Thermochemistry
Definitions
• Energy – capacity for doing work or
supplying heat.
• Thermochemistry – study of energy
changes that occur during phase changes
and chem. rxns.
• Chem. Potential Energy – energy stored in
chemical bonds.
Example
Lots of energy
stored in bonds!
Energy difference.
5473 kJ/mol
Little energy stored
in bonds.
Heat
• Represented by q.
• Energy that transfers from one object to
another because of a Temp. difference
between them.
• Heat flows from warm  cool until the
two objects are at the same Temp.
Exothermic vs. Endothermic
• In exothermic processes, the system loses
heat as its surroundings warm up.
– q has a negative value b/c the system is losing
heat.
• In endothermic processes, the system gains
heat as its surroundings cool down.
– q has a positive value b/c the system is gaining
heat.
Potential Energy 
Potential Energy Diagram of Ice
Melting at 0ºC.
Water
Ice
Time 
Is the melting of ice
an endothermic or
an exothermic
process? How can
you tell?
Measuring Heat Flow
• SI Unit of heat flow: Joule (J)
• Common unit used in chemistry: calorie
(cal)
– Amt. of heat needed to raise 1 gram of water
by 1ºC.
– 1 cal = 4.184 J
• Food Calorie (capital “C”) = 1000 cal, or 1
kilocalorie = 4184 J
What do Calories mean in food?
• 10 grams of sugar has 41 Calories.
– When 10 grams of sugar are burned, 41 kcal
(170 kJ) of energy are released.
– Your body “burns” food for energy.
– In order to use the energy available in 10
grams of sugar, you must do 41 kcal worth of
work.
Heat Capacity
• Amount of heat needed to raise an object’s
temperature by 1°C.
– Depends on the chemical composition and the
mass of the object.
• EXAMPLE: 1 gram of water requires 1
cal to raise its temperature by 1°C.
– 100. g of water require 100. cal to raise the
temp. by 1°C.
Heat Capacity
Same temperature change
10 g H2O
1 g H2O
Specific Heat (c)
• Amt. of heat needed to raise 1 gram of a
substance’s temperature by 1ºC.
– Expressed in J/g ºC, or cal/g ºC
• The higher a substance’s specific heat, the
more energy it takes to heat it.
• Substance’s with low specific heats heat up
and cool down quickly (most metals, e.g.)
Some Specific Heats
Substance
Water
Grain alcohol
Ice
Steam
Chloroform
Aluminum
Iron
Silver
Mercury
Specific Heat
J/gºC
cal/g ºC
4.18
1.00
2.4
0.58
2.1
0.50
1.7
0.40
0.96
0.23
0.90
0.21
0.46
0.11
0.24
0.057
0.14
0.033
Specific Heat (c)
• c = heat / (mass x change in Temp.)
• c = q / (m x ΔT)
• q = m x c x ΔT
Example Problem
• The temperature of a 95.4-g piece of Cu
increases from 25.0ºC to 48.0ºC when the
Cu absorbs 849 J of heat. What is the
specific heat of Cu?
– SOLUTION: q = m x c x ΔT
•
•
•
•
849 J = (95.4 g) c (48.0ºC – 25.0ºC)
849 J = (95.4 g) c (23.0ºC)
849 J = (2190 gºC) c
c = 0.388 J/gºC
• Based on what you know about metals,
does this answer make sense?
Example Problem
• When 435 J of heat is added to 3.4 g of
olive oil at 21ºC, the temperature increases
to 85ºC. What is the specific heat of olive
oil?
– SOLUTION: q = m x c x ΔT
•
•
•
•
435 J = (3.4 g) c (85ºC – 21ºC)
435 J = (3.4 g) c (64ºC)
435 J = (220 gºC) c
c = 2.0 J/gºC
Example Problem
• How much heat is required to raise the
temperature of 250.0 g of mercury by
52ºC? The specific heat of mercury is 0.14
J/gºC.
– SOLUTION: q = m x c x ΔT
– q = (250.0 g)(0.14 J/gºC)(52ºC)
– q = 1800 J = 1.8 kJ
Enthalpy Changes
• Enthalpy (H) – the heat content of a system
at constant pressure.
• Enthalpy change (ΔH) – the heat that
enters or leaves a system at constant
pressure.
• q = ΔH
• Neg. ΔH = exothermic process
• Pos. ΔH = endothermic process
Thermochemical Equations
• Enthalpy change can be written as a
reactant or a product.
– Reactant  endothermic
– Product  exothermic
• Example: The reaction of calcium oxide
with water is exothermic.
– It produces 65.2 kJ of heat per mole of CaO
reacted.
– CaO(s) + H2O(l)  Ca(OH)2(s) + 62.5 kJ
An Exothermic Reaction
CaO(s) + H2O(l)  Ca(OH)2(s) + 62.5 kJ
CaO(s) + H2O(l)
ΔH = -65.2 kJ
Ca(OH)2(s)
Thermochemical Equations
2NaHCO3(s) + 129 kJ  Na2CO3(s) + H2O(g) + CO2(g)
Na2CO3(s) + H2O(g) + CO2(g)
ΔH = +129 kJ
2NaHCO3(s)
Thermochemical Equations and
Stoichiometry
• You can use thermochemical equations in
stoichiometry.
– How much heat energy is produced when 55.0 grams
of ethanol is burned completely?
•
•
•
•
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g) + 1300. kJ
Given: 55.0 g C2H5OH
Want: kJ
Conversion factors:
– 1 mol C2H5OH produces 1300. kJ when burned
– 1 mol C2H5OH = 46.07 g/mol
1 mol C 2H5 OH
1300. kJ
x
55.0 g C2H5OH x
46.07 g C 2H5 OH 1 mol C 2H5 OH
= 1550 kJ
Thermochemical Equations and
Stoichiometry
• 0.500 grams of methane gas are burned completely beneath a
container that holds 100. grams of water, originally at 20.0º. If
all of the heat from the combustion reaction goes into the
water, what will the water’s final temperature be?
–
–
–
–
–
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 803 kJ
First find out how much total heat is released.
Given: 0.500 g CH4(g)
Want: kJ
Conversion factors:
• 1 mol CH4 = 16.05 g CH4
• 1 mol CH4 produces 803 kJ when completely burned
0.500 g CH4 x
1 mol CH4
803 kJ
x
= 25.0 kJ
16.05 g CH4 1 mol CH4
Thermochemical Equations and
Stoichiometry
• The combustion of 5.00 grams of methane releases 250. kJ of heat.
– Now we’ll calculate how hot the water in the container will get if it
absorbs all of the heat.
– First convert 25.0 kJ to J
25.0 kJ x
–
–
–
–
–
–
1000 J
= 2.50x104 J
1kJ
q = m x c x T
2.50x104 J = (100. g) (4.18 J/gºC) T
2.50x104 J = (418 J/ºC) T
T = 59.8ºC
The water will get 59.8ºC warmer.
The final temperature will be 20.0ºC + 59.8ºC = 79.8ºC.