Chemical Energetics

• Energetics of a Reaction: Chemical reactions involve a transfer of energy between the system (the chemical reaction) and its surroundings.

Energy Level Diagrams

• Energy level diagrams show the relative energies of the reactant and product.

• The energy change of a reaction is represented through the difference in height between the reactant and its product.

• Activation energy: the minimum energy required for the reaction to take place

Endothermic energy level diagram:

• Energy is gained by the system; higher activation energy required

Exothermic energy level diagram:

• Energy is lost by the system; lower activation energy required

Bond Energy

• This is the amount of energy required or released when a bond is formed or broken respectively. The unit measure of this energy is kJ/mol.

• Formula for energy change:

• If overall heat energy value is negative, reaction is exothermic

• If overall heat energy value is positive, reaction is endothermic

Production of Energy

• Fuel****: substance that can be used as a source of energy.

• Burning fuels to form oxides is an exothermic reaction.

• The heat from burning fuels is used in power plants to create steam from water and turn turbines.

• A combustion process requires the presence of a fuel, oxygen and heat.

A good fuel is:

1. Cheap

2. Available in large quantities

3. Liquid at room temperature

4. Have high efficiency (produce a large amount of energy)

5. Does not produce polluting gases

Hydrogen

• Produced by reacting methane gas with steam

• Used in fuel cells and rockets

Fuel Cell

• In this electrochemical cell, fuel loses electrons at one porous electrode while oxygen gains electrons at the alternate porous electrode.

• The product is water: 2H2 + O2 → 2H2 O

Reaction at anode:

2H2 → 4H+ + 4e-

Reaction at cathode:

4H+ + O2 + 4e- → 2H2 O

• The flow of the electrons, through the electric circuit, from the cathode to the anode generates a current.