Chemical compounds have an internal energy obtained when they were originally formed.

To get them to react further may require more energy for the new reaction to take place.

Bond energy is the amount of energy required to break the bonds in one mole of a particular covalent bond of a substance in the gaseous state.

Bond energy - breaking and making bonds 1

Before the reaction, the reactants have energy, but are stable & do not react.

Bonds in the reactants need to be broken & energy – called activation energy, is required for this.

As new bonds form during the reaction, energy is released.

Sometimes the energy released is greater than the activation energy & we call the reaction an exothermic reaction.

If less – it is called an endothermic reaction.Heating CuSO4

2

A + B C + D

A + B

C + DReactants

Products

Energ

y

Course of reaction

Exothermic reaction – net energy liberated

1 2

3

1 = activation energy

2 = energy liberated

3 = ∆H = heat of reaction

Exothermic reactions

3

A + B C + D

A + B

C + D

Reactants

Products

Energ

y

Course of reaction - time

Endothermic reaction – net energy absorbed

1 2

3

1 = activation energy

2 = energy liberated

3 = ∆H = heat of reaction

Activation energy in reactions

Endothermic & exothermic reactions

4

-

+

0

Ep Forces of attraction.

Forces of repulsion.

complete changeE

D

Forming the H2 molecule 5

Forming the H2 molecule

6

Cl Cl

Cl Cl

Bond

Cl Cl

Cl Cl

The Cl2 molecule is stable & requires energy to break the bond.

This is a endothermic process. Energy supplied is called the bond energy.Making new bonds is an exothermic process as energy is liberated.

Bond energy liberated measures strength of bond.

7

The bond strength is an indication of how much energy is required to break a bond or how much is released when the new bond is formed.

This table gives an idea of relative sizes of bond energies (measured in kJ.mol-1)H-H 436 H-C 414 C-C 347 O=O

498Methane is a gas that burns in O2 to form CO2, H2O & liberates energy.

Energy must be supplied (match- activation energy) to start the reaction.

Establish if this is endothermic or exothermic.8

CH4 + 2O2 CO2 + 2H2O + energy

Breaking Energy input Making Energy output bonds kJ.mol-1 bonds kJ.mol-1

4 C-H 4 X 414 4 X O-H 4 X 464 bonds = 1656 bonds = - 1856

2 O=O 2 X 498 2 X C=O 2 X 803 bonds = 996 bonds = - 1606 Totals = 2652 = - 3462

As 1 mole of CH4 burns, there is thus a net decrease in energy of -810 kJ.mol-1 & ∴ it is an exothermic reaction with a net loss of energy.

Net energy change Bond energies

9

CHH HH H

O OO O

C

H

H H

H

O O

O O+

Breaking bonds – activation energy

Reactants

CO OH

HO

HH

O

More energy released – bond energy - exothermic

Products

CH4 + 2O2 CO2 + 2H2O + energy

Separate atoms

Burning methane 10

No. of bonds Bond order

Bond strength (kJ∙mol-1)

Bond length (nm)

Single (C-C) 1 348 0,154

Double (C=C) 2 612 0,134

Triple (CΞC) 3 837 0,120

In general, the longer the bond, the less energy required to break the bond.

Bond length & bond energies

11

1. Bond energy

2. Bond length

3. Size of bonded atoms

4. Number of bonds (bond order)

Energy to break bonds 12