What factors determine an atom's oxidation state?

[Comeback City]

I understand the concept of oxidation states and how to find them, but what confuses me is how an atom "decides" which oxidation state to choose. Or even "what it is at all" that decides the oxidation state. Specifically, the transition metals with multiple oxidation states seem to be the tougher ones. An example of this "decision" I found was in my Holt, Rinehart, and Winston Modern Chemistry textbook for my chem class. When talking about different synthesis reactions between Iron and Oxygen to form Iron Oxide, it states;

"In the product of the first reaction, iron is in an oxidation state of +2. In the product of the second reaction, iron is in an oxidation state of +3. The particular oxide formed depends on the conditions surrounding the reactants."

The specific part of that is the "conditions surrounding the reactants" part. What are these conditions that determine oxidation state? When I asked my teacher, she said that temperature could possibly be one, but wasn't certain. Any insight is much appreciated!

 

[Borek]

I am afraid there is no simple answer. That is, the simple answer is: whatever oxidation state is most stable at given conditions is typically the winner. But finding out what is the most stable configuration can be tricky. More or less that's what the whole chemistry is about.

"Conditions" in your example will be the relative amount of reactants (Fe/O ratio) and temperature in the simplest case, but if the reaction takes place in water pH starts to play an important role. Plenty of different cases possible, depending on what the exact reaction is.

 

[Comeback City]

Thanks for the answer! So if we took a look at the example of Iron Oxide again, and the oxidation state came down to stability, could you single out one Iron Oxide compound that could be said to be the most stable, given normal conditions (i.e. Room temperature, pH of 7)?

 

[Borek]

The easiest way to find out the most stable form in water solution is to use Pourbaix diagram (easy to google). Note, that to use them you need to establish what is the redox potential in the solution - it can depend on several factors, most often it is just a matter of the solution being saturated with atmospheric oxygen.

 

[Comeback City]

The easiest way to find out the most stable form in water solution is to use Pourbaix diagram (easy to google). Note, that to use them you need to establish what is the redox potential in the solution - it can depend on several factors, most often it is just a matter of the solution being saturated with atmospheric oxygen.

Ah. I never knew what those diagrams meant, but they now make more sense. Also, I was unaware of redox potentials before you mentioned them. After a quick search, the general definition seems to be "the measure of how much a substance wants to obtain or lose an electron in reduction or oxidation." Is this accurate?

 

[Borek]

the general definition seems to be "the measure of how much a substance wants to obtain or lose an electron in reduction or oxidation." Is this accurate?

In general it can be defined much more precisely, but yes, as a first approximation sounds OK.

 

[Comeback City]

In general it can be defined much more precisely, but yes, as a first approximation sounds OK.

Alright. I'll read more about it. Thanks for the help!

 

[HAYAO]

Very good question!

Whether a reaction occur or not depends entirely on how well the two states (initial and final) are correlated, and what kind of conformation it needs to take (usually requires form of heat energy) for the reaction to proceed. We call the route in which a reaction occur by "reaction coordinate". Qualitatively, reaction coordinate has to do with what kind of path in a multi-dimension space it needs to take so that initial state can reach the final state. Most of the time, if the "saddle point" or the energy potential wall, which a species must go over for transitioning between initial and final state, is energetically favorable at a certain temperature, the reaction proceeds.

In the case of oxidation states, one species might decide to stay in a certain oxidation state simply because it is energetically favorable to do so. In this case, a species need to changes its conformation and structure so that it is closer to the structure of final state. This change in structure is what causes the potential energy to rise, and sufficient energy must be supplied to the system for this structure to go through a reaction. This "structure" changes usually have to do with vibration of the species.

Also, some reaction have multiple steps. The same argument applies here.

 

[Comeback City]

Very good question!

Whether a reaction occur or not depends entirely on how well the two states (initial and final) are correlated, and what kind of conformation it needs to take (usually requires form of heat energy) for the reaction to proceed. We call the route in which a reaction occur by "reaction coordinate". Qualitatively, reaction coordinate has to do with what kind of path in a multi-dimension space it needs to take so that initial state can reach the final state. Most of the time, if the "saddle point" or the energy potential wall, which a species must go over for transitioning between initial and final state, is energetically favorable at a certain temperature, the reaction proceeds.

