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

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.

This is a great 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:
    Getting Energy from Combustion
    <http://www.physics.ohio-state.edu/~wilkins.5/energy/Resources/Lectures/combenergy.html>
    [Wednesday, 13-Dec-2017 23:17:38 EST]
    Edited by: wilkins@mps.ohio-state.edu on Tuesday, 04-Oct-2011 09:20:36 EDT