Oxidation and Reduction

Oxidation originally meant that oxygen atoms were added to an element or compound, but since then scientists have expanded the definition.

Now oxidation refers to any time a negative charge is taken from a molecule. Reduction is the opposite of oxidation, instead of a negative charge being taken away, one is added.

Whenever oxidation is happening, reduction is happening at the same time. You have to get the negative charge from somewhere. But because the name stayed the same while the definition changed, oxidation can be confusing. Most of the time, what you’ll want to follow are the Hydrogen atoms (H). Hydrogen is moved around a lot in chemical reactions because it only has one electron and only needs one more to fill its requirements for total electrons. That makes it very versatile and relatively low energy to transfer from molecule to molecule. If hydrogen is added to a molecule, say a Carbon (C) then the carbon is being reduced. It has gained an electron from the Hydrogen. But if a Hydrogen is being taken from the Carbon, then the carbon is being oxidized, or having electrons taken away.

Hydrogen in its solitary state is a positively charged atom, but Oxygen all by its lonesome is negatively charged. They are in a manner of speaking opposites. So in oxidation (in the original meaning) positive oxygen was added. Turn that on its head and if Hydrogen is taken away then that is also oxidation. Blast it all, this is confusing. There are cases where neither hydrogen or oxygen are involved and cases where only charges (single electrons) are moved around too, but we’ll leave that for the college professors.

Here’s an example

Methane-gas-formation-oxidation

That means water plus carbon makes methane. The carbon (C) is being reduced when the H atoms, with their electrons, are added to it, and the Water (H2O) is being oxidized, since the H, with its electrons is being taken away. If your kids are the sort to get a thrill from it, you can tell them this is the reaction that happens to make the gas you pass. (My kids are the sort to get a thrill so I told them.)

Here’s another one

This complicated looking reaction means that sugar (C6H12O6) plus oxygen (O2) makes carbon dioxide (CO2) plus water (H2O). The sugar is being oxidized and the oxygen is being reduced. That’s the reaction that happens in your cells to give them energy to do their jobs. When the sugar molecules are broken down in your cell, breaking the bonds releases energy and that’s the energy your body runs on. The carbon dioxide gas and water are by-products. You breathe out carbon dioxide and pee out the water (plus other stuff).

Now here’s something way cool, check out this reaction

Can you guess what this reaction is? Look carefully and compare it with the cellular respiration equation we did just above. In this one, the carbon dioxide and water plus energy from the sun are being turned into sugars and oxygen. Yep, that’s photosynthesis. In this reaction the carbon is being reduced and the water is being oxidized. (Follow the electrons that are attached to the Hydrogen atoms).

Another Oxidation Reaction

Now I’ll write two half reactions so you can see more clearly how the electrons are moving. A half reaction is one step in a reaction even though in reality the whole reaction is happening all at once:

Here we start with zinc (Zn) in a solid form (s). As the reaction happens we get zinc with a positive 2 charge (2+) in an aqueous (aq) solution plus 2 negatively charged electrons (2e-). As the reaction goes on we have our two negative electrons added to some copper (Cu) with a plus two charge (2+) in an aqueous solution and we get solid copper. This is the reaction that takes place in a regular household battery. The zinc is being oxidized while the copper is being reduced. Remember, follow the negative charges to know which is which.

Experiment to demonstrate oxidation and reduction at home

How to oxidize pennies with vinegar and salt.

  1. Pour 1/2 cup vinegar into a small glass dish
  2. Then add one teaspoon salt. Stir.
  3. Drop in about 15 pennies (the ones minted before 1982 work best)
  4. Let the solution sit for about 10 minutes. the pennies will get shiny as the oxidized layer on the top is eaten away by the vinegar, an acid with lots of hydrogens floating around just waiting to reduce.
  5. Take the pennies out of the vinegar solution and lay them on a paper towel or clean rag to dry. Don’t wash them or rub them to dry them off.
  6. Now drop an ungalvanized iron nail into the vinegar solution. Let it sit for half an hour.

When you come back to look at your pennies and your nails, the pennies will be blue and the nails will be brown.

The pennies with the vinegar and the salt speeding things up are helping the pennies to oxidized very quickly. The blue stuff forming on the surface is the oxidized copper.

Cu + O2 —-> CuO (have your high schooler balance this equation)

The iron nails react with the copper ions, now in solution in your vinegar to coat the nails with copper. The iron is being reduced and the copper is being oxidized.

Additional Layers

  • Look at those cellular respiration and photosynthesis reactions again. Discuss with your child why they think the earth has this particular complimentary set of reactions.
  • Can you follow the energy cycle in the photosynthesis to cellular respiration equations? Why do you think plants need energy from light? Where does that energy go?
  • What’s up with batteries? Why do you want to move the electrons from zinc to copper? What good does that do? Think it over. Here’s a hint: electricity is moving electrons.
  • Why pennies older than 1982? Before 1982 pennies were made with 95% copper. Since 1982, pennies have contained only 2.5% copper. Why do you think that is? What is the government doing to our money and why? Has the same thing happened to other coins? To paper money? Research more about this and discuss what you find.
  • When you pass gas, it’s not you that made the gas, but who? Find out what’s going on in your digestive tract to make all that air.

More From Layers of Learning

An experiment with vinegar and a turkey bone.

An experiment with vinegar and a turkey bone.

Chemical labels to use to create your own home chemistry set

Chemical labels to use to create your own home chemistry set

Unit 2-14 has more great chemistry experiments.

Unit 2-14 has more great chemistry experiments.

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