According to IUPAC a photochemical reaction is a reaction caused by absorption of light. I still can't understand how we should consider light as a part of a reaction. Is a catalyst or a reactant?

In some reactions there is the $hν$ notation above the reaction arrow. For example we may have the following reaction:

$$\ce{A ->[$hν$] B}$$

I want to understand how light affect reactions and chemical equilibriums. I read somewhere about photoisomerization and that favors the less thermodynamic stable product and that make me think of how is it possible for the value of equilbrium constant to change (considering interoconversion as a "reaction").

Light in photoabsorption

When a molecule absorbs a photon it moves to an excited state. This process is associated with a probability factor, lets call it $P$. We can write such a transition as:

$$\ce{A + $hν$ -> A^*}$$

Suppose we have a box with molecules $\ce{A}$ that initially is a closed system. That is photons can enter the box. Now we use a laser and pump photons of specific wavelength into the box and after that we isolate the system, that is no energy can go in or out of the system. Does the above arrow $\ce{->}$ implies a reaction? I was thinking that if $P = 0.2$ then after the light enters the box we will have $[\ce{A}^*] = 0.2$ and $[\ce{A}] = 0.8,$ so the equilibrium constant will be $\displaystyle K = \frac{0.2}{0.8} = 0.25$ (I have omitted concentration units for simplicity).

But this doesn't make sense because first a new thermal equilibrium will be reached and the relative concentrations (populations) will be in accordance with Boltzmann distribution. Secondly if it is indeed a reaction then we must include light also. Also, the concentration $[\ce{A^*}]$ should depend on the incident light intensity.

So is photon absorption just a physical process and therefore the chemical equilibrium concept doesn't apply? I thought that if photoabsorption reaches an equilibrium such that:

$$\ce{A <=> A^*}$$

then for interconversion from cis to trans of a compound $\ce{A}:$

$$\ce{A_\textit{cis} <=> A_\textit{trans}}$$

by populating the excited state of the one isomer the equilibrium position should change. But still this doesn't make sense because Gibbs free energy change of the reaction take into account both ground and excited states of both products and reactants.

Light in chemical reactions

First the notation $hν$ can't be thought to be a catalyst as it doesn't make sense because isn't regenerated. For example, the chlorination of methane to yield chloromethane

$$\ce{CH4 + Cl2 ->[$hν$] CH3Cl + HCl}$$

uses light to initiate the reaction. But it isn't regenerated in any other step so it shouldn't be a catalyst. Then how one should think about light in a chemical reaction? It is a reactant? In other words could we write the chlorination of methane in the following way?

$$\ce{CH4 + Cl2 + $hν$ -> CH3Cl + HCl}$$

If that is the case can we find an equilibrium constant that includes the concentration of photons? Because for every reaction there must be a corresponding equilibrium constant according to thermodynamics.

I am asking the above because as I said I read about photoisomerization and I couldn't understand how is it possible to favor a thermodynamically less stable product via radiation.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. (May I suggest to any others who are tempted to comment here: if you feel your comment should be seen by others -- and many comments should -- then please post an answer, or comment on whichever answer is relevant to your point. If it isn't, then post it in chat, or don't post it.) $\endgroup$ Commented Jan 5, 2021 at 2:22

5 Answers 5


It is neither reactant nor catalyst, and equilibrium concepts do not apply

There are processes other than photochemistry that to go in a direction away from equilibrium. They require mechanical or electrical work, and there is no established way to incorporate these into a chemical equation.

If we write a conceptual equation for charging a battery, it could look like this: $$\text{empty battery} \ce{->} \text{charged battery}$$ The external power source is neither a reactant nor a catalyst; it is work done on the system, which serves to move the system away from chemical equilibrium.

If we write a conceptual equation for a refrigerator (or more generally, a heat pump), it could look like this: $$\text{warm body + warm body} \ce{->} \text{cold body + hot body}$$ Again, the mechanical work done by the compressor is neither a reactant nor a catalyst, and it serves to move the system away from thermal equilibrium.

For both processes, you could not say that in the presence of work, an equilibrium is reached. It is just the opposite - we are moving away from the equilibrium.

