Energy & Chemical Reactions

Energy is needed to break bonds apart.Energy is released when bonds are formed.

So how do exothermic reactions give out energy to the surroundings?

In any chemical reaction, the SAME NUMBER of bonds are broken as are made.

Exothermic Reactions

These are drawn as graphs.

The x axis shows time, the y axis shows energy.

Energy

Time

Energy Level Diagrams

They are used to show exothermic and endothermic reactions.

Exothermic ReactionsAn exothermic energy diagram looks like this.

Energy

Time

reactants

products

Energy released}

Exothermic ReactionsCombustion and respiration are exothermic reactions. e.g. burning methane (natural gas).

CH4 + 2O2 2H2O + CO2

Energy

Time

2H2O + CO2

Energy released}CH4 + 2O2

Bond EnergyIf the SAME NUMBER of bonds are made and broken,

how can energy be given out in an exothermic reaction?

The answer is that different chemical bonds have different amounts of energy in them.

Carbon is joined to a hydrogen by a strong bond.

412 kJ of energy is needed to break this bond,

and to separate the atoms

The same amount of energy is given out when the bond is formed.

H O has a bond energy of 463 kJ

C==O has a bond energy of 743 kJ

CH4 + 2O2 2H2O + CO2

H-O bond = 463

4 of these = 1852

C=O bond = 743

2 of these = 1486

Total energy = 3338 kJ

released

C-H bond = 412

4 of these = 1648

O=O bond = 496

2 of these = 992

Total energy 2640

kJ required

Calculating Bond Energies

More energy is released by bond making than is required for bond breaking, so 698 kJ of energy is given out to the surroundings in this reaction, so this is an exothermic reaction.

Activation EnergyMost exothermic reactions need a bit of energy to get them started, like lighting methane with a match.

This energy is known as ACTIVATION ENERGY. This energy breaks the bonds and allows atoms to separate.

This changes the shape of the energy diagram to this.

Energy

Time

reactants

products

} Activation energy

Energy

Time

CH4 + 2O2

2H2O + CO2

Separate atoms (with most energy): 1 C, 4 H, and 4 O

Why is activation energy needed ?

Energy is needed to break bonds so atoms can become separate. The separate atoms can then react and rejoin into

different compounds. If the new compounds require less energy to form, the reaction is exothermic.

Endothermic ReactionsExamples are photosynthesis and most electrolysis reactions.

The energy level diagrams look like this.

Energy

Time

reactants

products

} Energy needed to make the reaction work.

Endothermic ReactionsIn these reactions, the energy required for bond breaking IS GREATER than the energy released

by bond making.

This means that to make endothermic reactions take place, extra energy is needed from an

external source.

Questions

(a) What energy source is needed for these reactions to occur:

(i) Photosynthesis? (ii) electrolysis?

(b) Which type of reaction are these examples of?

(c) What can you say about the number of bonds broken and made during a chemical reaction?

(d) Sketch energy level diagrams to show exothermic and endothermic reactions.

(e) What is an exothermic reaction?

(f) Why are some reactions exothermic?

(g) What is Activation Energy, and why is it needed?

(h) Add this to the correct energy diagram sketch.