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Chemistry LibreTexts

1.5 Endothermic and Exothermic Reactions

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  • All chemical reactions (and physical changes and nuclear changes as well) involve energy changes.

    Exothermic reactions are reactions in which there is a net release of energy. When energy is released, an energy term will appear on the product side of the equation.


    The thermite reaction is a highly exothermic reaction between ferric oxide and aluminum, producing aluminum oxide and molten iron:

    \(\ce{Fe_2O_3(s) + 2Al(s) \rightarrow Al_2O_3(s) + 2Fe(l) + 847.6 kJ}\)

    If you had this reaction occurring in your hand (don't try this at home!!!) your hand would feel warmer because the reaction is releasing heat.
    Endothermic reactions are reactions that require a net input of energy. This is indicated by writing the energy term on thereactant side of the equation.
    Consider the reaction when sulfur trioxide decomposes into sulfur trioxide and oxygen. This reaction requires a net input of 198 kJ of energy in order to occur:
    \(\ce{2SO_3(g) + 198kJ \rightarrow 2SO_2(g) + O_2(g)}\)
    If you held this reaction in your hand, your hand would feel colder because the energy required for the reaction to occur would be taken from the surroundings, in this case your hand.
    [Note: This reaction actually requires more energy than 198 kJ to occur - we'll revisit this in the next module. ]

    Energy changes that occur during chemical reactions are very important to our society. Select one of the following research topics to investigate. Be sure to provide a bibliography, giving your sources of information. Present your findings in one of the following formats (or come up with your own presentation idea):

    • A formal essay
    • A web site
    • A podcast
    • A short play
    • A video
    • A commercial
    • A series of print advertisements


    Research Topic Ideas

    • What are some alternatives to fossil fuels as energy sources? List pros and cons of these alternatives. Optionally, select one alternative energy source such as hydrogen fuel cells and give a more in depth analysis of how it works and its costs and benefits.
    • Investigate the chemistry behind hot and cold packs. What chemical reactions are involved?

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