Thermochemical
Stoichiometry
• A change in energy is one of
the indicators that a chemical
reaction has taken place
• Energy change is usually
identified by temperature
change
– Exothermic – energy released
– Endothermic – energy absorbed
Exothermic Reactions
• The energy released to make the
products is more than the energy used
to break down the reactants
– Breaking bonds is always endothermic
– Making bonds is always exothermic
• When energy
gy used is less than energy
gy
released - exothermic
• Temperature change
– Potential energy in the compounds is
converted to kinetic energy and lost
to the system
– Surrounding parts of system heat up
Endothermic Reactions
• Energy required to break down the
reactants is greater than the
energy released to form the
products
• Where does the energy come
from?
– from the surrounding system
• How does the temperature of the
system change?
– decreases
1
Enthalpy
• The heat that is absorbed or
released in a reaction
(assuming constant pressure)
• Measured in kilojoules
• Treated like any other product
or reactant in the equation
Calculating Heat of
Reaction
• The ∆H (change in enthalpy)
depends on the number of
moles of reactant or product
gy is a reactant in an
• Energy
endothermic reaction
– Will be a positive answer
• Energy is a product in an
exothermic reaction
– Will be a negative answer
Mole to Energy Ratio
• When calculating using a ∆H,
the mole to energy ratio
replaces the mole to mole
ratio in the calculation
2
Example
• When cement (calcium oxide) is
mixed with water to make concrete,
the ΔH for the formation of calcium
hydroxide is -65.2 kJ. You will
show this in the chemical equation
by including the ΔH as a product of
the reaction
• CaO + H2O  Ca(OH)2 + 65.2 kJ
• Is this an endothermic or
exothermic reaction?
• Is your answer going to be positive
or negative?
How much heat is given
off/absorbed if you start with
50.0g of calcium oxide?
• CaO + H2O  Ca(OH)2 + 65.2 kJ
• 50.0g
50 0 C
CaO
O / 1 mole
l C
CaO/65.2
O/65 2 kJ
1
/ 56g CaO / 1 mole CaO
• - 58.2 kJ
Baking soda (sodium hydrogen
carbonate) decomposes when it is heated,
making it useful in baking. The carbon
dioxide released in the reaction cause a
cake to rise during baking. This process
is endothermic and the heat of reaction is
129 kJ
• 2NaHCO
2N HCO3 + 129 kJ  Na
N 2O + 2CO2 + H2O
• How much energy is absorbed/released when
112g of baking soda decompose
• 112g NaHCO3/ 1 mole NaHCO3/ 129 kJ
1
/ 84g NaHCO3 / 2 moles NaHCO3
• 86.0 kJ
3