Energy and Chemical Reactions Energetics

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Energy and Chemical ReactionsEnergetics

• Using Bond Enthalpies to calculate energy changes
  in a reaction

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Energy Changes

• Breaking chemical bonds requires energy
• Forming new chemical bonds releases energy

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Exothermic and Endothermic Processes

• Exothermic processes release energy
• C3H8 (g) 5 O2 (g) ? 3 CO2 (g) 4H2O
  (g) 2043 kJ
• Endothermic processes absorb energy
• C(s) H2O (g) 113 kJ ? CO(g) H2 (g)

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Exothermic reactions
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Examples of exothermic reactions

• Combustion
• Neutralization acid base ? salt water

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Endothermic Reactions
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Energy Changes in endothermic and exothermic
processes

• In an endothermic reaction there is more energy
  required to break bonds than is released when
  bonds are formed.
• The opposite is true in an exothermic reaction.

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Enthalpy

• Enthalpy is the heat absorbed or released during
  a chemical reaction where the only work done is
  the expansion of a gas at constant pressure
• DH

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Enthalpy Changes

• The change in enthalpy is designated by the
  symbol DH.
• If DH lt 0 the process is exothermic.
• If DH gt 0 the process is endothermic.

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Bond Enthalpies

• The energy to required to break a covalent bond
  in the gaseous phase is called a bond enthalpy.
• Bond enthalpy tables give the average energy to
  break a chemical bond. Actually there are slight
  variations depending on the environment in which
  the chemical bond is located

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Bond Enthalpy Table

• The average bond enthalpies for several types of
  chemical bonds are shown in the table below

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Bond Enthalpies

• By comparing the energies for breaking bonds in
  the reactants and forming bonds in the products
  one can determine the enthalpy change for a
  reaction

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Bond Enthalpies

• Energy is required to break chemical bonds.
  Therefore when a chemical bond is broken it
  carries a positive charge
• Energy is released when chemical bonds form.
  When chemical bonds are formed their energy
  change carries a negative charge

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Exothermic reaction more energy is released in
forming the bonds than in breaking the bonds.
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The endothermic reaction absorbs more energy
breaking bonds than it releases in bond formation.
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Bond Enthalpy Calculations

• Example 1 Calculate the enthalpy change for the
  reaction N2 3 H2 ? 2 NH3

• Draw the structural formulas
• List the number and types of each kind of bond
  broken and formed
• Bond breaking is endothermic, so add these
• Bond formation is exothermic, so subtract these
• Add the result to find the enthalpy of the
  overall reaction.

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• Bonds broken
• NN 945
• H-H 3(435) 1305
• Total 2250 kJ
• Bonds formed
• 2x3 6 N-H 6(390) - 2340 kJ
• Net enthalpy change
• 2250 - 2340 - 190 kJ

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Another problem

• Example 2 Calculate the enthalpy change for the
  reaction
• O2 2 H2 ? 2H2O

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Add bonds broken and subtract bonds formed

• Bonds broken (add)
• O O 502
• 2 H-H 2(142)
• Bonds formed (subtract)
• 4 H-O 4(464)