We know that ozone is photolyzed by light with a wavelength of 330 nm or less to make oxygen molecule and oxygen atom. Nitrogen oxides also produce oxygen atom upon photolysis.
In the stratosphere, oxygen atom most often reacts with molecular oxygen to regenerate ozone. The troposphere, unlike the stratosphere, contains considerable concentrations of water vapor and water reacts faster with O than does O2.
The product, OH, is a very reactive molecule. In the daytime when photolysis is possible, it is always present in a very small but constant concentration in air. Draw the Lewis structure. Does this tell you why it is so reactive?
Oxidation of Hydrocarbon
Hydrocarbons are released continually from living things or through the decomposition of living things on Earth. Only methane has a high enough concentration to be listed in the chart of molecules in the atmosphere. Why is that?
All hydrocarbons react in air to form carbon monoxide and then carbon dioxide through a series of reactions. The first step is always the reaction between the hydrocarbon and hydroxyl radical. With alkanes, the hydroxyl radical abstracts a hydrogen atom and forms a carbon-centered radical.
Because an O-H bond is stronger than a C-H, this step is exothermic.
With alkenes and alkynes, the electron-deficient hydroxyl radical adds to the multiple bond.
The carbon-centered radical then reacts with molecular oxygen. There are many steps after this that, ultimately, gives carbon dioxide and water.
Enthalpy of Combustion
The reactions that produce carbon and dioxide and water from hydrocarbons in the atmosphere release the same amount of energy as the combustion of those hydrocarbons. We can measure the enthalpy of combustion by burning the hydrocarbons in a calorimeter. This enthalpy can also be calculated using Hf values.
Let's consider the combustion (or air oxidation) of ethane. How much heat is released by transforming 1 mol of ethane to carbon dioxide and water? The first step is to show the balanced equation for the reaction of 1 equivalent of ethane.
CH3CH3 + 3.5 O2 2 CO2 + 3 H2O
Next we consult a table of enthalpy of formation values.
Molecule
Hf (kJ/mol)
CH4
-74.5
CH3CH3
-84.7
CH2=CH2
52.3
CHCH
226.7
Molecule
Hf (kJ/mol)
CH3CH2CH3
-104.5
CH3CH=CH2
20.6
CO2
-393.5
H2O(g)
-241.8
Using those values, we compute the enthalpy of the reaction. Because the enthalpy is a thermodynamic state function, it doesn't depend on the pathway. The combustion of ethane in a boiler and the air oxidation of ethane must release the same amount of energy because they reactions start from the same molecules and finish with the same molecules.
How much energy is released when ethyne is oxidized to CO2?
Professor Patricia Shapley, University of Illinois, 2012
Hf values.
2 CO2 + 3 H2O
CH