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BOND ENERGY, BOND ORDER, BOND LENGTH AND ENTHALPY OF REACTION

Morris Louis�Beta Kappa, 1961

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BOND ENERGY, BOND ORDER, BOND LENGTH AND ENTHALPY OF REACTION

Morris Louis�Beta Kappa, 1961

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BOND. COVALENT BOND.

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BOND BREAKING = ENDOTHERMIC

  • Energy is always required to break a bond. Bond breaking is always endothermic (+ΔH; ΔH means “change in enthalpy”).

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BOND MAKING = EXOTHERMIC

  • Energy is always released when a bond is formed. Bond formation is always exothermic (-ΔH).

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BOND ENERGY (ALSO CALLED BOND ENTHALPY)

  • The energy required to break a bond is called bond energy.

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Example: the C—H bond in a compound has a bond energy of 413 kJ/mol

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BOND ENERGY TABLE

You can find tables of bond energies like this one in your book or online. Remember, bond energies are the energies required to break the bond (endothermic).

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ENERGY DIAGRAMS

Activation Energy: the energy required to break the bonds present in the reactants so that they can react to form products.

ΔHrxn, Enthalpy of Reaction: the overall energy change of the reaction

Energy released when new bonds are formed.

This is an exothermic reaction because overall, the reaction releases energy.

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ENERGY DIAGRAMS

Activation Energy: the energy required to break the bonds present in the reactants so that they can react to form products.

ΔHrxn, Enthalpy of Reaction: the overall energy change of the reaction

Energy released when new bonds are formed.

This is an endothermic reaction because overall, the reaction absorbs energy.

Transition State

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Bond Order and Bond Energy

Bond order is the number of bonds between two atoms.

  • single bond = bond order of 1
  • double bond = bond order of 2
  • triple bond = bond order of 3

How does bond order relate to the strength (bond energy) of a bond?

Bond energy is directly related to bond order!

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Resonance and Bond Order

BO = number of bonds / number of bonding groups sharing bonds

BO = 4 bonds / 3 bonding groups = 4/3 = 1.33

BO = 4 bonds / 2 bonding groups = 4/2 = 2

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Resonance and Bond Order

What is the bond order of the bonds in a O3 (ozone) molecule?

(Remember: BO = number of bonds / number of bonding groups sharing bonds)

BO = 3 bonds / 2 bonding groups = 3/2 = 1.5

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Resonance and Bond Order

Given the bond energies in the table to the right, which of the following would you expect to be the bond energy between the carbon atom and one of the oxygen atoms in CO32-?

  1. 167 kJ/mol
  2. 455 kJ/mol
  3. 795 kJ/mol
  4. 1252 kJ/mol

Bond

Bond Energy (kJ/mol)

C-O

360

C=O

611

C≡O

1072

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Bond Length and Bond Order

The higher the bond order, the shorter the bond length. This in an inverse relationship.