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ReDox reactions

What is a redox reaction?

1st definition: Oxidation is gain of oxygen. Reduction is loss of oxygen

2nd definition: Oxidation is loss of hydrogen. Reduction is gain of hydrogen

Modern definition: Oxidation is loss of electrons. Reduction is gain of electrons.

“The Elephant Toothpaste experiment earned its name from the foamy byproduct of the rapid decomposition of hydrogen peroxide. Mixing yeast with hydrogen peroxide catalyzes its decomposition into water and oxygen gas via an exothermic reaction. Adding some dish soap causes the explosion of foam!”

This demonstration is based on the decomposition of hydrogen peroxide into water and oxygen gas.

Reactions like these that are both oxidations and reductions are known as disproportionation reactions:

2 H2O2(aq) -> 2 H2O(l) + O2(g)

Left on its own at room temperature, this reaction happens at a rate so slow that, for practical purposes, it may as well not even exist. A catalyst is added to speed things along.



Science Bob's Crazy Foam Experiment https://sciencebob.com/

Science Bob’s Crazy Foam Experiment


What is reduction potential?

Think of a compound as being like a large magnet. A large magnet attracts nails more strongly, and holds them more tightly, than a small magnet. Similarly, a compound with a large (positive) reduction potential attracts electrons more strongly than a small (or even a negative) reduction potential.


Myth: Redox reactions are different from other chemical reactions, like combustion, decomposition, replacement, etc.

Fact: Single-replacement rxns and combustion rxns are always redox. Other types are sometimes redox.

Sodium and fluorine bonding ionically to form sodium fluoride. Sodium loses its outer electron to give it a stable electron configuration, and this electron enters the fluorine atom exothermically. The oppositely charged ions are then attracted to each other. The sodium is oxidized, and the fluorine is reduced.

“LEO the lion says GER”
Loss of Electrons is Oxidation, Gain of Electrons is Reduction.

“RED CAT” and “AN OX”
Reduction occurs at the Cathode, and the Anode is for Oxidation


Oxidation and reduction overview


"NaF" by Wdcf - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:NaF.gif#/media/File:NaF.gif

“NaF” by Wdcf – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:NaF.gif#/media/File:NaF.gif





Question: How can I recognize ReDox reactions? Are any of these Redox: Salt used to melt snow? Alka Seltzer tablet in water? Gasoline to dissolve grease?

Answer: Ask yourself – Is a chemical substance used up by this process? Does it make a new chemical substance? If the answer to both questions is “no”, you don’t have a chemical change – and so, it can’t be classified as a reaction (redox or otherwise).

1. Is the salt used up? If you want to say “yes”, take a look at the road or sidewalk after the water dries up. Is the snow used up? Is water a new chemical substance? If you want to say “yes”, ask yourself if water and snow have different chemical formulas.

Are electrons transferred? To answer, you’ll have to write down the chemical equation for the process. Then assign oxidation numbers to the atoms, in all  reactants and products. If the oxidation numbers for any of the atoms in the reactants are different from the oxidation number for the same atoms on the product side, electron transfer must have occurred, and you have a redox reaction.

For example, 2 Na + Cl2 = 2 NaCl is a redox reaction, because the sodium’s oxidation number changes from 0 to +1, and the chlorine’s oxidation number changes from 0 to -1.

All combustion reactions and all formation reactions (in which compounds form from elements) are redox reactions. But there are other possibilities, too.

Hint: Alka Seltzer tablets contain sodium bicarbonate and citric acid. When dropped into water, sodium citrate, carbon dioxide, and water are formed. Look up the chemical formulas for each of these substances, assign oxidation numbers, and decide whether this is a redox reaction or not.

Author: Fred Senese senese@antoine.frostburg.edu


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