Alcohol-based fuels for automobiles lead to the production of formaldehyde (CH \(_{2} \mathrm{O} )\) in exhaust gases. Formaldehyde undergoes photodissociation, which contributes to photo-chemical smog: $$\mathrm{CH}_{2} \mathrm{O}+h v \longrightarrow \mathrm{CHO}+\mathrm{H}$$ The maximum wavelength of light that can cause this reaction is 335 \(\mathrm{nm}\) . (a) In what part of the electromagnetic spectrum is light with this wavelength found? (b) What is the maximum strength of a bond, in \(\mathrm{kJ} / \mathrm{mol},\) that can be broken by absorption of a photon of 335 -nm light? (c) Compare your answer from part (b) to the appropriate value from Table \(8.3 .\) What do you conclude about \(\mathrm{C}-\mathrm{H}\) bond energy in formaldehyde? (d) Write out the formaldehyde photodissociation reaction, showing Lewis-dot structures.

When we study light and its interactions with matter, a core concept we encounter is the electromagnetic spectrum. This spectrum represents the entire range of electromagnetic radiation, from the longest wavelengths (like radio waves) to the very shortest (like gamma rays). Electromagnetic radiation is a form of energy that travels through space at the speed of light, roughly 299,792 kilometers per second.
In the middle of the spectrum lies the visible light, which is what we can see with our own eyes. Stretching beyond the blue end of visible light, we encounter ultraviolet (UV) radiation. UV has shorter wavelengths and, therefore, more energy compared to visible light. In our textbook problem, it's mentioned that formaldehyde can absorb a photon with a wavelength of 335 nanometers (nm). This places the light in the UV region, which ranges from about 10 to 400 nm. UV light's high energy makes it capable of breaking chemical bonds, leading to phenomena such as photodissociation.

The strength of a chemical bond can be understood through the concept of bond dissociation energy (BDE), which is the amount of energy required to break a bond between two atoms and separate them into individual, gaseous atoms. Measured in kilojoules per mole (kJ/mol), BDE is a crucial factor in chemical reactions.
Let's relate this to our formaldehyde problem. Sunlight or UV light has enough energy to break certain chemical bonds. We calculated that a photon with a wavelength of 335 nm has an energy of about 358 kJ/mol. This value represents the maximum BDE that can be overcome by the photon; in other words, any chemical bond with energy less than or equal to this amount is susceptible to being broken upon absorbing such a photon. The concept of BDE is significant in fields such as environmental chemistry, where the breakdown of pollutants or other chemicals can lead to the formation or reduction of harmful substances.

Photochemical smog is a type of air pollution that's formed when sunlight acts on airborne pollutants, such as nitrogen oxides and volatile organic compounds (VOCs). This reaction results in a mixture of several secondary pollutants, including ozone, peroxyacetyl nitrates, and aldehydes. A classic example of these series of reactions is the photodissociation of formaldehyde, as noted in the textbook exercise.
When the bond within a formaldehyde molecule is broken by UV light, it creates radicals, which are highly reactive species. These radicals can further react with other molecules in the air, perpetuating a cycle of complex chemical reactions that lead to the hazardous cocktail we call photochemical smog. Smog is a public health concern because of its potential to cause or aggravate respiratory problems and other health issues, as well as its role in environmental degradation.

The Lewis-dot structure is a simple but powerful tool used in chemistry to represent the valence electrons of atoms within molecules. In formaldehyde (CH₂O), we depict carbon with four valence electrons, oxygen with six, and each hydrogen with one. These dots are arranged to show how the electrons are shared (in bonds) or unpaired.
In our problem, the photodissociation of formaldehyde leads to the splitting of a C-H bond. The Lewis-dot structure helps us visualize how the electrons are redistributed when formaldehyde absorbs sufficient energy (from a UV photon) to break that bond. By understanding how electrons are shared and moved around, chemists can predict the products of chemical reactions and explain the molecular basis of observed phenomena, such as the formation of photochemical smog.