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The Energy of a Chemical Reaction

The Energy of a Chemical Reaction

Author: Michael Correia

HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.

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What is Enthalpy?

Bond Energies

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Bond Energy Table and further explanation.

Step by Step

Look at example 1.2 in the link above.

Try It

Use the equation and Lewis structures from 0:40 of Professor Dave's video and the bond energies from Table 1 in the link above to determine the change in enthalpy (capital deltaH) of the reaction.

Concept to Know

Did you use Dave's suggestion (capital deltaH = products - reactants) and get a positive number? A positive capital deltaH indicates an endothermic reaction, which absorbs heat from the surroundings. However, we know that combustion reactions give off heat and are therefore exothermic. What's wrong with Dave's explanation?

Dave has made the common mistake of interpreting the bond energies as energy stored within the bonds. In fact, the bond energies represent the energy required to break a bond (when it is a reactant) and the energy released by the formation of that bond (when it is a product). 

Energy required to break bonds is an endothermic process (+capital deltaH). Energy released to break bonds is an exothermic process (-capital deltaH).

So the correct formula for determining enthalpy of a reaction is:


capital deltaH = reactants - products

Would you like to know more?


Big Idea


Develop a storyboard that shows how the combustion of methane absorbs and releases energy. Use bond energies to explain how the reaction is exothermic so that we can help Professor Dave understand his mistake.