Ch. 15
Thermochemistry
Unit 9 Vocabulary
• Thermochemistry - concerned with heat
changes that occur during chemical reactions
• Energy- capacity of an object to do work or
produce heat
• Kinetic energy- the energy an object has due
to its motion
• Potential energy- stored energy
• Temperature- describes amount of motion of
particles
Unit 9 Vocabulary
• Heat - describes amount of energy
transferred from one object to another
• Endothermic process- heat absorbing
process
• Exothermic process- heat releasing
process
• Specific heat- Amount of heat it takes to
raise 1g of a substance by 1°C
The Flow of Energy
Heat is transferred from one object to
another.
Cranking up the Heat:
Basic Thermodynamics
• Energy is required in all
biological and chemical
processes.
Kinetic Energy
–the energy an object has due to its
motion
Potential Energy

stored energy
Energy in the universe is
constant, though it
changes forms.
Law of Conservation of energy – energy
can neither be created nor destroyed
Heat vs. Temperature
• Temp. and heat are not the same thing.
• Temp. describes amount of motion of particles
• Heat describes amount of energy transferred
from one object to another
Describing Energy Changes
• Heat flow is from the warmer to the
cooler object
Heat Flow
Heat Flow
Describing Energy Changes
Hot day 50°C = 122°F
Energy transferred from fire to beans.
Change in energy for beans would be
positive.
Beans lost energy, it was transferred to its surroundings. This change
of energy for the beans would be negative, (change = ∆)
Describing Energy Changes
Endothermic reactions – absorbs heat, ∆E is
positive, the surroundings cool down
Surroundings
Heat
Exothermic reactions – releases heat, ∆E is
negative, the surroundings heat up
Surroundings
Heat
Units used to describe Energy:
• calorie – called “little calorie” – (cal)
• kilocalorie – called “Big calorie”- (CAL)
– 1000 calories – same as a food calorie
• Joule – the SI unit of heat - (J)
CONVERSION FACTOR
• 1 cal = 4.184 joules
1 cal
4.184 J
4.184 J
1 cal
Heat Units: Conversion Problems
1. Convert 32.7 cal to joules.
2. Convert 6.24 J to cal.
Specific Heat -
• Amount of heat it takes to raise 1g of a
substance by 1°C
– Metals have a low specific heat
– Water has a high specific heat
Learning Check
1. On a sunny day, the concrete deck around an
outdoor swimming pool becomes hot, while the
water stays cool. This is because…
A. The deck has a higher specific heat than the
water
B. The deck has a lower specific heat then the
water
C. Both objects have the same specific heat
Learning Check
2. Two objects are sitting next to each other in
the sunlight. Object A gets hotter than object
B.
A. Object A has a lower specific heat than object
B
B. Object A has a higher specific heat than object
B
C. Both objects have the same specific heat
Specific Heat of Water
Matches definition: Amount of heat it takes to raise 1g
of a substance by 1°C
4.18 J/g × °C
or
1 cal/g × °C
Formula for Specific Heat
q = mc ΔT
q = amount of heat – measured in cal or J
m = mass – measured in g
C = specific heat – units are J/g x °C or cal/g x
°C
ΔT = change in temp – measured in °C
Specific Heat Problems:
1. Aluminum has a specific heat of 0.9020 J/g × °C. How
much heat is lost when a piece of aluminum with a
mass of 23.984 g cools from a temperature of 415.0°C
to a temperature of 22.0°C?
Specific Heat Problems:
2. 850 calories of heat are applied to a 250 g sample
of liquid water with an initial temperature of 13.0
°C.
Find: a) the change in temperature and
b) the final temperature.
(remember, the specific heat of liquid water, in calories,
is 1.00 cal/g x °C.)
Specific Heat Problems:
3. The temperature of a sample of water increases by 69.5 °C
when 24,500 J are applied. The specific heat of liquid
water is 4.18 J/g x °C. What is the mass of the sample of
water?
Specific Heat Problems:
4. When 34,700 J of heat are applied to a 350 g sample
of an unknown material the temperature rises from
22.0°C to 173.0°C. What must be the specific heat of
this material?
Energy and Change of State
• When energy is added to a solid substance, its
temperature increases until its melting point is
reached. Upon the addition of more energy, the
substance begins to melt.
 The temperature remains the same until all the
substance has melted. ****
Enthalpy of Fusion (DHfus)
• Energy required to melt 1 gram of a substance at
its melting point
Heating Curve: ( For Water)
T
e
m
p
(°C)
DHfus
0
-10
-20
Melting point
Heating Ice
(solid)
Energy
Energy and Change of State
• After the substance melts and if heating
continues, the temperature will continue to
rise until it reaches its boiling point.
۞The temperature remains the same until all
the substance has boiled.***
Enthalpy of Vaporization (DHvap)
• Energy required to vaporize 1 gram of a
substance at its boiling point
DHvap
Boiling Point
100
T
e
m
p
(°C)
DHfus
0
-10
Melting Point
Heating Ice
-20
Energy
Heating Water
(liquid)
► Important Formulas to Know ◄
When a substance is being heated, use the
following formula:
q = (m)(c)(DT)
When a substance is melting or boiling, use
the following formula:
q = (m) (DHfus or DHvap)
q = (m)(c)(DT)
(C) = Specific Heat:
- of ice = 2.06 J/g x °C
- of water = 4.18 J/g x °C
- of steam = 2.02 J/g x °C
q = (m) (DHfus or DHvap)
• Enthalpy Values for Water:
DHfus = 334 J/g
DHvap = 2260 J/g
Learning Check:
Sample Problem
How much energy is required to convert 40g of
ice at -10°C to steam at 150°C ?
• Use Both Formulas
• Draw a graph
Sample Problem #1
How much heat is required to melt 5.67g
of ice?
Q = (m)(ΔH fus)
Q = (5.67g)(334 J/g)
Q=
Sample Problem #2
How much energy is required to boil 85g of
water?
Sample Problem #3
How much energy is required to raise the
temperature of 25g of water from 15°C to
75°C ?
Sample Problem #4
What is the final temperature when 1250 J of
energy is added to 15g of water at 25°C ?
Practice
1. How much energy is required to boil 50 grams of
water?
2. What is the final temperature when 2500 J of heat is
added to 35 grams of water at 15 degrees Celsius?