How can I recognize redox reactions?

I can't figure out if the following are examples of Redox reactions or not: Salt used to melt snow? Alka Seltzer tablet in water? Gasoline to dissolve grease?

A redox reaction is a chemical change that involves a transfer of electrons. Ask yourself these questions about the processes you're trying to classify:
  • 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).

    For example, look at the first process you've listed. 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 this question you'll have to write down the chemical equation for the process and assign oxidation numbers to the atoms in all of the 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

General Chemistry Online! How can I recognize redox reactions?

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