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Understanding Exothermic and Endothermic Reactions Through Bond Energies

In chemical reactions, reactant bonds break, and new bonds form in products, with energy changes indicating reaction types. Exothermic reactions, like methane combustion, release energy. Examining bond energies helps us deduce where this energy comes from—bond breaking or forming. The energy measurement includes a unit (Joules), quantity, and sign indicating direction. For instance, breaking the bonds in hydrogen fluoride (HF) releases energy, resulting in a negative enthalpy change. Understanding these concepts is crucial in energy chemistry.

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Understanding Exothermic and Endothermic Reactions Through Bond Energies

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  1. Page 34 • In a chemical reaction, bonds in the reactants are broken and the atoms rearrange to form new bonds in the products. Flames or heat associated with the combustion reaction indicate that the reaction is exothermic. • Where does the energy in an exothermic reaction come from? (Do you think it comes from bond breaking or bond forming? What evidence do you have for your answer?)

  2. Bond Breaking and Bond Making

  3. Heat Potential energy

  4. Average Bond Energies(per mole of bonds)

  5. Direction • Every energy measurement has three parts. • A unit ( Joules or kiloJoules). • A number how many. • and a sign to tell direction. • negative - exothermic • positive- endothermic

  6. One more time..... • Consider the combustion of methane: • CH4 + 2 O2 → CO2 + 2 H2O (all gases)

  7. In this example, the bonds to break are: • 4(C-H) and 2 (O=O) • The bonds to make are 2(C=O) and 4(H-O)

  8. That translates into 4(414) + 2(498.7) = +2653.4 kJ And 2(745) + 4(460) = -3330 kJ So DH = -676.6 kJ/mol

  9. H2 + F2 2HF How much energy is associated with this reaction? Does it require energy or release energy? Is it exothermic or endothermic?

  10. So: • to break H-H bond requires 436.4 kJ/mol • to break F-F bond requires 156.9 kJ/mol • to make 2 H-F bonds releases 2(568.2 kJ/mol)

  11. But which energy is going in and which is coming out? • The first two steps are endothermic; energy is required; we give them + signs. The last step is exothermic; energy is released; we give it a - sign. • To get the overall change in enthalpy, DH, we add the values to get -543.1kJ/mol. • The actual value as measured experimentally is -543.1 kJ/mol!!! (per mol of what?)

  12. Try this problem Heat Potential energy

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