[OP] So is photon absorption just a physical process and therefore the chemical equilibrium concept doesn't apply?

Photochemical reactions can move a reaction away from equilibrium (as in the example of cis/trans isomerization the OP mentions). This makes it a non-equilibrium process, so equilibrium concepts need to be expanded. If you want to describe the situation under certain irradiation conditions, you might use the term photostationary cis:trans ratio (see section on stilbene in https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/photchem.htm).

[OP] I read somewhere about photoisomerization and that favors the less thermodynamic stable product and that make me think of how is it possible for the value of equilbrium constant to change (considering interoconversion as a "reaction").

The equilibrium constant does not change. The reaction just does not go to equilibrium (or in fact moves away from equilibrium).

[OP] First the notation hν can't be thought to be a catalyst as it doesn't make sense because isn't regenerated.

Things written on top of the reaction arrow are not necessarily catalysts. In general, that spot is used for reaction conditions, such as solvent, high temperature, or "reflux".

[OP] Then how one should think about light in a chemical reaction? It is a reactant?

Some textbooks write "heat" as reactant or product, which already does not make sense (but is used in combination with Le Chatelier principle to memorize how equilibrium constant change with temperature). For photochemical reactions, however, there is a stoichiometric aspect to the role of photons; for every reaction, one photon has to be captured.

A comprehensive published treatment

The abstract of this paper (Perspective – life and death of a photon: an intuitive non-equilibrium thermodynamic distinction between photochemistry and thermochemistry) addresses some of the misconceptions for photochemical reactions. It is a bit long, so I am breaking it up into smaller parts.

First, it states that photons are not chemical reactants in photochemical reactions:

Neither the thermodynamically determined probability isotherm nor its kinetically manifest rate isotherm can be applied to photo-absorptive reactions such that the participants, including photons, may be treated as if they were chemical reactants. Photons and chemical reactants differ from each other fundamentally: firstly, a photon's energy is absolute and, in all instances of practical relevance to the present paper, independent of its surrounding electrochemical field, while the energy of a chemical reactant is relative and defined by its surrounding field; secondly, while both photons and chemical reactants can and do engage in entropy creation, only chemical reactants can engage in entropy exchange.

It then goes on to address the mistake of treating photochemical processes using equilibrium concepts:

Clarification of these differences requires identification and abandonment of fundamental historical errors in photochemical thought deriving from inappropriate overreach of analogies drawn between light and ideal gases, and including: treatment of photo-absorption as a reversible chemical reaction; attribution to light of thermal potential, or temperature (as distinct from the idealised abstraction of a ‘temperature signature’); attribution to light of exchangeable entropy content.

Then, it addresses how entropy plays a role in these processes:

We begin by addressing widespread misapprehensions concerning the perennially misunderstood concept of entropy and the frequently overlooked distinction between entropy creation and entropy exchange. Armed with these clarifications, we arrive at a useful perspective for understanding energy absorption and transfer in photosynthetic processes which, through the chemical ‘kidnapping’ of metastable excited states within structured metabolic pathways, achieves outcomes which the Second Law denies to thermal chemical reactions

  • $\begingroup$ "For both processes, you could not say that in the presence of work, an equilibrium is reached. It is just the opposite - we are moving away from the equilibrium". Could we say that we have an equilibrium with a constraint like when a box is partitioned by a fixed adiabatic wall to two smaller boxes with different temperature at the two sides? I mean if the refrigerator stops then the temperatures will again be equal like if we substitute the adiabatic wall with one that allows heat transfer. $\endgroup$
    – Anton
    Commented Jan 4, 2021 at 21:59
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    $\begingroup$ @Anton I would prefer the technical term photostationary to a made up own like "equilibrium with a constraint". I edited the answer and added a link to a page on photochemical reactions. $\endgroup$
    – Karsten
    Commented Jan 4, 2021 at 23:35
  • $\begingroup$ So light just excites the reactants and reduces the activation energy? $\endgroup$
    – Anton
    Commented Sep 17, 2021 at 12:26
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    $\begingroup$ @Anton - still thinking about this nine months later, I'm impressed. Light specifically excites the reactants but not the products. This raises the rate of the forward reaction whereas the reverse reaction keeps going at the same rate (very low, if you want to make some product). The outcome is a ratio of product to reactant that is higher than that at equilibrium. In that sense, it reduces the activation energy. $\endgroup$
    – Karsten
    Commented Sep 17, 2021 at 19:11

Reactants and products of chemical reactions (as well as catalysts, solvents and other chemicals that might take part in a reaction) are types of matter. Matter can be thought of as something that intrinsically has both a mass and a volume, although the linked Wikipedia article will go into greater details and varying attempts at defining matter.

