Bond Energies
<ul><li>Breaking a bond uses energy – an endothermic process </li></ul><ul><li>Making a bond releases energy – en exotherm...
<ul><li>For diatomic molecules this is straightforward. The bond dissociation energy of hydrogen for example will always b...
<ul><li>The average (or mean) bond dissociation enthalpy of all 4 bonds is then used for the C-H bond. </li></ul><ul><li>T...
<ul><li>Calculate the enthalpy change for the following reaction </li></ul><ul><li>C 2 H 4(g)  + H 2 O    CH 3 CH 2 OH (g...
<ul><li>Start by drawing out the compounds. Now decide which bonds need to be broken and which new bonds will be made. In ...
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5 Bond Energies

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5 Bond Energies

  1. 1. Bond Energies
  2. 2. <ul><li>Breaking a bond uses energy – an endothermic process </li></ul><ul><li>Making a bond releases energy – en exothermic process </li></ul><ul><li>The enthalpy change of a reaction is the overall difference in energy between the energy used in breaking the bonds in the reactants and the energy released in forming new bonds in the products. </li></ul><ul><li>Standard enthalpy of bond dissociation is the energy absorbed when one mole of bonds is broken </li></ul>
  3. 3. <ul><li>For diatomic molecules this is straightforward. The bond dissociation energy of hydrogen for example will always be the same. </li></ul><ul><li>However for polyatomic molecules this is not the case. </li></ul><ul><li>Take CH 4 for example. </li></ul><ul><li>Once the first bond to be broken the remaining bonds are in a slightly different environment and the enthalpy change for removing the second hydrogen will not be identical to that for breaking the first bond. Similarly for the remaining two hydrogens. </li></ul>H H C H H
  4. 4. <ul><li>The average (or mean) bond dissociation enthalpy of all 4 bonds is then used for the C-H bond. </li></ul><ul><li>This is the enthalpy change given in data tables. There will be some error incurred using these as the mean bond energy for ethane for example will be slightly different to that of methane, but the error is small and not important for our calculations. </li></ul><ul><li>When using bond energies to find enthalpy changes the compounds must always be in the gaseous state. </li></ul>
  5. 5. <ul><li>Calculate the enthalpy change for the following reaction </li></ul><ul><li>C 2 H 4(g) + H 2 O  CH 3 CH 2 OH (g) </li></ul><ul><li>Given the following average bond dissociation energies </li></ul><ul><li>Bond KJ/Mol </li></ul><ul><li>C-C 347 </li></ul><ul><li>C=C 612 </li></ul><ul><li>C-H 413 </li></ul><ul><li>O-H 464 </li></ul><ul><li>C-O 358 </li></ul>
  6. 6. <ul><li>Start by drawing out the compounds. Now decide which bonds need to be broken and which new bonds will be made. In this example the following bonds are broken/made. </li></ul><ul><li>Bonds broken KJ/Mol Bonds Made KJ/Mol </li></ul><ul><li>1 x C=C 612 1 x C-H 413 </li></ul><ul><li>1 x O-H 464 1 x C-O 358 </li></ul><ul><li>1 x C-C 347 </li></ul><ul><li>total broken +1076 total made -1118 </li></ul><ul><li>Difference = +1076 – 1118 = -42 KJ/Mol </li></ul>H H H  H + H H O H C H C H H H H H O C C
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