Energy and Heat

8/12/2019 Energy and Heat

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CHEMISTRY

Chapter 4

Energy and Heat

Engr. Yvonne Ligaya F. Musico



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Energy



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Energy

Makes objects move

Makes things stop

Energy from sun plants foods we

eat energy to liveWe need energy to “do work”



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Work is done when

Go up stairs

Play soccer

Lift a bag of groceries

Ride a bicycle

Breathe

Heart pumps blood

Water goes over a dam



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Potential Energy

Is stored energy

Examples

Water behind a dam

Compressed spring

Chemical bonds in gasoline or coal Food



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Kinetic Energy

Is energy of motion

Examples

Hammering a nail

Water flowing over a dam

Working out

Boxing

Burning gasoline



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Some Forms of Energy

Mechanical

ElectricalThermal (heat)

Chemical

Radiant (light)



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Laws of Conservation

Law of Conservation of EnergyDuring any ordinary chemical or physical change,

energy can be converted from one form to another,

but

energy is neither created no r destroyed

Law of Conservation of Matter (Mass)

During a chemical change there is no detectable

increase or decrease in the total quantity of matter.mass is neither created nor destroyed



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Learning Check

A. A car uses more fuel when the air

conditioner is on.

True (1) or False (2)

B. An apple hanging on a tree has potential

energy that can turn into bothmechanical

or chemical energy.True (1) or False (2)



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Solution

A. A car uses more fuel when the air

conditioner is on. True (1)

B. An apple hanging on a tree has potential

energy that can turn into both mechanical

or chemical energy. True(1)



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Temperature

Particles are always moving.

When you heat water, the water molecules

move faster.

When molecules move faster, the

substance gets hotter.When a substance gets hotter, its

temperature goes up.



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Learning Check

Suppose you place water in a freezer.

A. The water particles move

1) faster 2) slower 3) the sameB. The water will get

1) hotter 2) colder 3) stay the

same

C. The temperature of the water will be

1) higher 2) lower 3) the same



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Solution

Suppose you place water in a freezer.

A. The water particles move

2) slower

B. The water will get

2) colder

C. The temperature of the water will be

2) lower



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Heat



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Heat

Energy that flows from something warm to

something cooler

A hotter substance gives KE to a cooler

one

When heat is transferred (lost or gained),there is a change in the energy within the

substance



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Learning Check

A. When you touch ice, heat is transferred

from

1) your hand to the ice

2) the ice to your hand

B. When you drink a hot cup of coffee, heat

is transferred from1) your mouth to the coffee

2) the coffee to your mouth

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Solution

A. When you touch ice, heat is transferred

from

1) your hand to the ice

B. When you drink a hot cup of coffee, heat

is transferred from

2) the coffee to your mouth

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Learning Check

When you heat 200 g of water for 1 minute, thewater temperature rises from 10°C to 18°C.

If you heat 400 g of water at 10°C in the same

pan with the same amount of heat for 1

minute, what would you expect the finaltemperature to be?

1) 10 °C 2) 14°C 3) 18°C

200 g 400 g

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Solution

2)14°C

Heating twice the mass of water using the

same amount of heat will raise thetemperature only half as much.

200 g 400 g

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Conversion of Units

Heat is measured in calories or joules

1 kcal = 1000 cal

1 calorie = 4.18J

1 kJ = 1000 J

The calorie (cal) is defined as the amount of heat required to raise or

lower the temperature of 1 gram (1 g) of water 10C.

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Specific Heat

Why do some foods stay hot longer than

others?

Why is the beach sand hot, but the water is

cool on the same hot day?

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Specific Heat

Different substances have different

capacities for storing energy

It may take 20 minutes to heat water to75°C. However, the same mass of

aluminum might require 5 minutes and

the same amount of copper may takeonly 2 minutes to reach the same

temperature.

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Specific Heat Values

Specific heat is the amount of heat needed to

raise the temperature of 1 g of a substance by

1°C

cal/g°C J/g°C

water 1.00 4.18

aluminum 0.22 0.90

copper 0.093 0.39

silver 0.057 0.24

gold 0.031 0.13

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Learning Check

A. A substance with a large specific heat

1) heats up quickly 2) heats up slowly

B. When ocean water cools, the surrounding air1) cools 2) warms 3) stays the same

C. Sand in the desert is hot in the day, and cool

at night. Sand must have a

1) high specific heat 2) low specific heat

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Solution

A. A substance with a large specific heat

2) heats up slowly

B. When ocean water cools, the surrounding air

2) warms

C. Sand in the desert is hot in the day, and cool

at night. Sand must have a

2) low specific heat

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Factors that affect how much an object

absorbs or losses heat

mass of substance

Temperature change T

initial temperature

final temperature

Composition of an object

Specific heat of the substance (C)

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Calculating Heat

etemperatur inchangeT

object anof capacityheat specificC

massm

heat of amount QWhere

T mC Q

:

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Heat Calculations

A hot-water bottle contains 750 g ofwater at 65°C. If the water cools to body

temperature (37°C), how many calories

of heat could be transferred to soremuscles?

