Rate of reaction? (UV and heat)

[kenny1999]

I remember when I was in high school, heat was very often said to be a factor of speeding up chemical reactions that we all know, because heat is actually energy, energy will excite the molecules and make them easier to collide with other molecules or leave its own molecules (breaking bond)

First, have I remembered these concepts correctly?

Second, How about UV? At that time, effect of UV on chemical reaction is not actually discussed, not even the basic concept. But for some particular reactions, teachers or books will say - it can be accelerated or affected by UV light, but some reactions never mention UV. It sounds like UV could affect one reaction but not another. However, heat or temperature is almost always said to be affecting any reaction of any kind

But isn't UV basically the same thing as heat on chemical reaction? It gives energy and break the bond? Molecule and bonding will not know about UV, they only know about energy...

In other words, could some reactions never, ever be affected by the presence of UV / sunlight?

 

[Borek]

But isn't UV basically the same thing as heat on chemical reaction?

No.

UV (or electromagnetic radiation in general) can heat up the reaction mixture, so in a way you can expect it to always influence the reaction rate, but in some cases UV is exactly what is needed to start the reaction (think UV cured resins for example). That more or less works for specific molecules that have such bond energy, that UV quant is exactly the amount of energy required to break it.

 

[kenny1999]

Hi, we know that UV will degrade plastic over a long period of time, e.g., weeks, months or years. I know that the mechanism will basically involve creation of free radical that will attack the polymer chains. However, isn't free radical highly reactive and very effective in trace amount? Why it often takes weeks or months of exposure before any
observable damage can be seen? For example, I am pretty sure that if I put a white plastic case under sunlight for a few hours or days I wouldn't observe any yellowing

 

[DrStupid]

However, isn't free radical highly reactive and very effective in trace amount?

Radicals do not only react with the polymer (resulting in depolymerisation) but also with the monomers (resulting in polymerisation). If the equilibrium is on the side of the polymer than transport processes become rate-determing.

 

[Chestermiller]

Is the reaction photolytic?

 

[TeethWhitener]

But isn't UV basically the same thing as heat on chemical reaction? It gives energy and break the bond? Molecule and bonding will not know about UV, they only know about energy...

The Woodward-Hoffmann rules were formulated specifically to explain why stereochemistry of products differed depending on application of heat vs light:
https://en.wikipedia.org/wiki/Woodward–Hoffmann_rules

 

[HAYAO]

Second, How about UV? At that time, effect of UV on chemical reaction is not actually discussed, not even the basic concept. But for some particular reactions, teachers or books will say - it can be accelerated or affected by UV light, but some reactions never mention UV. It sounds like UV could affect one reaction but not another. However, heat or temperature is almost always said to be affecting any reaction of any kind

In most cases, the effect of ambient UV is negligibly smaller than heat in chemical reactions. UV in common places is practically quite low in power density, too low for most chemical reactions. That's why most people don't mention it. However, if you do provide enough time to let the photoreaction proceed, you will have to understand their effects.

If you are in a lab, which in most cases the sunlight is kept at minimum, most lamps have negligibly small amount of UV light. It does, however, depending on what reactants you are using because some chemicals react to UV light more sensitive than others. For example, benzene absorbs light in a deeper region of UV light such that typical ambient light conditions do not affect them at all. If you are working with these chemicals, then UV is probably not a factor.

But isn't UV basically the same thing as heat on chemical reaction? It gives energy and break the bond? Molecule and bonding will not know about UV, they only know about energy...

Not at all. As @TeethWhitener mentioned, photo-induced reaction may not always yield the same result as heat-induced reaction. For example, [2+2] cycloaddition of ethylene does not happen with heat. It only happens by photochemical reaction. Reaction of certain reactants with oxygen tend to be a lot faster with UV in many chemicals if they have high spin-orbit coupling (or a sensitizer is used) since oxygen is triplet state at ground state while as most compounds are singlets.

 

[kenny1999]

In most cases, the effect of ambient UV is negligibly smaller than heat in chemical reactions. UV in common places is practically quite low in power density, too low for most chemical reactions. That's why most people don't mention it. However, if you do provide enough time to let the photoreaction proceed, you will have to understand their effects.

If you are in a lab, which in most cases the sunlight is kept at minimum, most lamps have negligibly small amount of UV light. It does, however, depending on what reactants you are using because some chemicals react to UV light more sensitive than others. For example, benzene absorbs light in a deeper region of UV light such that typical ambient light conditions do not affect them at all. If you are working with these chemicals, then UV is probably not a factor.Not at all. As @TeethWhitener mentioned, photo-induced reaction may not always yield the same result as heat-induced reaction. For example, [2+2] cycloaddition of ethylene does not happen with heat. It only happens by photochemical reaction. Reaction of certain reactants with oxygen tend to be a lot faster with UV in many chemicals if they have high spin-orbit coupling (or a sensitizer is used) since oxygen is triplet state at ground state while as most compounds are singlets.

Hi. the effect of ambient UV is negligibly smaller than heat in chemical reactions?

Sorry but I want to take an example of a few PP (Polypropylene) items in my home one is a washing bowl often placed near cooking flame (about 20 cm) for many years and I cook so often, it is still good now like new. Another PP container is put near windows and exposed to ambient sun, usually only a few months in a year because there is almost no good sunlight in other seasons here, but it is , already yellowed so much

 

[HAYAO]

Hi. the effect of ambient UV is negligibly smaller than heat in chemical reactions?

Sorry but I want to take an example of a few PP (Polypropylene) items in my home one is a washing bowl often placed near cooking flame (about 20 cm) for many years and I cook so often, it is still good now like new. Another PP container is put near windows and exposed to ambient sun, usually only a few months in a year because there is almost no good sunlight in other seasons here, but it is , already yellowed so much

20 cm from cooking flame is not that hot...and I expect it to happen like for couple of minutes each time. And you are literally exposing the PP in sunlight for almost full season. Not a good comparison.

Meanwhile, typical laboratory reactions using heat are like couple of hours at most (sometimes overnight).