Covalent Bond Strength

1
Covalent Bond Strength

• Most simply, the strength of a bond is measured
  by determining how much energy is required to
  break the bond.
• This is the bond enthalpy.
• The bond enthalpy for a ClCl bond,
• D(ClCl), is measured to be 242 kJ/mol.

2
Average Bond Enthalpies

• This table lists the average bond enthalpies for
  many different types of bonds.
• Average bond enthalpies are positive, because
  bond breaking is an endothermic process.

3
Average Bond Enthalpies

• NOTE These are average bond enthalpies, not
  absolute bond enthalpies the CH bonds in
  methane, CH4, will be a bit different than the
• CH bond in chloroform, CHCl3.

4
Enthalpies of Reaction

• Yet another way to estimate ?H for a reaction is
  to compare the bond enthalpies of bonds broken to
  the bond enthalpies of the new bonds formed.

• In other words,
• ?Hrxn ?(bond enthalpies of bonds broken) ?
• ?(bond enthalpies of bonds formed)

5
Enthalpies of Reaction

• CH4(g) Cl2(g) ???
• CH3Cl(g) HCl(g)
• In this example, one
• CH bond and one
• ClCl bond are broken one CCl and one HCl bond
  are formed.

6
Enthalpies of Reaction

• So,
• ?Hrxn D(CH) D(ClCl) ? D(CCl) D(HCl)
• (413 kJ) (242 kJ) ? (328 kJ) (431 kJ)
• (655 kJ) ? (759 kJ)
• ?104 kJ

7
Bond Enthalpy and Bond Length

• We can also measure an average bond length for
  different bond types.
• As the number of bonds between two atoms
  increases, the bond length decreases.

8
Work

• When a process occurs in an open container,
  commonly the only work done is a change in volume
  of a gas pushing on the surroundings (or being
  pushed on by the surroundings).

9
Work

• We can measure the work done by the gas if the
  reaction is done in a vessel that has been fitted
  with a piston.
• w -P?V

10
Enthalpy

• If a process takes place at constant pressure (as
  the majority of processes we study do) and the
  only work done is this pressure-volume work, we
  can account for heat flow during the process by
  measuring the enthalpy of the system.
• Enthalpy is the internal energy plus the product
  of pressure and volume

H E PV
11
Enthalpy

• When the system changes at constant pressure, the
  change in enthalpy, ?H, is
• ?H ?(E PV)
• This can be written
• ?H ?E P?V

12
Enthalpy

• Since ?E q w and w -P?V, we can substitute
  these into the enthalpy expression
• ?H ?E P?V
• ?H (qw) - w
• ?H q
• So, at constant pressure the change in enthalpy
  is the heat gained or lost.