# Energy from Moleules

## Atoms and Moleulce

Take home points on atoms and molecules.

1. Atoms and Molecules exist -- subnanometer scale.
2. Molecules are bound atoms.
3. Energy can be extracted from molecules.

Scanning tunneling microscope allowed seeing atoms well. [1986 Nobel Prize: Binnig and Rohrer]

## Energy from Combustion

How does burning a stable fuel yield energy?

### Three questions

1. What does stable mean?
2. In combustion: what vanishes; what appears?
3. Can generated energy be simply predicted?

Consider methane CH4 burning in oxygen O2.
Is this correct? CH4 + O2 ⇒ CO2 + H2O. No! Problem:
LHS: 4 H & 2 O; RHS: 2 H & 3 O.
CH4 + 2 O2 ⇒ CO2 + 2 H2O? Yes!

Balancing the equation: number of each atom species is conserved in reaction.

Note: every molecule in reaction is stable.First we need to define energy of a molecule.

## Binding energy of a molecule.

Definition: Molecular binding energy is
difference between energy of molecule and
its elements in their most stable state.

Chemists call this Enthalpy of Formation; the complete definition prescribes the measurement.

Clearly defined or not; measurements are hard. In 30 years numbers have changed 1/2 percent.

Methane combustion reaction:
CH4 + 2 O2 ⇒ CO2 + 2 H2O.

Since O2 is the most stable form of oxygen, its binding energy is zero.

Units: kJ/mol. What is a mol? It is standard number of molecules (about 6 1023). This unit eases balancing chemical equations; imagine using kJ/kg.

## Table for some simple molecules

CH4 Molecule BindingEnergy

Are NO and NO2 stable? No and Yes. They are quite reactive but neverless quite stable when isolated.

### Combustion Energy from Methane

CH4+2 O2 CO2+ 2 H2O
-75+0 = -394+2(-242)+"heat"
Rearranging heat on left hand side
heat = 394+2(242)-75
= 803 kJ/mol
CH4 (methane) has 0.016 kg/mol.
Therefore, heat = 50 MJ/kg.

Coal liberates about 27 MJ/kg (lignite, with 15 MJ/kg for anthracite). Only by "cleaning" coal can combustion products be mainly CO2 and H2O.

Methane is an excellent source of energy:

• lots of energy/weight;
• efficient combustion process;
• no pollution (in nitrogen-free combustion);
• relatively easy to transport.

## Can we get more energy?

Ethylene C2H4: +52.3 KJ/mol, 0.0284 kg/mol
C2H4 +3 O2 2 CO2+2 H2O
52+0 = 2(-394)+2(-242)+"heat"
heat = 2(394)+242+52
= 1324 kJ/mol
= 46.6 MJ/kg.

Even though ethylene is not bound, its greater weight per mole counter balances that to give about same energy.

Messages:

1. Heat generated by molecule burning into more stable end products.
2. Light atoms have the potential to be produce more energy.

Some words:

• Exothermic: gives off energy in reaction
• Endothermic: needs energy for reaction to occur