EnthalpyChemical 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

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 ComplexAn 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 energyis minimized during a reaction.

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1.4 Potential Energy Diagrams

Potential Energy Profiles

Potential Energy Diagramgraphical 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 energyDiagram.

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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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