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#1
Old 04-06-2006, 08:22 AM
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Chemistry: How do you find/recognize redox reactions?

Disclaimer: This was chemistry homework, but I'm already done with the assignment and I just want to better understand the topic.

In chemistry, how would you find or recognize a redox reaction when you don't know the specific chemical ingredients or changes involved? For our assignment, we were tasked with observing and recording redox reactions that happen in our everyday lives.

I understand, vaguely, that these reactions involve gaining/losing electrons or gaining/losing oxygen. I'm fine with that, but I don't understand how we're supposed to know whether a process undergoes redox reactions from observation alone. Am I supposed to be able to determine the chemical makeup of a substance and the interactions it undergoes with the environment just by looking at it? Or is there some strategy to this that I just don't know?

Thanks for help. I'm completely confused
#2
Old 04-06-2006, 12:20 PM
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Well, a lot of redox reactions involve colour change for a start. The way you worded it is a bit vague. Do you mean questions like "Which of the following are likely to be redox reactions: a yellow-brown gas turns dark brown as it is heated (Or was it cooled? Been too long since I did chemistry), two clear solutions react to form a cloudy precipitate, a nail rusts in the rain, an unknown liquid is titrated against 2.0 mol NaOH, causing the phenolphthalein indicator changes from purple to colourless"? Descriptive chemistry questions like that are pretty common and you'll just have to learn the common types of reactions and what they are typically like. If you meant something else, then I dunno.
#3
Old 04-06-2006, 12:47 PM
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You are right that you have to know something about the chemistry of your environment to know whether a redox reaction is taking place, but you probably already know enough. As you mentioned, a redox reaction involves a reactant gaining or losing an electron. You also mentioned that it is also usually a redox reaction when something gains or loses an oxygen.

You already know that iron(Fe) reacts with oxygen to produce rust (Fe2O3). You breath in oxygen and breath out carbon dioxide. Whithout knowing the multiple metabolic processes involved, you can pretty much deduce that a redox reaction is occuring. When you burn something, the carbon is being oxidized by oxygen to produce carbon dioxide. Where do you suppose the electrical current produced by a battery comes from?

It probably would have been a good idea to look online for some examples. Other examples that may be less obvious. Any plated jewelry you have was plated through a redox reaction. Anything that is plated at all. Bleach oxidizes organic molecules usually making them colorless. Hydrogen peroxide also oxidizes organics.
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Old 04-06-2006, 01:25 PM
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I think the idea may have been to "recognize" already-known redox reactions as something that happens around you.

Any cleansing involving bleach is a redox, for example; if you have studied Cl's oxidation states and know that bleach includes several Cl-states you could recognize it, if you haven't studied them, you wouldn't (I actually used to make bleach for a living; we only put the Cl into it in one form but by the time the bottles got filled it had managed to get itself through the whole catalog in different proportions). Christopher gives others.

Any time something burns, that's a redox right there.



++++++++++++

The way to realize that a reaction is a redox when you have it on paper is by looking at the electronic structures for each atom "before" and "after". If any atom ended with a different charge than it started with, it's a redox.



In Organic Chemistry you see a lot of "redox-ish" mechanisms where an atom doesn't get its charge changed by a whole unit and there's no oxygen involved, but one or more atoms have lower differential charge at the end of the reaction than before: "Additions" are usually this kind of thing. In the end Chemistry is just redistribution of electrical charges. If the redistribution affects the electronic structure of individual atoms, it's a Redox; if it doesn't, it's not.
#5
Old 04-06-2006, 09:28 PM
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Sorry, I didn't make myself very clear. I know how to look for specific instances of an example if I'm given a long list of already-known redox reactions, but it seemed like the teacher wanted us to look for NEW instances of redox. As in: Go out there, look at the world, and figure out where (not-previously-given) redox reactions are occurring.

Iron, chlroine, combustion and respiration were all examples we were given, but supposing they weren't... how would I know, just by looking at them, that they were undergoing redox?

Were these examples first chemically studied and THEN labeled as redox reactions, or did a chemist one day go "Wow, look at that rust... it must be redox!" out of the blue? If it's the latter, how would a first-year chemistry student be able to do the same...?
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