 1

From 1 atm to a little over 5 atm there is only

carbon dioxide gas in the cylinder.

Step 2

Liquid forms when the pressure is a little greater

than 5 atm.

The liquid that is formed is

Step 3

As the pressure increases a little, the boiling stops and only liquid is present in the cylinder.

Step 4

Some solid forms, and solid and liquid exist together in equilibrium in the cylinder.

Step 5

When the pressure increases a little bit past this point, all of the liquid solidifies, and there is only solid in the