1.4 Potential Energy
Diagrams
Enthalpy
Chemical energy is most commonly converted to heat, we use the symbol, ΔH to
symbolize a change in energy available as heat. The symbol is sometimes read as “delta
H” or an enthalpy change.
Representing Combustion Reactions
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1.4 Potential Energy Diagrams
Energy of Chemical Bonds
Bond Energy
Endothermic
Exothermic
Representing the ΔH value associated with a chemical reaction.
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1.4 Potential Energy Diagrams
Transition State Theory
Activated Complex
An intermediate state that is formed during the conversion of reactants into products. The
activated complex forms when potential energy is at a maximum and kinetic energy
is minimized during a reaction.
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1.4 Potential Energy Diagrams
Potential Energy Profiles
Potential Energy Diagram
graphical representation of the energy changes that take place during a chemical reaction.
The enthalpy changes can be followed along the vertical axis, which measures potential
energy in kJ/mol. The horizontal axis is called by a variety of names such as the “reaction
coordinate” or the “progress of reaction.” It is critical to remember it is not a time axis.
.
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1.4 Potential Energy Diagrams
Potential Energy Profiles - 2
Potential Energy Diagram (con’t)
It is important to be aware that the net energy evolved or absorbed during the reaction is
independent of the activation energy.
Enthalpy Change
While altering factors such as concentration, surface area, or temperature will affect the
rate of a chemical reaction, they will not affect the appearance of a potential energy
Diagram.
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1.4 Potential Energy Diagrams
Reversible Reactions
Many chemical reactions are reversible under certain conditions.
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1.4 Potential Energy Diagrams
Reversible Reactions - 2
Summary
• ΔH for the forward and reverse reactions have the same magnitude and opposite
signs.
• Ea is always smaller for the endothermic reaction.
• Altering temperature, pressure, concentration, or surface area will have no effect
on a potential energy diagram. There is only one thing that will alter a diagram
and that is discussed below.
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1.4 Potential Energy Diagrams
Activation Energy and Catalysis
Catalysts
Potential Energy Profiles and Catalysts
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