Take home points on atoms and molecules.
Scanning tunneling microscope allowed seeing atoms well. [1986 Nobel Prize: Binnig and Rohrer]
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.
|Oxygen molecule: what is it?|
|Chemist view of hydrogen and oxygen forming wather.||tr>|
|Propane combusting with oxygen to from carbon dioxide, water, heat and light|
How does burning a stable fuel yield energy?
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.
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.
Are NO and NO2 stable? No and Yes. They are quite reactive but neverless quite stable when isolated.
|CH4||+||2 O2||⇒||CO2||+||2 H2O|
|Rearranging heat on left hand side|
|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:
Ethylene C2H4: +52.3 KJ/mol, 0.0284 kg/mol
|C2H4||+||3 O2||⇒||2 CO2||+||2 H2O|
Even though ethylene is not bound, its greater weight per mole counter balances that to give about same energy.
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