In the case of oxidation states, one species might decide to stay in a certain oxidation state simply because it is energetically favorable to do so. In this case, a species need to changes its conformation and structure so that it is closer to the structure of final state. This change in structure is what causes the potential energy to rise, and sufficient energy must be supplied to the system for this structure to go through a reaction. This "structure" changes usually have to do with vibration of the species.

Also, some reaction have multiple steps. The same argument applies here.

That's a good explanation of it. One quick question though: if one oxidation state is energetically LESS favorable, can it be assumed that the substance will take this oxidation state only if sufficient energy is provided? Or would it just go back to the conditions surrounding the reaction and Pourbaix diagrams that Borek mentioned?

 

[HAYAO]

That's a good explanation of it. One quick question though: if one oxidation state is energetically LESS favorable, can it be assumed that the substance will take this oxidation state only if sufficient energy is provided? Or would it just go back to the conditions surrounding the reaction and Pourbaix diagrams that Borek mentioned?

If you are comparing only one state that is more favorable than the current oxidation state, then yes. However, the yield is dominated by Boltzmann distribution. Therefore, even at high temperature, the yield of energetically less favorable state is still lower than the initial state.

Although I have written the above post based on very simple case of two species, in reality, there are multiple possible outcome with numerous reaction paths. The reaction path that requires the least amount of effort (energy), is usually the dominant resulting species. That does not mean, however, that other reaction paths that require more effort does not happen. It can certainly happen depending on the temperature. This is why you almost always observe byproduct in a reaction. (I am ignoring electronic correlation right now for simplicity of the discussion).

For your information, catalysts actually provides new reaction path of lower saddle point energy (requires less effort). Although in high school chemistry and etc., they teach you that catalysts lowers the activation energy of reaction, it does not mean that it lowers the one of the same reaction path. To be precise, pH can also be considered as a form of catalyst since they change a state of a species in question. They provide new path to a reaction, changes the energy potential of the entire system. This is why you expect different species under different pH condition. Pourbaix diagram shows pH dependent potential energy of states. These are different form what I was talking about above.

 

[Comeback City]

To be precise, pH can also be considered as a form of catalyst since they change a state of a species in question. They provide new path to a reaction, changes the energy potential of the entire system. This is why you expect different species under different pH condition. Pourbaix diagram shows pH dependent potential energy of states. These are different form what I was talking about above.

So could any condition/factor that changes the state of the system (temperature for example) be considered a catalyst? Or is it just specific to pH? (Btw thanks for the great answers!)

 

[Borek]

I am not convinced I agree with the HAYAO notion of pH being a "catalyst". It would be easier to classify it - together with TP - as "external conditions" (as in many cases it will not take part in the reaction, instead it will influence relative concentrations of acids/bases present in the solution). However, in many cases H⁺ (or OH^-) are just reactants.

 

[Comeback City]

I am not convinced I agree with the HAYAO notion of pH being a "catalyst". It would be easier to classify it - together with TP - as "external conditions" (as in many cases it will not take part in the reaction, instead it will influence relative concentrations of acids/bases present in the solution). However, in many cases H⁺ (or OH^-) are just reactants.

Would you say a catalyst should be an actual substance, then?

 

[HAYAO]

I am not convinced I agree with the HAYAO notion of pH being a "catalyst". It would be easier to classify it - together with TP - as "external conditions" (as in many cases it will not take part in the reaction, instead it will influence relative concentrations of acids/bases present in the solution). However, in many cases H⁺ (or OH^-) are just reactants.

Of course not because it's not. I'm just trying to give a better image of how potential energy curve is affected.

 

[Comeback City]

Of course not because it's not. I'm just trying to give a better image of how potential energy curve is affected.

So is it more that it acts like a catalyst than it actually being one?

 

[HAYAO]

So is it more that it acts like a catalyst than it actually being one?

Yes, in a sense that it changes the potential energy surface of the system.