Lesson 33 Energy and Phase Change

Objectives:- The student will define the three general

phases of matter.- The student will understand how matter

can change phases.- The student will be able to interpret phase

diagrams.- The student will quantify changes in energy

associated with a phase change.

• I. Melting and boiling occur at specific temperatures

– a.       All state changes are reversible

•      i.      Melting and freezing

•                  ii.      Boiling and condensing

•                  iii. Sublimation and deposition

• b. All substances have standard melting and boiling points at normal atmospheric pressure.

•         i.      Water melts at 0oC, and boils at 100oC.

•         ii.      Substances which are solid at room temperature have melting points somewhere above room

temperature.

•      iii.      Substances which are gases at room temperature have boiling points well

below that of room temperature.

•        iv.      Melting and boiling points are physical properties often used to identify

substances.

II. Modeling melting, heating and boiling

a.       Remember we discussed the “kinetic theory of matter” previously

              i.      All matter is made of particles in constant motion

                        ii.     In a solid, they are vibrating in place

                        iii.    In a liquid, they are sliding around one another

iv. In a gas, they are flying around, freely, and unattached.

b. Solid to liquid            i.      In the solid, the particles are locked into their

positions, but they are vibrating in place.            ii.      As heat is applied to the substance, the particles

begin to gain energy, and move faster and faster. The temperature of the substance increases

            iii.      Eventually, the particles gain enough energy to be able to break free from one another, and begin to move around. This is melting.

            iv.      While this is occurring, no change in overall temperature is observed.

            v.      When all of the particles have broken free from their original positions, the substance has completely melted. The temperature of the substance begins to rise again.

c. Liquid to gas

         i.      In the liquid, the particles are sliding around one another.

            ii.      As heat is applied, the particles begin to gain energy, and slide around faster and faster. The temperature of the substance increases.

            iii.      Eventually, some particles gain enough energy to completely separate from the particles around them. This is boiling. No temperature change is observed at this time.

            iv.      When all of the particles have broken free, if heat is still being applied, the temperature of the gas begins to increase again.

III. Phase Diagramsa. Much of the information above can be shown through a phase

diagram.b. Phase diagrams show how the states of matter in a system are affected

by changes in temperature and pressure.c. The solid-vapor line, AB, gives the temperature and pressure at which

matter is in equilibrium between the solid and vapor phase.d. The liquid-vapor line, BD, gives the temperature and pressure at which

matter is in equilibrium between the liquid and vapor phase.e. The solid-liquid line, BC, gives the temperatures and pressures at

which matter is in equilibrium between the solid and liquid phase.f. Point B is the triple point, all three phases are in equilibrium at this

point.g. Point D is the critical point, only the gas phase exists and

temperatures and pressures above this point.h. Tm is the normal melting point.i. Tb is the normal boiling point.j. The slope of the BC line indicates

whether the matter will expand or contract when it freezes. If the line has a negative slope it will expand, and a positive slope indicates it will contract.

k. Based on the phase diagram above for water, determine the states of matter that exist for water under the following conditions:

i. 50 C and 0.1 kPaii. -30 C and 50 kPaiii. 105 C and 1000 kPaiv. 30 C and 100 kPa

HOT STUFF

PREDICT!

IV. Amounts of heat transferred can be measured.a. The q equation above is useful for measuring energy or heat involved when there is a

temperature change.b. We also would like to consider the change in heat when a substance is changing states. The formulas for this are:

i. For melting/freezing – H = grams x Hfusion

ii. For boiling/condensing – H = grams x Hvaporization

iii. Hfusion is the energy required to melt one gram of a substance at its melting point. This value is unique for each substance.iv. Hvaporization is the energy required to vaporize one gram of a substance at its boiling point. This value is also unique for each substance.

v. As you can see, the formulas are identical except for the Hfus or Hvap included. These values must be given to you for the particular substance you are using.

vi. A unique phenomena occurs when a substance undergoes a phase change. The matter will hold a constant temperature until the transition is complete for the entire piece of matter.

vi. See examples in class.

Constants needed for problems:

Cp ice = 2.06Cp water = 4.184Cp water vapor = 2.02

Hvap = 2260 J/g

Hfus = 334 J/g

Problem:

• How much energy would it require to heat a 45.5 gram block of ice at -22.5 C until it was all converted into water vapor at 133 C?

Questions:1. Explain what occurs in each of the following situations

at the molecular level:a. Ice meltingb. Water freezingc. Water boilingd. Steam condensing

2. Define and explain the uses of a phase diagram.3. Define enthalpy of fusion and vaporization.4. How much energy is required for a 33.4 gram block of

ice to be heated from -22 C to 180 C? Cp ice = 2.06, Cp water = 4.184, Cp water vapor = 2.02,

Hvap = 2260 J/g, Hfus = 334 J/g5. How much heat is required to melt 5.67 g of Iron (II)

oxide, FeO, if its enthalpy of fusion is 32.2 kJ/mole?

6. How much energy is necessary to convert 10.0 g of ice at -10.0 C to steam at 150C?

7. You have a 46.0g sample of water at a temperature of -58 C. How many joules of energy are necessary to:

a. heat the ice to 0.0 C?b. melt the ice?c. heat water from 0 to 100.0 C?d. boil the water?e. heat the steam from 100.0 C to 114 C?

8. How much energy is needed to melt 25.4 g of I2? The enthalpy of fusion of I2 is 61.7 J/g.

9. How much energy will be needed to melt 4.24 g of Pd? The enthalpy of fusion of Pd is 162 J/g.

10. In the phase diagram below for substance X, determine the following: ( Normal pressure is 101 kPa)

a. critical temperature b. critical pressure

c. triple point temperature d. triple point pressure

e. normal melting point f.normal boiling point