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# The Energy of a Chemical Reaction

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Author: Michael Correia
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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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Tutorial

## Bond Energies

Bond Energy Table and further explanation.

Look at example 1.2 in the link above.

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 (H) of the reaction.

Did you use Dave's suggestion (H = products - reactants) and get a positive number? A positive H 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 (+H). Energy released to break bonds is an exothermic process (-H).

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

H = reactants - products

## Assignment

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.

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