Chapter 5: Chemical Energetics

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Last updated 11:43 AM on 8/7/26
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4 Terms

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Enthalpy VS Enthalpy Changes

Enthalpy:

  • It is the total chemical energy of a substance (heat content)

  • Chemical energy consists of:

    • Kinetic energy

    • Potential energy (main component of C.E.)

      • Main component due to electrostatic attraction between the particles

Enthalpy Change:

  • It is the energy change

    • During a chemical reaction, bonds are broken and new bonds are formed

    • Bond breaking → requires energy

    • Bond forming → releases energy

  • Symbol of enthalpy change: ΔH

  • Units of ΔH: J mol–1 or kJ mol–1

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Endothermic VS Exothermic

Endothermic:

  • ΔH → +ve (ΔH>0)

  • Products have higher energy content than the reactants

  • Absorbs heat from the surroundings

  • Temperature of the surroundings decreases

Exothermic:

  • ΔH → -ve (ΔH<0)

  • Products have lower energy content than the reactants

    • Products are more energetically stable than reactants (energy level of products is lower than the reactants)

  • Releases heat to the surroundings.

  • Temperature of the surroundings increase

*Don't say: “heat energy" → wrong!!

<p>Endothermic:</p><ul><li><p class="has-focus">ΔH → +ve (ΔH&gt;0)</p></li><li><p class="has-focus">Products have higher energy content than the reactants</p></li><li><p class="has-focus">Absorbs heat from the surroundings</p></li><li><p class="has-focus">Temperature of the surroundings decreases</p></li></ul><p class="has-focus"></p><p class="has-focus is-empty">Exothermic:</p><ul><li><p class="has-focus is-empty">ΔH → -ve (ΔH&lt;0)</p></li><li><p class="is-empty has-focus">Products have lower energy content than the reactants</p><ul><li><p class="is-empty has-focus">Products are more energetically stable than reactants (energy level of products is lower than the reactants)</p></li></ul></li></ul><ul><li><p>Releases heat to the surroundings.</p></li><li><p>Temperature of the surroundings increase</p></li></ul><p class="has-focus"></p><p class="has-focus">*Don't say: “heat <em>energy</em>" → wrong!!</p>
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Energy Profile Diagram

*Different from energy level diagram

Activation energy (Ea)

  • It is the minimum energy which the reacting particles must possess in order to overcome the energy barrier before becoming products

<p>*Different from energy level diagram</p><p></p><p class="has-focus">Activation energy (Ea) </p><ul><li><p class="has-focus">It is the minimum energy which the reacting particles must possess in order to overcome the energy barrier before becoming products</p></li></ul><p></p>
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Thermochemical Equation