Regardless of how matter is usually defined, photons (and energy in general) are not matter. They are considered to have zero mass and it does not occupy volume. Thus, they have no real place in chemical equations which deal primarily with matter.

Instead, photons in photochemical reactions are best thought of as an energy source that will transfer a discrete amount of energy onto a certain molecule. While terms such as photocatalysed are thrown around in the chemical literature a lot, these are best understood as an analogy rather than an accurate description.

For the record, even though catalysts are often written on top of reaction arrows it is always possible to spell out a detailed step-by-step mechanistic cycle starting with the original catalyst, taking various steps in which the catalyst and reagents are modified and ending with a reaction that regenerates the catalyst. These reactions are necessarily balanced and, as written in the first paragraph, entirely involve matter.

  • 1
    $\begingroup$ So you would agree, I guess, that including "heat" as a reactant or product is silly as well... $\endgroup$
    – Karsten
    Commented Jan 4, 2021 at 19:46
  • $\begingroup$ @KarstenTheis Absolutely! $\endgroup$
    – Jan
    Commented Jan 6, 2021 at 11:14
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    $\begingroup$ I think you are on thin ice to claim that photons are not matter, they have energy hence mass (but zero rest mass) and are influenced by gravity and act like particles and like waves. Molecules/atoms also can behave like waves, as in double split experiments. Molecules also impart energy to cause reactions by collisions, e.g Lindemann, RRKM etc models of unimolecular reactions where inert molecules/atoms play this part. $\endgroup$
    – porphyrin
    Commented Mar 18, 2021 at 15:31

Photolytic dissociation of molecules like $\ce{Cl2}$ is basically a first order reaction in the concentration of the dissociating molecule, with the rate constant equal to the photon flux $\phi$ multiplied by the absorption cross section $A_x$ of the dissociating molecule: $$-\frac{\mathrm d[\ce{Cl2}]}{\mathrm dt}=+2\frac{\mathrm d(\ce{Cl^.})}{\mathrm dt}=k[\ce{Cl2}]$$with $$k=\phi A_x$$After that, the odd chlorine atoms can react with methane in a second reaction: $$\ce{CH4 + Cl^. -> CH3^. + HCl}$$and the $\ce{CH3^.}$ radical can participate in subsequent reactions.

Of course the equation for the rate constant would also have to be integrated over wavelength.

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    $\begingroup$ In short, the chlorination is photo-induced, but the reaction itself is a plain old radical chain reaction. $\endgroup$
    – Karl
    Commented Jan 3, 2021 at 15:58

Don't worry too much about wordings of whether light is a reactant or not. It essentially boils down to semantics. What is a photon anyway? A packet of energy? then what is a packet? Fenyman (Nobel laureate, Physics) wrote a story somewhere that as a newly minted PhD, his father asked him what a spontaneous emission (of photon) is.

He said: “How do you . . . think of a particle photon coming out [of the atom] without it having been there in the excited state?”

I thought a few minutes and I said: “I’m sorry. I don’t know. I can’t explain it to you.”

Taken from a 500 paged book Our Changing Views of Photons: A Tutorial by Bruce W. Shore by Oxford University Press.

You can realize the complexity!

All you need to worry is the mathematics and kinetics of photochemical reactions. Is light a catalyst, is it a reactant, is it a product? There are several reactions which emit light. Should I call that as one of the "products". These are all filler words. Yes, the number of photons are important and their energy is important in a reaction which is affected by light. See Einstein's laws in photochemistry.

