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Lesson 4 – Representing Enthalpy
Changes
Method One
THERMOCHEMICAL EQUATIONS WITH ENERGY TERMS

a balanced chemical equation that includes the heat transferred to
or from the surroundings
Ex.
H2 (g) + ½O2 (g)  H2O(l) + 285.8 kJ
MgCO3 (s) + 117.3 kJ  MgO(s) + CO2(g)

To write a thermochemical equation, you must know
–the balanced chemical equation
–the molar heat of reaction for one substance
–if the reaction is endothermic or exothermic
Sample Problem 1
From a calorimetry experiment, a student determines that the
specific heat of solution of KBrO3 (s) is an endothermic 0.25 kJ/g.
Calculate the molar heat of solution and write the
thermochemical equation.
Sample Problem 2
The combustion of magnesium releases 24.7 kJ per gram of
magnesium. Calculate the molar heat of combustion of
magnesium and write the thermochemical equation.
Sample Problem 3
Use the following thermochemical equation to calculate the molar
heat of combustion of hexane.
2C6H14 (l) + 19O2 (g)  12CO2 (g) + 14H2O (g) + 7086kJ
Method Two
THERMOCHEMICAL EQUATIONS WITH ΔH VALUES

The enthalpy of reaction is indicated by a separate expression
beside the chemical equation
»Remember: exothermic reactions have -ΔH
endothermic reactions have +ΔH
Examples:
H2 (g) + ½O2 (g)  H2O(l)
Horxn = -285.8 kJ
MgCO3 (s)  MgO(s) + CO2(g)
Horxn = 117.3 kJ
Method Three
MOLAR ENTHALPY OF REACTION

the quantity of heat transferred in a reaction per mole of a
specified substance

Hr (kJ/mol)

to calculate the molar heat of reaction:
Hr = specific heat of reaction x molar mass

standard molar enthalpy of reaction (Hro): molar enthalpy
that is determined at SATP
Sample Problem 4
Ethane is the second largest component of natural gas. If its
specific heat of combustion is -51.85 kJ/g, what is the molar
heat of combustion of ethane?
Method Four
POTENTIAL ENERGY DIAGRAM

a graphical representation of the energy transferred during a
physical or chemical change

exothermic reactions: products are lower in energy than
reactants (energy lost to surroundings)

endothermic reactions: products are higher in energy than
reactants (energy absorbed from surroundings)
Exothermic Reactions
Endothermic Reactions
Reactants
Ep
Products
H
Products
Reaction Progress
Ep
H
Reactants
Reaction Progress