1. Bond Energy and Calculations
Learning Objectives
23 February 2019
Bond Energy and Calculations
Understand that bonds are broken and made in chemical reactions
Learn how to calculate the energy needed and released
Key Words –
Exothermic
Endothermic
Energy level diagram
What to remember?
ENERGY IS NEEDED TO BREAK A BOND
ENERGY IS NEEDED TO MAKE A BOND

2. Energy Level Diagram Exothermic reactions Energy is given out
The products have less energy than the reactants
Combustion and neutralisation are exothermic

3. Energy Level Diagrams
Exothermic reactions
energy

4. Energy Level Diagrams
Exothermic reactions
energy
course of reaction

5. Energy Level Diagrams Exothermic reactions energy reactants
course of reaction

6. Energy Level Diagrams Exothermic reactions energy reactants products
course of reaction

7. Energy Level Diagrams Exothermic reactions energy given out
∆H is negative
energy
reactants
products
course of reaction

8. Energy Level Diagrams Endothermic reactions Energy is taken in
The products have more energy than the reactants
The energy is taken in from the surroundings

9. Energy Level Diagrams
Endothermic reactions
energy

10. Energy Level Diagrams
Endothermic reactions
energy
course of reaction

11. Energy Level Diagrams Endothermic reactions energy reactants
course of reaction

12. Energy Level Diagrams Endothermic reactions energy products reactants
course of reaction

13. Energy Level Diagrams Endothermic reactions energy products
energy taken in
∆H is positive
reactants
course of reaction

14. Endothermic reactions
Summary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative
∆H is positive
Products have less energy than reactants
Products have more energy than reactants

15. ∆H How much energy is given out or taken in?
Energy is needed to break chemical bonds
Energy is given out when bonds are made
∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

16. Bond energies The energy needed to break a chemical bond
Different chemical bonds have different bond energies
Chemical bond
Bond energy, kJ/mole
H―H
436
O=O
496
O―H
463

17. Working out ∆H
Draw an energy level diagram with all the reactants and products on it
2 H-O-H
H―H
O=O +
energy
2H2 + O2
2H2O
course of reaction

18. Bond
Bond Energy (kJ mol-1)
H-H
+436
Cl-Cl
+242
C-H
+413
H-Cl
+431
O-H
+464
C-O
+358
H-Br
+366
Br-Br
C-C
+193
+ 348

19. Multiple Bonds Bond Energy (kJ mol-1)
C=C
+614
C C
+839
C = N
+615
C N
+891
S=O
+523
C = O
+799
C O
+1072
O=O
+496
N=N
+418
N N
+941
S = S

20. Working out ∆H Show the bond energies for all the bonds
∆H = ∑ ∆H(bonds broken) − ∑ ∆H(bonds formed)
436 +
O=O
energy
436 H O H O
course of reaction

21. Working out ∆H Show the bond energies for all the bonds 436 + 496
energy
436 H O H O
course of reaction

22. Working out ∆H Show the bond energies for all the bonds 436 + 496
energy
436
463 +
463 H O
course of reaction

23. Working out ∆H Show the bond energies for all the bonds 436 + 498
energy
436
463 +
463
463 +
463
course of reaction

24. Working out ∆H Add the reactants’ bond energies together energy 1368
463 +
463
463 +
463
course of reaction

25. Working out ∆H Add the products’ bond energies together energy 1368
1852
course of reaction

26. Working out ∆H ∆H = energy in ― energy out 1368 1852 ― energy 1368
course of reaction

27. Working out ∆H ∆H = energy in ― energy out 1368 1852 -484 ― energy
course of reaction

28. Working out ∆H ∆H = energy in ― energy out ∆H = -484 energy 1370 1852
course of reaction

29. Example 1 Hydrogen + Chlorine Hydrogen Chloride H2(g) + Cl2(g) 2HCl(g)
H-H Cl-Cl H-Cl + H-Cl

30. H H Cl Cl Intermediates H-H and Cl-Cl H-Cl and H-Cl Reactants Products
678
Intermediates
862
431
431
436
242
H-H and Cl-Cl
H-Cl and H-Cl
Reactants
Products
678 – 862 = – 184

31. Is this endothermic or exothermic?
The total energy (∆H) =
Energy to make bonds – Energy to break bonds
= 678 – 8/62
= – 184 kJ mol-1
The answer is NEGATIVE
Is this endothermic or exothermic?

32. Example 2 Hydrogen Bromide Hydrogen + Bromine 2HBr(g) H2(g) + Br2(g)
H-Br + H-Br H-H Br-Br

33. H H Br Br Intermediates H-Br and H-Br Reactants H-H and Br-Br Products
732
Intermediates
366
366
H-Br and H-Br
629
Reactants
193
436
732 – 629 = 103
H-H and Br-Br
Products

34. Is this endothermic or exothermic?
The total energy (∆H) =
Energy to make bonds – Energy to break bonds
= 732 – 629
= 103 kJ mol-1
The answer is POSITIVE
Is this endothermic or exothermic?

35. Working out ∆H Summary The energy values have units of kJ/mole
In the exam, you will be given the energy values and all the bonds to make or break
Energy goes in to break bonds
Energy goes out when bonds are made
∆H is energy in – energy out