Q = mC T=( 750 g )(1.00 cal/g°C )( 28°C )

= 21,000 cal



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Solution

How many kcal are needed to raise the temperature of

120 g of water from 15°C to 75°C?

2) 7.2 kcal

120 g x (75°C - 15°C) x 1.00 cal x 1 kcal

g°C 1000 cal

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Energy and Nutrition

1 Calorie (nutritional) = 1 kcal

1 Cal = 1000 cal

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Caloric Food Values

Carbohydrate = 4 kcal/g

Fat = 9 kcal/g

Protein = 4 kcal/g

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Foods and Calories

Food Carbo Fat Protein Energy(kcal)

carrots,

1 cup 11 0 1 50

banana 26 0 1 110Egg 0 6 6 80

chicken

(no skin) 0 3 20 110

beef (3 oz) 0 5 22 130

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Learning Check

1.0 cup of whole milk contains 12 g of

carbohydrate, 9.0 g of fat, and 9.0 g of

protein. How many kcal (Cal) areobtained?

1) 48 kcal

2) 81 kcal3) 165 kcal

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Solution

3) 165 kcal

12 g carbo x 4 kcal/g = 48 kcal

9.0 g fat x 9 kcal/g = 81 kcal

9.0 g protein x 4 kcal/g = 36 kcal

Total kcal = 165 kcal



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Heating and CoolingCurves

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Heating Curve for Water

120 °Csteam

100 °C water steam

50°C liquid water

0 °C ice liquid

-10 °C ice

Heat added

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Learning Check

A. The flat lines on a heating curve represent

1) a temperature change

2) a constant temperature

3) a change of state

B. The sloped lines on a heating curve

represent


2) a constant temperature

3) a change of state

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Solution

A. The flat lines on a heating curve

represent

2) a constant temperature3) a change of state

B. The sloped lines on a heating curverepresent


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Temperature Changes

T(1) beginning temp.1

T(1)

T(f) final temp of both

T°C

T(2)

T(2) final temp. 2

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Cooling Curve

Using the heating curve of water as a guide,

draw a cooling curve for water beginning

with steam at 110°C and ending at -20°C.

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Learning Check

A. Water condenses at a temperature of

1) 0°C 2) 50°C 3) 100°C

B. At a temperature of 0°C, water1) freezes 2) melts 3) changes to a gas

C. When a gas condenses, heat is

1) released 2) absorbedD. Freezing is

1) endothermic 2) exothermic

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Solution

A. Water condenses at a temperature of

3) 100°C

B. At a temperature of 0°C, water

1) freezes 2) melts

C. When a gas condenses, heat is

1) releasedD. Freezing is

2) exothermic

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Learning Check

Is energy absorbed (1) or released (2) in each

of the following:

____A. Ice to liquid water

____B. Water vapor to rain

____C. Water to ice

When it rains, the air becomes

1) warmer 2) cooler 3) does not change

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Solution

Is energy absorbed (1) or released (2) in each of the

following:

_1__A. Ice to liquid water

_2__B. Water vapor to rain

_2__C. Water to ice

When it rains, the air becomes

1) warmer

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Learning Check

Complete using the terms gains or loses

In the cooling coils of a refrigerator, liquid

Freon ___________ heat from the food and

changes to a gas

Food ___________heat and becomes colder

In the back of the refrigerator, Freon _________ heat and condenses back to a

liquid

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Solution

Complete using the terms gains or loses

In the cooling coils of a refrigerator, liquid Freon

absorbs heat from the food and changes to a gas

Food loses heat and becomes colder

In the back of the refrigerator, Freon loses heat

and condenses back to a liquid

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Combining Heat Calculations

To reduce a fever, an infant is packed in 1250 g

of ice. If the ice at 0°C melts and warms to

body temperature (37.0°C) how much heat in

joules is absorbed?

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Combining Heat Calculations

Step 1: Diagram the change of state

37°C

T = 37.0°C - 0°C = 37.0°C

0°C S L

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Step 2: Calculate the heat to melt ice

(fusion)

= 1250 g ice x 334 J

1 g ice

= 418,000 J

Step 3: Calculate the heat to warm the

water from 0°C to 37°C

= 1250 g x 37.0°C x 4.18 J

g °C

= 193,000 J

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Total: Step 2 + Step 3= 418,000 J + 193,000 J

= 611,000 J

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Learning Check

A. Why do drops of liquid water form on a

glass of iced tea ?

B. When it snows, the air temperature seemswarmer. How can that be?

C. How much heat is needed to change 1.00g of water at 0° to steam at 100°C?

1) 540 cal 2) 640 cal 3) 720 cal

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Solution

A. Why do drops of liquid water form on a glass of iced

tea?

Condensation of water in the air that cools

B. When it snows, the air temperature seems warmer.How can that be?

Condensation is exothermic; heat is released.

C. How much heat is needed to change 10.0 g of water

at 0° to steam at 100°C?

2) 640 cal



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Thanks for listening

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Energy and Heat