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]
Current flow depends on tip-atom distance. Picture contrast is measure of current flow.
7 x 7 Si
reconstruction

stm diagram

Forming a molecule on a surface

Fe = iron atom on silver surface (Ag);
CO - for carbon-monoxide on Ag.
(A) Tip induces detachment of CO and bonding to tip;
(B) tip with CO move toward Fd atom on surface
(C) tip reverses process, detaching CO to surface; (D) single Fe-CO bond formed.
(A) Five Fe atoms and five CO molecules on Ag surface.
(B) formation of Fe-CO shown in whole figure above.
(C) Another Fe-CO formed.
(D) Second CO moved to Fe-CO to form Fe(CO)2.
Molecule. etc.
Oxygen molecule: what is it?
Carbon dioxide
Nitrogen dioxide
ethane Ethane
Chemist view of hydrogen and oxygen forming wather.
Methane
Ethane
Propane
Butane
Propane combusting with oxygen to from carbon dioxide, water, heat and light

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.
How about?
   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

Binding Energy in kJ/mol
Molecule CH4 C2H6 C3H8 COCO2 NONO2 H2O
Binding
Energy
-75-85-104 -111-394 +91+34 -242

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:

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

    What about other "alkanes"?

    Methane: -74.8 kJ/mol; 0.0160 kg/mol
    CH4 + 2 O2 ⇒ CO2 + 2 H2O
    [-74.8 -(-393.5) -2(-241.8)]/0.0160
    = 50.1 MJ/kg
    Ethane: -84.7 kJ/mol; 0.0301 kg/mol
    C2H6 + 3.5 O2 ⇒ 2 CO2 + 3 H2O
    ½ [-84.7 -2(-393.5) -3(-241.8)]/0.0301
    = 23.7 MJ/kg
    Propane: -103.9 kJ/mol, 0.0302 kg/mol
    C3H8 + 5 O2 ⇒ 3 CO2 + 4 H 2O
    (1/3)[-103.9 -3(-393.5) -4(-241.8)]/0.0441
    = 15.4 MJ/kg
    Butane: -124.7 kJ/mol; 0.0581 kg/mol
    C4H10 + 6.5 O2 ⇒ 4 CO2 + 5 H2O
    ¼[-124.7 -4(-393.5) -5(-241.8)]/0.0581
    = 11.4 MJ/kg


    To cite this page:
    Energy From Molecules
    <http://www.physics.ohio-state.edu/~wilkins.5/energy/Resources/Lectures/energyfrommolecules.html>
    [Wednesday, 13-Dec-2017 23:12:13 EST]
    Edited by: wilkins@mps.ohio-state.edu on Tuesday, 25-Sep-2012 14:29:31 EDT