Integrated SequencePhysics Chemistry Organic Biology

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Virtual Textbook of Organic Chemistry - Combustion

Wikipedia - Combustion
Characterization of the combustion reaction in a variety of contexts.

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Special points of emphasis

Work, Energy, and Power


Periodic Properties

Periodic Properties

The Chemical Bond


Chemical Thermodynamics and the Equilibrium State

Reactions of Alkanes


In analyzing chemical change thermodynamically, try to get as clear a picture as you can of the system (the matter involved) before and after the reaction. A very important reaction of alkanes is combustion, in which a system comprised of carbon, hydrogen (the alkane) and oxygen (in the air) will undergo a transformation through which old bonds are broken and new bonds are formed. Initially carbon atoms are bound to other carbons and to hydrogen atoms (alkane molecules) and oxygen atoms are bound to other oxygen atoms (in oxygen molecules). In the final state, following combustion, however, carbon atoms will be bound to oxygen atoms (carbon dioxide) and hydrogen atoms will be bound to oxygen atoms (in water). It is a valid conceptual tool in thermodynamics to imagine an imaginary pathway from the initial to the final state. Changes in functions of thermodynamic state (internal energy, enthalpy, free energy) are path independent. Try to imagine pulling apart the carbon and hydrogen atoms of the alkane molecules and the oxygen atoms of oxygen molecules (breaking all the bonds) and letting those then fall together to form carbon dioxide and water. Pulling the atoms apart (breaking bonds) involves the input of energy (bond dissociation energy), because the bonding electrons represent a negative charge density attracting and holding the positive nuclei of bonded atoms together. As the atoms are separated, the system rises from the potential energy well whose depth is the strength of the bonds. Now our imaginary system is comprised of independent, nonbonded atoms.

Now let's take the next imaginary step, allowing the atoms to form the molecules of the reaction product. They are falling together into new potential energy wells, the carbon-oxygen and hydrogen-oxygen bonds of the product. These new bonds are stronger bonds, though, than the bonds of the reagent, so the system has undergone a net internal energy decrease.

Why are carbon-oxygen and hydrogen-oxygen bonds (CO2 and H2O) stronger than carbon-carbon, carbon-hydrogen, and oxygen-oxygen (CnHm and O2)? As a general rule, the greater the electronegativity difference between bonded atoms, the stronger the bonds. This is a really useful thing to know, so I’ll say it again. Generally the greater the electronegativity difference between bonded atoms, the stronger the bonds. In combustion, relatively weak, low-electronegativity-difference bonds (carbon-carbon, carbon-hydrogen, and oxygen-oxygen) are being superseded by stronger, high-electronegativity-difference bonds (carbon-oxygen and hydrogen-oxygen).

Why does electronegativity difference contribute to bond strength? Because as the atoms form the bond, not only is there the potential energy decrease of a normal covalent bond, the electron glue holding the nuclei together, but there is an additional decrease in electrostatic potential energy when the electronegative atom (oxygen) pulls the bonding electrons inward towards itself, an extra depth to the energy well. This concept is very important. It is the underlying coherence of the topic of oxidation-reduction, which systematizes these electronegativity-energy relationships. In oxidation-reduction terms, for combustion, we say that oxygen is reduced as it gains electron control at the expense of carbon.

Reactions of Alkanes


Bioenergetics and Cellular Respiration

The overall stoichiometry in oxidative metabolism is the same as combustion. Oxidative metabolism renders a significant portion of the free energy change involved, as oxygen gains electron control from carbon, into a form which can power life processes, ATP. The stoichiometry of the oxidative metabolism of a nutrient molecule is the same as if it were combusted by burning; the molecule ultimately reacts with oxygen gas to form carbon dioxide and water, but unlike burning in air, metabolism occurs through enzymatic processes that allow the changes to occur in coupled equilibrium with the phosphorylation of ADP, both direct, substrate level phosphorylation, and indirect, through the chemiosmosis of electrons from NADH and FADH2 in the mitochondria.

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