REPRESENTING ENTHALPY CHANGES
1.
2.
3.
ΔH Notation
Thermochemical Equation
Potential Energy Diagram

the ΔH is written outside the balanced
chemical equation
RECALL:
+ ΔH means ENDOTHERMIC
- ΔH means EXOTHERMIC
2 SO2 + O2  2SO3
ΔH = -197.8 kJ
“When 2 moles of SO2 react with 1 mole of O2,
2 moles of SO3 are produced and 197.8 kJ of
energy are released.”
1)What is the molar enthalpy of formation for
SO3?
2)What will the ΔH be if 4 moles of SO2 react
with excess O2?

a balanced chemical equation which includes
the ΔH for the rxn as either a reactant (LEFT
side) or product (RIGHT side)
exothermic
endothermic
- ΔH
energy quantity as
a PRODUCT
+ΔH
energy quantity as
a REACTANT
1) Write a thermochemical equation for the
following:
N2 + 3H2  2 NH3
(i.e., ΔH notation)
ΔH = -92.38 kJ
2) Write the thermochemical equation using ΔH
notation:
CO2 + 393.5 kJ  C + O2
(i.e., thermochemical equation)

all stoichiometric ratios apply to the energy
component
2 Fe + 3 CO2 + 6.39 kJ  Fe2O3 + 3 CO
“When 2 mol of Fe react with 3 mol of CO2 ,
6.39 kJ of energy are consumed. 1 mol of
Fe2O3 and 3 mol of CO are produced”.
1) Rewrite the thermochemical equation so that
the coefficient in front of the Fe is a 1
2) How many kJ are required to completely
react 8.00 g of Fe?
Exothermic
Energy
Rxn Coordinate
Exothermic
Energy
Rxn Coordinate
Exothermic
Energy of the
Reactants
-ΔH
energy released
Energy
Energy of the
products
Rxn Coordinate
Endothermic
Energy
Rxn Coordinate
Endothermic
Energy of the products
Energy
+ΔH
energy absorbed
Energy of the
Reactants
Rxn Coordinate
Construct a PE diagram for the reaction:
N2 + O2 + 180.8 kJ  2NO
Endothermic
2NO
Energy
ΔH = 180.8 kJ
N2 + O2
Rxn Coordinate