Thermochemistry
INTRO TO THERMOCHEMISTRY
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Chemical reactions involve changes in energy
• Breaking bonds releases energy
• Forming bonds requires energy
The energy involved in chemistry is real and generally a
measurable value
• Energy units are numerous, but we will
concentrate on the Joule (SI base unit) and the
calorie (little c, big C is the food Calorie or a
kilocalorie)
• 1 calorie = 4.184 Joules
Thermochemistry
• The study of energy changes that occur
during chemical reactions and changes of
state.
The Universe
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Can be divided into 2 “parts”
System- the part you are investigating
Surroundings- the rest of the universe
In a thermo-chemical experiments the
region in immediate vicinity of the
system are the surroundings.
Chemical Potential Energy
• the energy stored in the
chemical bonds of a
substance
• the kinds of atoms and
their arrangement in the
substance determine the
amount of energy stored in
the substance.
There are three methods used to
transfer heat energy.
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Conduction – transfer of heat through
direct contact
Convection – transfer of heat through
a medium like air or water
Radiant – transfer of heat by
electromagnetic radiation
Heat
• Symbol = q
• energy that transfers from one object to another
because of temperature difference
• Heat always flows from a warmer object to a cooler
object.
An Ice Cold Spoon
energy
transfer
A Hot Spoon
If 2 objects remain in contact,
heat will flow from a warmer
object to the cooler object until
the temperatures are equal.
AKA:
HEAT
WHAT IS HEAT?
• Hot & cold, are automatically associated with the words
heat and temperature
• Heat & temperature are NOT synonyms
• The temperature of a substance is directly related to the
energy of its particles, specifically its:
Kinetic Energy defines the temperature
Particles vibrating fast = hot
Particles vibrating slow = cold
Different type of materials may have the same
temp, same mass, but different conductivity.
•They are affected by the potential energy
stored in chemical bonds or the IMFs holding
molecules together
•It is possible to be at same temp (same KE) but
have very different thermal energies.
•The different abilities to hold onto or release
energy is referred to as the substance’s
heat capacity
Unit for Measuring Heat Flow
• calorie (cal) = heat needed to raise the
temperature of 1 g of pure water by 1º C
• 1 Calorie = 1 kilocalorie = 1000 calories
• calorie = c, dietary calorie = C
• 1 Calorie = energy in food
• a doughnut has 500 Calories or 500,000 cal
Unit for Measuring Heat Flow
• Joule, the SI unit of energy
• One joule of heat raises the temperature of 1 g
of pure water 0.2390º C
• 1 J = 0.2390 cal
• 4.184 J = 1 cal
Units for Measuring Heat
The Joule is the SI system unit for
measuring heat:
kg  m
1 J  N m 
2
s
2
The calorie is the heat required to raise the
temperature of 1 gram of water by 1 Celsius
degree
1 calorie  4.18 Joules
Heat
DEFINITION
THE FLOW OF THERMAL ENERGY FROM
SOMETHING WITH A HIGHER TEMP TO
SOMETHING WITH A LOWER TEMP
UNITS
MEASURED IN JOULES OR CALORIES
THROUGH WATER OR AIR = CONVECTION
TYPES
THROUGH SOLIDS = CONDUCTION
TRANSFERRED ENERGY BY COLLISION WITH
PHOTON = RADIANT ENERGY
Heat Capacity
The measure of how well a material absorbs
or releases heat energy is its heat capacity
•Physical property unique to a particular material
•The heat capacity depends on both its mass and its chemical
composition.
•The greater the mass; the greater the heat capacity.
•It can be thought of as a reservoir to hold heat, how much
it holds before it overflows is its capacity
•Water takes 1 calorie of energy to raise temp 1 °C
•Steel takes only 0.1 calorie of energy to raise temp 1 °C
Specific Heat
The amount of heat
required to raise
the temperature of
one gram of
substance by one
degree Celsius.
q = mCT
Calculations Involving Specific Heat
q
c  
m T
p
OR
q  c m  T
c = Specific Heat
q = Heat lost or gained
T = Temperature change
m = Mass
p
Table of Specific Heats
Law of
Conservation of Energy
• In any chemical or physical
process, energy is neither created or
destroyed
• If energy of the system decreases
the energy of the surroundings must
increase by the same amount so that
the total energy of the universe
remains unchanged.
Endothermic Process
• a reaction in which heat energy is absorbed
• process in which energy is absorbed as it
proceeds, and surroundings become colder
• the system gains heat as the surroundings cool
down
• typically feels cooler the longer the reaction
proceeds
•
q is positive
Endothermic Reactions
Exothermic Process
• a reaction in which heat energy is generated
• process in which energy is released as it
proceeds, and surroundings become warmer
• typically feel warm as the reaction proceeds
• the system loses heat as the reaction
progresses
• q is negative
Exothermic Reactions
HEAT
HEAT
HEAT
HEAT
Calorimetry
• The precise measurement of heat flow into or
out of a system for chemical and physical
purposes
• The heat released by the system is equal to the
heat absorbed by its surroundings.
• Conversely, the heat absorbed by a system is
equal to the heat released by it’s surroundings.
Calorimeter
Key Points
• Δ = Change
• q=mcΔT
• q= heat capacity
• q= heat
• KE = ½ mv2
• m = mass
• Heat Capacity
• c = specific heat
• 1 J = 0.2390 cal
•
• 4.184 J = 1 cal
ΔT = change in
temperature
• Calorimetry
Enthalpy
• H
• The heat content of a system at a constant
pressure.
• The heat released or absorbed by a reaction at
constant pressure is the change in enthalpy.
• Heat and Enthalpy are used interchangeably.
• So…… q = ΔH
CHANGE IN HEAT ENERGY (ENTHALPY)
The energy used or produced in a chemical
reaction is called the enthalpy of the reaction
•Burning a 15 gram piece of paper produces a
particular amount of heat energy or a particular
amount of enthalpy
•Enthalpy is a value that also
contains a component of direction
(energy in or energy out)
Heat lost is the out-of direction; ie exo-
CHANGE IN HEAT ENERGY (ENTHALPY)
The energy used or produced in a chemical
reaction is called the enthalpy of the reaction
•Burning a 15 gram piece of paper produces a
particular amount of heat energy or a particular
amount of enthalpy
•Enthalpy is a value that also
contains a component of direction
(energy in or energy out)
Heat lost is the out-of direction; ie exoHeat gained is the in-to direction; ie endo-
Exothermic and Endothermic
Processes
Enthalpy Change
qsurr  mcT
qsys  H  qsurr  mcT
Thermochemical Equations
• The enthalpy change for a reaction can be
written as a reactant or product.
• The Heat of Reaction is the enthalpy change
for the chemical equation exactly as it is
written.
CaO(s) + H2(l) Ca(OH)2 (s) + 65.2 kJ
Heat of Combustion
• The heat of reaction for the complete burning
of one mole of a substance.