Can Light Cause Oxidation?

[hazelm]

At Yahoo's Ask page where people try  to answer each other's questions someone asked if canola oil needed to be refrigerated.  The answer was "no" but:

 

(quote)  Keep it away from strong light, which will oxidize it.

Canola oil, like most food oils, goes rancid. This is what the chemists would call oxidation. The reaction that rusts iron is oxidation.  (unquote)

 

Is this correct?  Does light cause oxidation?  I thought rust was caused by heat and humidity on iron.  Is rancidity of oils the same as oxidation of iron?

 

Thank you.

[exchemist]

At Yahoo's Ask page where people try  to answer each other's questions someone asked if canola oil needed to be refrigerated.  The answer was "no" but:

 

(quote)  Keep it away from strong light, which will oxidize it.

 

Canola oil, like most food oils, goes rancid. This is what the chemists would call oxidation. The reaction that rusts iron is oxidation.  (unquote)

 

Is this correct?  Does light cause oxidation?  I thought rust was caused by heat and humidity on iron.  Is rancidity of oils the same as oxidation of iron?

 

Thank you.

Hello Hazel. Light cannot itself cause oxidation, as for that you need a source of oxygen or equivalent. But light can certainly accelerate the oxidation reaction between oxygen in the air and some substances, including oils and fats.

 

It does this by becoming absorbed by certain bonds in the molecule, and thereby kicking electrons into "excited states", that are more reactive than the normal "ground state" of the bond. You may have heard  of "free radicals" for instance - this is one type of excitation. In this condition they are more likely to react with passing oxygen molecules. 

 

So while light does not itself oxidise things, it can make them more reactive and thus likely to get oxidised.

 

More here about "rancidification":  https://en.wikipedia.org/wiki/Rancidification

Edited May 6, 2018 by exchemist

[hazelm]

Hello Hazel. Light cannot itself cause oxidation, as for that you need a source of oxygen or equivalent. But light can certainly accelerate the oxidation reaction between oxygen in the air and some substances, including oils and fats.

 

It does this by becoming absorbed by certain bonds in the molecule, and thereby kicking electrons into "excited states", that are more reactive than the normal "ground state" of the bond. You may have heard  of "free radicals" for instance - this is one type of excitation. In this condition they are more likely to react with passing oxygen molecules. 

 

So while light does not itself oxidise things, it can make them more reactive and thus likely to get oxidised.

 

More here about "rancidification":  https://en.wikipedia.org/wiki/Rancidification

Thank you, exchemist.  That does help.  I shall read Wiki tomorrow.  Late now.

[exchemist]

Thank you, exchemist.  That does help.  I shall read Wiki tomorrow.  Late now.

OK. One further thought, to give this a bit of context: it is commonplace in school level chemistry to talk about two ways of making reactions take place, or go faster. One is to heat up the reactants. This gives the molecules more kinetic energy so that more of them can react when they encounter each other. The other is to shine a light in, which does what I was saying earlier. 

[hazelm]

OK. One further thought, to give this a bit of context: it is commonplace in school level chemistry to talk about two ways of making reactions take place, or go faster. One is to heat up the reactants. This gives the molecules more kinetic energy so that more of them can react when they encounter each other. The other is to shine a light in, which does what I was saying earlier. 

Don't you sometimes wonder what else goes on in our hidden environment?    By the way, back to your first instance of excitation,, I wondered if heat off the light brought on some of the reaction.  Doesn't light always give off some heat?

Edited May 7, 2018 by hazelm

[exchemist]

Don't you sometimes wonder what else goes on in our hidden environment?    By the way, back to your first instance of excitation,, I wondered if heat off the light brought on some of the reaction.  Doesn't light always give off some heat?

Haha, that is a question involving quite some science, actually. Yes,  when a photon is absorbed, it excites one atom or molecule in just one, very particular, way. A visible wavelength photon might excite an electron to a higher orbital within the atom or molecule. An Infra-red photon might excite the vibration of a molecule in a particular way. A microwave photon might excite a rotation of a molecule.

 

However, what subsequently can happen is the energy that starts off in this one initial "degree of freedom" gets progressively distributed around, as a result of the thermal motion of matter, and winds up in bits and pieces among electronic excitation, vibration and rotation.  The way in which this happens can be quite complicated, especially the conversion of electron excitation into vibration and rotation.

 

But in the end, once the system has reverted to equilibrium, the energy will be distributed in the same way is by heating up the substance. So it has got warmer.

 

The advantage of light in chemical reactions is that, because they get absorbed by one specific bit of the molecule, they can trigger one desired reaction rather than a whole lot of them, which you sometimes can get with heat, which is a bit of a blunderbuss approach by comparison.    

Edited May 7, 2018 by exchemist

[hazelm]

Haha, that is a question involving quite some science, actually. Yes,  when a photon is absorbed, it excites one atom or molecule in just one, very particular, way. A visible wavelength photon might excite an electron to a higher orbital within the atom or molecule. An Infra-red photon might excite the vibration of a molecule in a particular way. A microwave photon might excite a rotation of a molecule.

 

However, what subsequently can happen is the energy that starts off in this one initial "degree of freedom" gets progressively distributed around, as a result of the thermal motion of matter, and winds up in bits and pieces among electronic excitation, vibration and rotation.  The way in which this happens can be quite complicated, especially the conversion of electron excitation into vibration and rotation.

 

But in the end, once the system has reverted to equilibrium, the energy will be distributed in the same way is by heating up the substance. So it has got warmer.

 

The advantage of light in chemical reactions is that, because they get absorbed by one specific bit of the molecule, they can trigger one desired reaction rather than a whole lot of them, which you sometimes can get with heat, which is a bit of a blunderbuss approach by comparison.    

Ha!  Someone with a great skill in creative writing could create a good novel of all that.  They could have little photons running around at night while we sleep and giggle while creating all sorts of havoc.  More fun.

 

But, seriously, I do appreciate all of this.  It is fascinating.  I know what photons are and, once I read it, I understand what they do -- until some young physicist  decides to "prove" that photons do not exist or some other such argument.  You know the ones I mean. 

 

I have not yet gotten to Wiki.  Had to run errands first - under a vast array of photons beating down on me.  Thank you again.  All done for now.

Edited May 18, 2018 by hazelm

[DaveC426913]

Ha!  Someone with a great skill in creative writing could create a good novel of all that. 

Or an industry that could change the world...

 

Say, photography.

 

The emulsion layers in film are sensitive to most of the range of visible light, allowing us to capture a projected image.

 

But they're not sensitive to all forms of light. That's why darkrooms can be lit by deep red lights, which humans can see, but film cannot.