Energy and chemical change

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Everything Science www.everythingscience.co.za
12. Energy and chemical change
Chemistry
Grade 11

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Energy changes in chemical reactions
When a reaction occurs, bonds in the reactants break and new bonds in the products form.
These changes involve energy. When bonds break, energy is absorbed and when new bonds
form, energy is released.
The bond energy is a measure of bond strength in a chemical bond. It is the amount of energy
(in kJ·mol−1
) that is needed to break the chemical bond between two atoms.
Enthalpy is a measure of the total energy of a chemical system for a given pressure and is
given the symbol H.

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Endothermic and exothermic reactions
If the energy that is needed to break the bonds is less than the energy that is released when
new bonds form, then the reaction is exothermic. The energy of the products is less than
the energy of the reactants.
An exothermic reaction is one that releases energy in the form of heat or light.
Combustion reactions and respiration are both examples of exothermic reactions.
If the energy that is needed to break the bonds is more than the energy that is released
when new bonds form, then the reaction is endothermic. The energy of the products is
greater than the energy of the reactants.
An endothermic reaction is one that absorbs energy in the form of heat or light.
Photosynthesis and the thermal decomposition of limestone are both examples of
endothermic reactions.

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Heat of reaction
The difference in energy between the reactants and the product is called the heat of reaction
and has the symbol ∆H. ∆H is calculated using:
● In an endothermic reaction, ∆H is a positive number (greater than 0).
● In an exothermic reaction, ∆H will be negative (less than 0).
Δ H=Eprod − Ereact
Type of reaction Exothermic Endothermic
Energy absorbed or
released
Released Absorbed
Relative energy of
reactants and products
Energy of reactants greater
than energy of products
Energy of reactants less than
energy of products
Sign of ΔH Negative Positive

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Energy graph for exothermic reactions

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Energy graph for endothermic reactions

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Activation energy
The activation energy is the minimum amount of energy that is needed to start a chemical
reaction.
The activated complex (or transition state) is the complex that exists as the bonds in the
products are forming and the bonds in the reactants are breaking. This complex exists for a
very short period of time and is found when the energy of the system is at its maximum.
The two examples below show how the activated complex is drawn:

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Energy graph for exothermic reactions

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Energy graph for endothermic reactions

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