1. Bond energies
The amount of energy needed to break or make a bond is
called the bond energy.
1 of 35 © Boardworks Ltd 2007

2. Bond energies
The amount of energy needed to break or make a bond is
called the bond energy.
Different chemical bonds have different bond energies.
For example:
2 of 35 © Boardworks Ltd 2007

3. Bond energies
The amount of energy needed to break or make a bond is
called the bond energy.
Different chemical bonds have different bond energies.
For example:
Bond
Bond
energy (kJ)
H–H 432
Cl – Cl 240
H – Cl 428
3 of 35 © Boardworks Ltd 2007

4. Bond energies
The amount of energy needed to break or make a bond is
called the bond energy.
Different chemical bonds have different bond energies.
For example:
Bond
Bond
energy (kJ)
H–H 432
Cl – Cl 240
H – Cl 428
The energy changes in a reaction can be calculated from
the bond energies of the reactants and the products.
4 of 35 © Boardworks Ltd 2007

5. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
5 of 35 © Boardworks Ltd 2007

6. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
6 of 35 © Boardworks Ltd 2007

7. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking
7 of 35 © Boardworks Ltd 2007

8. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking
= H – H + Cl – Cl
= 432 kJ + 240 kJ
= 672 kJ
8 of 35 © Boardworks Ltd 2007

9. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking energy from bond-making
= H – H + Cl – Cl
= 432 kJ + 240 kJ
= 672 kJ
9 of 35 © Boardworks Ltd 2007

10. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking energy from bond-making
= H – H + Cl – Cl = H – Cl + H – Cl
= 432 kJ + 240 kJ = 428 kJ + 428 kJ
= 672 kJ = 856 kJ
10 of 35 © Boardworks Ltd 2007

11. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking energy from bond-making
= H – H + Cl – Cl = H – Cl + H – Cl
= 432 kJ + 240 kJ = 428 kJ + 428 kJ
= 672 kJ = 856 kJ
total energy change = energy out – energy in
11 of 35 © Boardworks Ltd 2007

12. Calculating bond energies
What are the energy changes in the reaction between
hydrogen and chlorine?
hydrogen + chlorine  hydrogen chloride
H2 + Cl2  2HCl
energy for bond-breaking energy from bond-making
= H – H + Cl – Cl = H – Cl + H – Cl
= 432 kJ + 240 kJ = 428 kJ + 428 kJ
= 672 kJ = 856 kJ
total energy change = energy out – energy in
= 856 kJ – 672 kJ
= 184 kJ
12 of 35 © Boardworks Ltd 2007