1. IB SL Chemistry Mrs. Page 2015-2016
5.3 Bond Enthalpy
IB SL Chemistry
Mrs. Page

2. Understandings:
Bond forming releases energy and bond breaking requires energy.
Average bond enthalpy is the energy needed to break 1 mol of a bond in a gaseous molecule averaged over similar compounds.

3. Applications and Skills
Calculation of the enthalpy changes from known bond enthalpy values and comparison of these with experimentally measured values.
Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.
Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.

4. Bond Enthalpy
The energy required to break 1 mole of bonds in a gaseous covalent molecule under standard conditions.
Bond breaking is endothermic (takes in energy from surroundings) (+ ∆H)
Also known as bond dissociation enthalpy
Data booklet Table 11
Average values from experimental values so only approximations

5. Bonds & Energy
You have to be able to draw the structural chemical formula to see all the bonds and add them up properly.
Bond length influences bond enthalpies as the longer the bond (larger atomic radii) the weaker the bond (remember the radii increases down a group and decreases across a period)
Bond strength is associated with how much energy it takes to break the bond

6. Bond Enthalpy (Bond Energy) Examples
If H-H is bonded, its bond enthalpy is 436 kJmol-1.
It is represented by H2(g)  2H(g)
1 mole of H2 molecules is broken apart to give 2 moles of gaseous hydrogen atoms.
Example 2: The H-Cl bond enthalpy is 431kJmol-1, which is represented by :
HCl(g)  H (g) + Cl(g) ΔHθ = +431 kJ mol-1

7. Bond Enthalpies can only be calculated for substances in the gaseous state.
Example: If we can consider Br2(l) 2Br(g), the ΔHθ = +224kJ mol-1. This does not represent the bond enthalpy of the Br-Br bond, as the reaction can be broken down into two process.

8. Br2(l) 2Br(g), ΔHθ = +224kJ mol-1 Continued…

9. Calculating Overall Bond Enthalpy Change for a Reaction
Breaking bonds requires energy (Endothermic). This is because energy is needed to break apart the bonds
Making bonds releases energy (Exothermic) because the potential energy of the bonds apart becomes kinetic energy as they come together, and they release heat (think of an object falling to Earth)
Reactants – products this time!

10. Overall Enthalpy Change for a Reaction
ΔHrxn = ΣBE(bonds broken) – ΣBE(bonds made)

11. Example Problem
Consider the reaction between ethene and bromine to produce 1,2-dibromoethane:
C2H4(g) + Br2(g)  C2H4Br2(g)
What is the enthalpy change for this reaction? If the species are drawn as structural formulas, then all the bonds can be seen clearly.

12. Example problem Continued…
Now, we imagine the reaction happening with all the bonds in the reactant being broken.
**Note: This is NOT how the reaction occurs, it is just a way of allowing us to work out the enthalpy change.

13. Example problem Continued…
Next, new bonds form to make the products.

14. Example problem Continued…
The bond energies can then be added up to work out the enthalpy change:
Bond Broken
Bond Energy/ kJmol-1
Number of Bonds
Total energy/ kJmol-1
C-H
412 4
1648
C=C
612 1
Br-Br
193
Total energy to break all bonds
2453
The total enthalpy change when all the bonds are broken is kJmol-1. This is positive, as breaking bonds is an endothermic process.

15. Example problem Continued…
Enthalpy released when the bonds are made:
Bond Broken
Bond Energy/ kJmol-1
Number of Bonds
Total energy/ kJmol-1
C-H
412 4
1648
C-C
348 1
C-Br
276 2
552
Total energy to break all bonds
2548
The total enthalpy change when all the bonds are made is kJmol-1. This is negative, as making bonds is an exothermic process.

16. Example problem Continued…
ΔHrxn = ΣBE(bonds broken) – ΣBE(bonds made)
ΔHθ = [(4 x 412) + (1 x 612) + (1 x 193) – [(4 x 412) + (1 x 348) + (2 x 276)] = -95kJmol-1

17. Bond Enthalpies and Enthalpies of Combustion
The complete combustion of propane can be represented by the equation:
CH3CH2CH3(g) + 5O2((g)  3CO2(g) + 4H2O(g)
Calculate the bond enthalpy from Table 11 of your Data Book

18. Bond Enthalpies and Enthalpies of Combustion
Bonds Broken
8 C-H = 8(414)
5 O=O = 5(498)
2 C-C = 2(346)
Total = +6494
Bonds Made
6 C=O = 6(804)
8 O-H = 8(463)
Total = -8528
Bond Enthalpy = kJmol-1

19. Bond Enthalpies and Enthalpies of Combustion
Bond Enthalpy = -2726
∆Hcө= -5470
Compare this value to the experimentally determined value for the enthalpy change of combustion for octane (Data Book Table 13)
∆Hcө= liquid state and Bond Enthalpy gaseous

20. You Try Calculate the bond enthalpy for the following reactions:
CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g)
4NH3(g)+ 3O2(g) 2N2(g)+ 6H2O(g)
[4(414) + 242] – [3(414) ]
= -99 kJmol-1
[12(391)+3(498)] – [2(945) + 12(463)]
= kJmol-1

21. CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g)
This reaction is overall exothermic because the final product is lower in energy than the reactants.

22. OZONE, O3 Ozone plays an important role in our atmosphere.
There are two types of ozone,
One that is critical for life on Earth found in Earth’s stratosphere. This ozone protects us from solar radiation by absorbing UV rays. Loss of this ozone is frequently referred to as a “hole in the ozone layer.”
The other type of ozone “bad ozone” is found closer to Earth’s surface in the troposphere. This type of ozone is created due to the burning of fossil fuels and act as a pollutant causing damage to living tissues.

23. OZONE PRODUCTION
Statospheric (“good”) ozone is created in the atmosphere.
Formation requires two steps:
UV radiation to splits an O2 molecule into 2 O atoms
The two oxygen atoms then combine with an oxygen molecule (O2) to form ozone

24. OZONE DEPLETION
Stratospheric ozone is also destroyed in the atmosphere in a process called photolysis (breakdown by light).
Ozone in the stratosphere protects life on Earth from UV radiation.
UV radiation is absorbed by ozone, breaking it down into O2(g) and an oxygen atom
Later we will learn how human interactions increase the rate at which ozone in depleted, creating holes in the ozone layer.

25. OZONE PHOTOLYSIS
The destruction of ozone is an endothermic reaction that requires energy (UV light) to occur
Reactant more stable than products
Potential energy

26. Extra Practice
Try this site to practice at home