Thermochemistry
• The study of the energy changes
in chemical reactions
–Absorption or release of heat
energy in chemical reactions
–Changes in physical state
Heat
• A form of energy that always
flows from a warmer object to
a cooler object
• We Indirectly measure heat by
measuring temperature changes
Energy
• The capacity to
do work or to
supply heat
• Chemical
potential energy
is energy stored
in chemicals
because of their
compositions.
Conservation of energy
• In any chemical or physical
process, energy is neither created
or destroyed.
• Energy may be converted between
stored energy and heat energy, or
accounted for as work.
Heat capacity
• A calorie is the quantity of
heat that raises the
temperature of 1 gram of
pure water 1oC.
• 1 Calorie = 1,000 calories = 1
Kcal
• 1 Joule = 0.239 calories
How many joules of energy
can you expect a 175
Calorie granola bar to
release?
One Joule = 0.239 calories
Specific Heat and Heat Capacity
• Specific heat is represented by
“C”
• C = the amount of heat needed
to raise the temperature of 1
gram 1 degree centigrade
Specific heat
• Specific heat is measured in
Joules and calories
• C “Water” = 1 calorie/g x oC
• C “ Al = 0.21 calorie/g x oC
Specific heat an Heat
capacity
• The lower the specific
heat - the faster the
substance will heat up
• lower the specific heat =
faster loss of heat
• Try some practice !! (go
to the bottom of the
Exothermic and Endothermic
• Exothermic = thermal energy
given off to the surroundings bond energy is decreased
• Endothermic = thermal energy
taken in from the surroundings
bond energy is increased
Calorimetry
• The accurate and precise
measurement of the heat change
for chemical and physical
processes
Enthalpy
• Another term for the potential
energy of a system
• Change in enthalpy is H
exothermic reactions
• The system feels hot
• Enthalpy change is negative
• Al + CuCl3 =>AlCl3 + Cu +
180 KCal
• H = - 180kcal
Endothermic reactions
• The system feels cold
• Potential energy - bond energy,
increases
•
H+
Calorimeters
devices used to measure the
amount of heat absorbed or
released during a chemical or
physical process
Enthalpy = Heat (almost!)
• When reactions are conducted
at constant pressure, thermal
energy released is exactly equal
to changes in bond energy
Heat released or absorbed = Q
Q= H = M x C x T
Thermochemical equations
An equation that includes the
heat change is a thermochemical equation.
Exothermic or Endothermic ?
• CaO(s)+ H2O ====> Ca(OH)2
Enthalpy change = -65.2 kJ
• Exothermic !!!
Exothermic or endothermic?
CaO(s)+H2O => Ca(OH)2 + 65.2KJ
Heat and changes of state
Molar heat of fusion:
+6.01 KJ/mole
Molar heat of solidification
-6.01 KJ/mole
Which process makes the surroundings
cold?
Which process makes the surroundings
warm?
Evaporation is endothermic –
the surroundings get cool
Molar heat of vaporization:
H= +40.7 Kj/mole
Molar heat of condensation :
H= - 40.7 Kj/mole
Heat of solution:
• The heat change caused by dissolution of
one mole of substance
• NaOH: * H= -445.1Kj/mol
Standard Heat of Formation
The change in enthalpy
that accompanies the
formation of one mole of
the compound from its
elements with all
substances in their
standard states at 25oC
Heat of formation
• Na + 1/2 Cl2  NaCl
D H= -411 Kj/Mole
• 1/2N2 + 3/2 I2  NI3 D H= +154.4 kJ/mol
What we mean by
formation
C (s) + O2 (g) ---> CO2 (g)
C (s) + (1/2) O2 (g) ---> CO (g)
H2 (g) + O2 (g) --> H2O2 (l)
H2 (g) + (1/2) O2 (g) ---> H2O (l)
C (s) + 2 H2 (g) + (1/2) O2 (g) ---> CH3OH (l)
Heats of Formation
What is not a Formation Reaction?
• (There is never a compound on the reactant
side, only elements.
• 6 CO2(g) + 6 H2O(l) --->
C6H12O6(s)+ 6 O2(g)
Heats of reaction
• Heat of the products minus the
Heats of the reactants.
• Try one !!!
• 6 CO2(g) + 6 H2O(l) --->
C6H12O6(s)+ 6 O2(g)
Heat of formation table pg 316
Hf C6H12O6(s) is -1270 kJ/mole
6 CO2 (g) + 6 H2O (l) 
C6H12O6 (s)
+ 6 O2(g)
C6H12O6(s) = +2815.8kj/mole
Does the answer make
sense?
• 6 CO2(g) + 6 H2O(l) --->
C6H12O6(s)+ 6 O2(g)
• Should this be an exothermic or endothermic
reaction? How do you know?
• What did you get ( + or -)
• +2815.8kj/mole kj/mole
CaO (s) + CO2 (g)  CaCO3(s)
H product – H reactants  H reaction
Try one !!!!
CaO (s) + CO2 (g)  CaCO3(s)
• -1,207.0 kj - (-635.1 + -393.5)
•
-1,207.0 kj – (-1028.6 kj)
•
-1,207.0 kj + 1028.6 kj
•
-178.4 kj
•
Exothermic
How about a physical
change?
• H2O (g) = H2O (l)
How about a physical
change?
H2O (g)  H2O (l)
•
•
•
•
-285.8kj - (-241.8kj)
-285.8kj + 241.8kj
-44.0kj
Exothermic
Heats of reaction
• 6 CO2(g) + 6 H2O(l) --->
C6H12O6(s)+ 6 O2(g)
–(-1270 kj + 0) – ((6 )(-393.5) +(6)(-285.8))
-1270 kj – (-2361.0 + -1714.8)
–-1270 – (-4075.8)
–2,805.8 kj
Practice time !!
• Lets try a few more: page 317
• 30 pg 317, a, b, and c
• (use the table on pg 1=316)
Hess’s Law
If you add two or more thermochemical
equations to give a final equation, then you
can also add the heat changes to give the
final heat change

Htotal = H1 + H2
C (diamond) + O2 (g) = CO2 (g)
H = -395.4 kJ
C (graphite) + O2(g) = CO2 (g)
H = -393.5 kJ
C (diamond) ====> C (graphite) ?
Htotal = H1 + H2
C (diamond) + O2 (g)  CO2 (g)
CO2(g)
H = -395.4 kJ

C (graphite) + O2 (g)
H = +393.5 kJ
C (diamond)  C (graphite) ?
H = -1.9 kj/mole
Exothermic !!
Remember !!!
• Heat of formation for free elements
is set at “O”
• Heat of formation for diatomic
molecules is “O”
Lets Practice (Hess’s law)
Lets practice!!!
Problem 57 & 58 page 322
• Thermochemistry worksheet
• Hess’s law worksheet
• Additional resources