general, organic, and biological chemistrycopyright © 2010 pearson education, inc.1 chapter 2...

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 1

Chapter 2 Energy and Matter

2.7Changes of State


Melting and Freezing

A substance is melting when it changes from a solid to a liquid is freezing when it changes from a liquid to a solid such as water has a freezing (melting) point of 0 °C


Heat of Fusion

The heat of fusion is the amount of heat released when 1 gram of liquid

freezes (at its freezing point) is the amount of heat needed to melt 1 gram of a

solid (at its melting point) for water (at 0 °C) is

334 J or 80 cal

1 g of water

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

Guide to Calculations Using Heat of Fusion (or Vaporization)

4


Calculations Using Heat of Fusion

How much heat in calories is needed to melt 15.0 g ofice at 0 °C ?

STEP 1 Given: 15.0 g of water(s)

change of state: melting at 0 °C

STEP 2 Plan: g of water(s) g of water(l)

STEP 3 Write conversion factors:1 g of water = 80 cal

1 g of water and 80 cal 80 cal 1 g of water


Calculations Using Heat of Fusion (continued)STEP 4 Set up the problem to calculate calories:

15.0 g water x 80 cal = 1200 cal

1 g water


Learning Check

How many joules are released when 25.0 g of water

at 0 °C freezes?

1) 334 J

2) 2000 J

3) 8350 J


Solution

3) 8350 J

STEP 1 Given: 25.0 g of water(l)

change of state: freezing at 0 °C

STEP 2 Plan: g of water g of water

STEP 3 Write conversion factors:1 g of water = 334 J 1 g of water and __334 J___ 334 cal 1 g of water


Solution (continued)

STEP 4 Set up the problem to calculate joules:

25.0 g water x 334 J = 8350 J (3) 1 g water


Sublimation

Sublimation occurs when a solid changes directly to a gas is typical of dry ice, which sublimes at 78 C takes place in frost-free refrigerators is used to prepare freeze-dried foods for long-term

storage


Evaporation and Condensation

Water evaporates when molecules

on the surface gain sufficient energy to form a gas

condenses when gas molecules lose energy and form a liquid


Boiling

At boiling, water molecules

acquire the energy to form a gas

bubbles appear throughout the liquid


Heat of Vaporization

The heat of vaporization is the amount of heat absorbed to vaporize 1 g of a liquid to gas at the

boiling point released when 1 g of a gas condenses to liquid at

the boiling point

Boiling Point of Water = 100 °C

Heat of Vaporization (water) = 2260 J or 540 cal

1 g of water

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

Comparing Heat of Fusion and Heat of Vaporization

14


Learning Check

How many kilocalories (kcal) are released when 50.0 gof water(g) as steam from a volcano condenses at 100 °C?

1) 27 kcal

2) 540 kcal

3) 2700 kcal


Solution

1) 27 kcal

STEP 1 50.0 g of water(g) = 50.0 g of water(l)

Change of state: water condensing at 100 °C

STEP 2 Plan: g of water(g) g of water(l)

STEP 3 Write conversion factors:

1 g of water = 540 cal 1 g of water and 540 cal

540 cal 1 g of water

1 kcal = 1000 cal 1 kcal and 1000 cal 1000 cal 1 kcal


Solution (continued)

STEP 4 Set up the problem to calculate kilocalories:

50.0 g water x 540 cal x 1 kcal = 27 kcal

1 g water 1000 cal


Summary of Changes of State


Heating Curve

A heating curve illustrates the changes of

state as a solid is heated uses sloped lines to show

an increase in temperature uses plateaus (horizontal

lines) to indicate a change of state


Learning Check

A. A plateau (horizontal line) on a heating curve represents

1) a temperature change

2) a constant temperature

3) a change of state

B. A sloped line on a heating curve represents





Solution

A. A plateau (horizontal line) on a heating curve represents



B. A sloped line on a heating curve represents



Cooling Curve

A cooling curve illustrates the changes of

state as a gas is cooled uses sloped lines to

indicate a decrease in temperature

uses plateaus (horizontal lines) to indicate a change of state


Learning Check

Use the cooling curve for water to answer each.

A. Water condenses at a temperature of

1) 0 °C 2) 50 °C 3) 100 °C

B. At a temperature of 0 °C, liquid water

1) freezes 2) melts 3) changes to a gas

C. At 40 °C, water is a

1) solid 2) liquid 3) gas

D. When water freezes, heat is

1) removed2) added


Solution

Use the cooling curve for water to answer each.

A. Water condenses at a temperature of

3) 100 °C

B. At a temperature of 0 °C, liquid water

1) freezes

C. At 40 °C, water is a

2) liquid

D. When water freezes, heat is

1) removed


Combined Heat Calculations

To reduce a fever, an infant is packed in 250 g of ice. Ifthe ice (at 0 °C) melts and warms to body temperature(37.0 °C), how many calories are removed?

STEP 1 Given: 250 g ice changes to water at 37.0 °C

Need: calories to melt at 0 °C and warm to 37.0 °C

STEP 2 T = 37.0 °C – 0 °C = 37.0 °C

37.0 °C

0 °C


Combined Heat Calculations (continued)

STEP 3 Conversion factors:

1 g of water (0 °C) = 80 cal

1 g of water (0 °C) and 80 cal

80 cal 1 g of water (0 °C)

1 g of water(l) = 1.00 cal/g °C

1 g of water(l) and 1.00 cal/g °C 1.00 cal/g °C 1 g of water(l)


Combined Heat Calculations (continued)

STEP 4 Set up problem:(1) Heat to melt ice (fusion) 250 g ice x 80 cal = 20 000 cal

1 g ice

(2) Heat the water(l) from 0 °C to 37.0 °C 250 g x 37.0 °C x 1.00 cal = 9250 cal

g °C

Total: 20 000 cal + 9250 cal = 29 000 cal (rounded)

general, organic, and biological chemistrycopyright © 2010 pearson education, inc.1 chapter 2...

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