Questions: Photochemistry and Photochemical Reaction Pathways

Question 1

A molecule is chemically stable at high temperatures but decomposes rapidly when irradiated with UV light of the appropriate wavelength. What is the best explanation?

• A. UV light delivers more total energy than thermal heating can at those temperatures
• B. Absorption of a photon places the molecule on an excited-state potential energy surface where decomposition proceeds without a barrier
• C. UV photons mechanically break covalent bonds by direct impact
• D. UV irradiation raises the local temperature of individual molecules beyond the thermal decomposition threshold

Answer: (B)

The key insight is that excited states are different chemical species on different potential energy surfaces — not just 'hotter' ground-state molecules. The ground-state molecule is stable because its potential energy surface has a high barrier to decomposition. After absorbing a photon, the molecule is on an excited-state surface where the bond may be repulsive with no barrier, allowing immediate dissociation. Thermal chemistry cannot access this pathway regardless of temperature, because heating only moves molecules along the ground-state surface.

Question 2

The quantum yield of a photochemical chain reaction is measured as 1,000. What does this mean?

• A. Each molecule absorbs 1,000 photons before reacting
• B. One absorbed photon initiates a radical chain reaction in which 1,000 product molecules are ultimately formed
• C. The measurement overcounts photon absorption by a factor of 1,000 due to scattering
• D. The reaction rate is 1,000 times faster than predicted from activation energy alone

Answer: (B)

Quantum yield = (number of molecules undergoing a process) / (number of photons absorbed). A value greater than 1.0 is possible only for chain reactions: the primary photochemical step consumes exactly one photon per molecule (Stark-Einstein law), but that one event can initiate a cascade of thermal chain reactions. Quantum yields in the hundreds to thousands are observed in photoinitiated radical chain reactions like HCl synthesis from H₂ and Cl₂.

Question 3

According to the Stark-Einstein law, the primary photochemical step requires exactly one photon per molecule that undergoes the primary process.

• T. True
• F. False

Answer: True

The law of photochemical equivalence states that each molecule activated in the primary photochemical step absorbs exactly one photon. This is consistent with the quantization of light: a single electronic transition is triggered by a single photon. Quantum yields above 1.0 arise from secondary thermal reactions downstream of the primary event, not from multi-photon absorption in the primary step (which requires extremely high light intensity and is a separate phenomenon).

Question 4

A photochemical reaction and a thermal reaction that produce the same product should proceed through the same transition state.

• T. True
• F. False

Answer: False

Photochemical reactions access excited-state potential energy surfaces that thermal reactions cannot reach. The pathways, transition states, and intermediates are fundamentally different. For example, photochemical and thermal cycloadditions follow opposite stereochemical rules (Woodward-Hoffmann rules), precisely because they proceed via different electronic surfaces. Producing the same final product does not mean the same route was taken.

Question 5

Why can a photon sometimes enable a chemical reaction that high temperatures cannot bring about, even when the thermal energy available would be comparable in magnitude?

Model answer: Temperature determines the energy distribution of molecules on the ground-state potential energy surface. No matter how high the temperature, molecules remain on that same surface — they simply have more kinetic energy along it. A photon does something different: it promotes the molecule to a completely different excited-state potential energy surface where the bonding structure, barrier heights, and reaction pathways are entirely different. A reaction that has a prohibitive barrier on the ground-state surface may have no barrier at all on the excited-state surface, making the reaction spontaneous once the photon is absorbed.

This is the core insight of photochemistry: photons provide access to new potential energy surfaces, not just more energy on the old one. The ozone photodissociation example illustrates it clearly — O₃ is thermally stable but photodissociates readily under UV because the excited state lands on a repulsive surface with no barrier to dissociation. Heating O₃ does not achieve the same result because the ground-state surface has a significant dissociation barrier.