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Bond enthalpy

Specification:

  • the term average bond enthalpy and the relation of bond enthalpy to the length and strength of a bond; bond-breaking as an endothermic process and bond-making is exothermic; the relation of these processes to the overall enthalpy change for a reaction

  • the terms: exothermic, endothermic, standard conditions, (standard) enthalpy change of reaction (∆r H), (standard) enthalpy change of combustion (∆cH), (standard) enthalpy change of formation (∆f H), (standard) enthalpy change of neutralisation (∆neutH)

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Key definitions:

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  • Bond enthalpy = is the total amount of energy needed to break a covalent bond. 

  • Enthalpy = heat energy per mole

  • Endothermic = Energy is absorbed from the surroundings. (bond breaking)

  • Exothermic = Energy is released to the surroundings. (bond making)

  • Standard conditions = 101 KPa and 298K at RTP

  • Enthalpy change of reaction = The enthalpy change when molar quantities react in a balanced equation under standard conditions.

  • Enthalpy change of combustion = Where one mole of substance is completely burnt in excess oxygen under standard conditions

  • Enthalpy change of formation = The change in enthalpy change during formation of one mole of a substance from its constituent element with all its substances in its standard state.

  • Enthalpy change of neutralisation = The enthalpy change when one mole of hydroxide ion reacts with one mole of hydrogen ions to form one mole of water under standard conditions in a solution of 1.0 mol/dm³

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Key points to note for bond enthalpy:

  • Bond breaking is endothermic because energy is taken in from the surroundings when a bond is broken.

  • Bond making is exothermic because energy is released to the surroundings when a bond is made.

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Relationship between bond length and bond strength:

  • Bond length and bond strength are directly proportional to each other.

  • An increase in bond length is due to the number of inner subshells present.

  • This is because the greater the bond length, the less energy it requires to break the bond.

  • This is because there is less attraction between the outer electrons and positive nucleus.

  • Hence requires less energy to break to break the bond.

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  • As you can see in the diagram, molecules like fluorine and chlorine have more charge density and have a smaller atomic radius than iodine.

  • This means that the outer electrons in fluorine and chlorine are more attracted to the nucleus = shorter bond length

  • Outer electrons in molecules like iodine are less attracted to the nucleus = longer bond length.

  • Greater the length = lower the bond strength

Calculating Bond enthalpy:

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Let’s have a look at an example:

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Calculate the bond enthalpy change of combustion of gaseous methanol from equation 5.2 and using the table of values provided.

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Steps to solving this problem:

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  1. If in the exam the molecular formula is given, draw out the structural formula so you can see all the bonds.

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2. Then you start by counting the number of bonds broken and made. Red represents the bonds broken and the green shows the total number of bonds made.

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3. Now you need to calculate the total energy needed for bond breaking and bond making processes.

 

  • Make sure to count all the bonds broken and made and their corresponding energies.

  • Treat both processes as individual calculations to avoid any errors like this:

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4. Now once we have calculated the individual values of the bond making and bond breaking processes, we need to apply this formula…

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Written by: Bansari Sanghvi

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