1

Mr. Shields Regents Chemistry U03 L03

2

Phase changes

This transition represents a change from one state of matter to another

Consider the following: Steam (G ) Water (L) Ice cube (S)

>100 deg C 0-100 deg C <0 deg C

A Change of state is also known as a PHASE CHANGE

3

Phase Changes

Consider Ice (s) going through a phase change to water (l).

H20 (s) H20 (l)

In both solid and liquid phases, the molecules are H2O. The identity of a substance during a phase change is not altered.

4

Phase Changes

 What’s happening to the molecules as we make these transitions?

Let’s consider the phase changes in our water example

Steam (G ) Water (L) Ice cube (S) >100 deg C 0-100 deg C <0 deg C

5

Phase Changes

As temperature decreases K.E. decreases

- The force of attraction between particles begins to play a bigger and bigger role

Molecules are like magnets- they feel a force of attraction between one another

So what exactly is going on?

6

Phase Changes

1. As T decreases Molecules begin to attract one another more strongly

2. The Molecules start arranging themselves in a more orderly fashion

- they grab on to each other and stay in place

Liquid phase solid phase

As they cool down further…

7

Phase Changes

3. Motion of the molecule becomes more restricted - The molecule is trapped in place by it’s neighbor

unorganized Little organization Highly organized

High KE (T) Lower KE Even Lower KE

8

Phase Change & Energy

There are 2 specific energy terms associatedWith Phase changes

Increasing T represents an Increase in EnergyDecreasing T represents a Decrease in Energy

Which energy term applies depends upon which way the temperature is changing

1. Temperature (T) is either increasing

2. Temperature (T) is either decreasing

9

Phase Change DiagramWe can show this relationship between Energy, T and Phase change in a Phase Change Energy diagram

In an endothermic phase change the particles absorb energy

In an exothermic phase change the particles release energy

Does the T of the phase increase or decrease?

Does the T of the phase increase or decrease?

Gas

Solid

LiquidEnergy AbsorbedEnergy Released

Endothermic

Exothermic

T decreases

T increases

10

Let’s look at the Liquid to Gas phase changes first

Phase Change – Liquid / Gas

When a liquid absorbs energy it’s temperature increase.Eventually the liquid becomes a gas (steam).

Is this an endothermic or exothermic process?

endothermic

exothermic

What about the reverse process (Gas to Liquid)?

11

Phase Change – Liquid / Gas

Gas

Solid

LiquidVaporization “endo”

Condensation “exo”

What happens if the gas is cooled (i.e. it gives up energy)?

The gas phase undergoes a phase change to the liquid phase.This is called CONDENSATION.

If enough energy is absorbed a liquid becomes a gas.This is called VAPORIZATION.

12

Let’s next look at Solid to Liquid phase changes

Phase Change – Solid / Liquid

When a solid absorbs energy it’s temperature increase.The solid becomes a liquid.

Is this an endothermic or exothermic process?

endothermic

exothermic

What about the reverse process (liquid to solid)?

13

Phase Change – Solid / Liquid

Gas

Solid

LiquidFusion“endo”

Solidification “exo”

If enough energy is absorbed a solid becomes a liquid

This is called FUSION (commonly called melting).

What if the liquid is cooled (i.e. gives up energy).What eventually happens?

The liquid becomes a solid. This is called SOLIDIFICATION,(commonly called freezing)

14

Let’s next look at Solid to Gas phase changes

Phase Change – Solid to Gas

When certain solids absorb energy they go directly from theSolid to Gas phase, i.e. they skip the Liquid phase. There areonly a few materials that do This.

endothermic

This process is known as SUBLIMATION. Is it an ENDOOr EXO phase change?

Iodine (I2) and dry Ice (CO2)

15

Sublimation of Iodine

Iodine solid

Iodine Gas

16

Sublimation is a unique Process. Certain criteria mustBe met for it to occur.

Phase Change – Solid to Gas

endothermic

So what kind of compounds SUBLIME?

Compounds with Weak intermolecular forces of attraction between molecules and High Vapor

Pressures

CO2 or I2

17

Let’s next look at the opposite of sublimation. In otherWords the process of going from Gas to Solid.

Phase Change – Solid to Gas

Since this is the reverse process the same criteria of weak Molecular forces and high VP still apply.

exothermic

This process is known as Deposition. It’s an exo process.In other words, heat is released.

18

Phase Change Quiz

Gas

Solid

LiquidEnergy AbsorbedEnergy Released

Endothermic

Exothermic

a

b

c

d

e

f

ENDOTHERMIC

a. Sublimation

b. Fusion

c. Vaporization

EXOTHERMIC

d. Deposition

e. Condensation

f. Solidification

T ? T ?