Thermochemistry

is the study of energy changes that occur

during chemical reactions and changes in
state.

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The law of conservation of energy

• states that in any chemical or physical process, energy is neither created

nor destroyed
• the energy of the universe remains unchanged

•If the energy of the system increases during

that process, the energy of the surroundings
must decrease by the same amount.
•If the energy of the system decreases during
that process, the energy of the surroundings
must increase by the same amount.

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Recognizing Endothermic and Exothermic
Processes

On a sunny winter day, the snow on a rooftop

begins to melt. As the melted water drips from
the roof, it refreezes into icicles. Describe the
direction of heat flow as the water freezes.

Is this process endothermic or exothermic?

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2 Solve Apply concepts to this situation.
First identify the system and the surroundings.

System: water
Surroundings: air

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2 Solve Apply concepts to this situation.
Determine the direction of heat flow.
• In order for water to freeze, its temperature
must decrease.

• Heat flows out of the water and into

the air.

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2 Solve Apply concepts to this situation.

Determine if the process is endothermic or exothermic.

• Heat is released from the system to the

surroundings.

• The process is exothermic.

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Athletes often use instant cold packs to soothe
injuries. Many of these packs use the dissociation of
ammonium nitrate in water to create a cold-feeling
compress.
Is this reaction endothermic or exothermic? Why?

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Athletes often use instant cold packs to soothe
injuries. Many of these packs use the dissociation of
ammonium nitrate in water to create a cold-feeling
compress.
Is this reaction endothermic or exothermic? Why?

The instant cold pack feels cold because it removes heat from
its surroundings. Therefore, the dissociation of ammonium
nitrate in water is endothermic. The system (the cold pack)
gains heat as the surroundings lose heat.

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Recognizing Endothermic and Exothermic
Processes

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Recognizing Endothermic and Exothermic
Processes

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Recognizing Endothermic and Exothermic
Processes

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Recognizing Endothermic and Exothermic
Processes

exothermic endothermic

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The specific heat capacity, or simply the specific
heat, is the amount of heat it takes to raise the
temperature of 1 g of the substance 1°C.
Specific Heats of Some Common Substances
• Water has a
very high
specific heat
compared with
the other
Specific heat
substances. Substance
J/(g·°C) cal/(g·°C)
• Metals Liquid water 4.18 1.00
generally have Ethanol 2.4 0.58
low specific Ice 2.1 0.50
heats. Steam 1.9 0.45
Chloroform
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 Heat Capacity and
Specific Heat of Water
Specific Heat

Just as it takes a lot of heat to raise the temperature

of water, water also releases a lot of heat as it cools.

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 Heat Capacity and
Specific Heat
Specific Heat of Water

Water in lakes and oceans absorbs heat from the air

on hot days and releases it back
into the air on cool days.

• This property of water is

responsible for moderate climates
in coastal areas.

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 Heat Capacity and
Specific Heat of Water Specific Heat

When a freshly baked apple pie comes out of the oven,

both the filling and the crust are at the same temperature.

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 Heat Capacity and
Specific Heat of Water Specific Heat

When a freshly baked apple pie comes out of the oven,

both the filling and the crust are at the same temperature.

• The filling, which is mostly water, has a

higher specific heat than the crust.
• In order to cool down, the filling must
give off a lot of heat.
• This release of heat is why you have to
be careful not to burn your tongue when
eating hot apple pie.

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 Heat will flow from the lava to the surroundings until the
lava and surroundings are at the same temperature. Air has
a smaller specific heat than water.

Where would lava cool more quickly in water or in air?

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 Heat will flow from the lava to the surroundings until the
lava and surroundings are at the same temperature. Air has
a smaller specific heat than water.

Where would lava cool more quickly in water or in air?

Water requires more energy to raise

its temperature than air.
Therefore, lava in contact with water
loses more heat energy than lava in
contact with air, allowing it to cool
more quickly.

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 Heat Capacity and
Specific Heat
Calculating Specific Heat

To calculate the specific heat (C) of a substance, you

divide the heat input by the mass of the substance
times the temperature change.

q heat (J or cal)
C= =
o
m  ΔT mass (g)  change in temperature ( C)

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Calculating the Specific Heat of a Substance

The temperature of a 95.4-g piece of copper increases from

25.0°C to 48.0°C when the copper absorbs 849 J of heat.
What is the specific heat of copper?

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 Evaluate Does the result make sense?
3

• Remember that liquid water has a specific heat of

4.18 J/(g·°C).
• Metals have specific heats lower than water.
• Thus, the calculated value of 0.387 J/(g·°C) seems
reasonable.

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The specific heat of ethanol is 2.4 J/(g·°C). A sample of
ethanol absorbs 676 J of heat, and the temperature rises
from 22°C to 64°C. What is the mass of ethanol in the
sample?

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The specific heat of ethanol is 2.4 J/(g·°C). A sample of
ethanol absorbs 676 J of heat, and the temperature
rises from 22°C to 64°C. What is the mass of ethanol in
the sample?

q q
C= m  ΔT m= C  ΔT
676 J
m= = 6.7 g ethanol
2.4 J/(g·°C)  (64°C – 22°C)

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