  • $\begingroup$ I voted down because you add some interesting points to the discussion but dismiss the point of the OPs question. $\endgroup$
    – Buck Thorn
    Commented Jan 3, 2021 at 7:54
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    $\begingroup$ @BuckThorn He's right, though. There are photoinduced reactions, photocatalysed reactions, luminescense, phosphoresence, chemical lasers, etc. Light needs to be treated differently in all those, and in none of it light can be treated just like a regular chemical reactant. $\endgroup$
    – Karl
    Commented Jan 3, 2021 at 16:07
  • 1
    $\begingroup$ Exactly, there is no point in discussing whether light is a "reactant" or "product". It is like the old age argument "How many angels can land on the head of the pin"? There is no answer. $\endgroup$
    – AChem
    Commented Jan 3, 2021 at 16:39
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    $\begingroup$ I agree with the point that the OPs question is too broad. There is no single answer, and there is room in different contexts to interpret energy from light as a reactant, product or catalyst, even if these are not universally useful or appropriate descriptions. But the first paragraph dismisses the question as having no answer ("semantics") and goes into some tangential discussion about the nature of photons. Agree the exact nature of the interaction with light is of importance, but why not say that instead? Otherwise very insightful as usual. $\endgroup$
    – Buck Thorn
    Commented Jan 4, 2021 at 9:29
  • $\begingroup$ Indeed there is no single answer, I upvoted at least three. $\endgroup$
    – Alchimista
    Commented Jan 4, 2021 at 12:10

I would consider photons to act like chemicals in reactions. The difference is that although they have to obey conservation of energy and linear and angular momentum, their number is not conserved so we can't balance a chemical equation by counting photons. It should be obvious that they can be absorbed by molecules and emitted as in a light stick.

There is still an effective chemical equilibrium for photons, think about blackbody radiation. As catalysts -- well, in an ordinary chemical reaction the (homogeneous) catalyst gets turned into another species at one stage of the reaction, with higher or lower energy, then gets regenerated at a later stage. The regeneration is inevitable because of the number conservation law the chemical catalyst obeys. If you think about the analogous situation with photons, chlorophyll fluoresces strongly in the infrared which could be considered as the first stage of catalysis of photosynthesis: a visible photon is absorbed and an infrared photon is emitted with the energy going towards a useful chemical reaction. The photon just leaves the reaction area never to be seen again rather than waiting around for something to bump its energy back up to a useful level. So even though we got photon in, photon out, we wouldn't consider it catalysis because the outgoing photon can't and never will be able to cause photosynthesis again (blueshifted mirrors excepted:).

But a laser could be considered catalytic because a photon stimulates the emission so it is regenerated along with an indistinguishable buddy after the reaction. If you read the above link you can see that consideration of chemical equilibrium of photons leads to prediction of lasers.

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    $\begingroup$ Sorry, this is reasoning by analogy, or in short, nonsense. $\endgroup$
    – Karl
    Commented Jan 3, 2021 at 16:01
  • $\begingroup$ Wouldn't it be more accurate to think of light as a catalyst rather than a chemical? After all post-absorption the energy of the photon might be dissipated in a myriad of ways whereas (as you note) conventional chemical products conserve the identity of the individual atoms in the reagents. $\endgroup$
    – Buck Thorn
    Commented Jan 4, 2021 at 9:36
  • $\begingroup$ While conventional definitions such as provided by the Wikipedia state that catalysts are regenerated if you weaken that condition (which is not central to their ultimate function of accelerating a particular reaction) then light can certainly fit the bill. Catalysts do not contribute to the composition of the product(s) of interest. $\endgroup$
    – Buck Thorn
    Commented Jan 4, 2021 at 9:49
  • $\begingroup$ As impressive as your rant may be :-) I don't think it is on topic or useful for the question at hand. I have removed it, but if you really want it back, there's always the rollback button. As for the downvotes, it doesn't seem all that bad: yes, people seem to disagree with you, but the sheer number of competing answers and comments already shows that it is quite a controversial topic. You shouldn't be ashamed of your contribution to the discussion, which was made in good faith. $\endgroup$ Commented Jan 5, 2021 at 2:17

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