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There are only four types of reaction ("Skeletal chemistry" 9 of 9)


Created by jrose, last updated 2015-11-13

"There is really only one type of reaction, but for convenience we think of all chemical change as one of four processes: [1] the transfer of a proton (acid-base reactions); [2] the transfer of an electron (redox reactions); [3] the sharing of electrons (radical reactions); and [4] the sharing of electron pairs (Lewis acid-base reactions). When you think about it, all four types of change are manifestations of a single process in which an electron falls into a hole. That is easily seen to be the case with redox and radical reactions: in Lewis acid-base reactions there are two electrons falling into a double hole, and Brønsted acid-base reactions are just special cases of Lewis reactions."


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Cabbage Indicator

[1-a] The transfer of a proton (acid-base reactions). A proton is just a hydrogen atom without its electron -- often written as the "H " ion. Cabbage has a pigment molecule called "flavin" (one type of molecule called an "anthocyanin"), and it changes structure based on whether it's in acidic or basic solutions. This structural change in enough molecules causes the color of the solution to change, which is why we call such chemicals acid-base "indicators."

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CHEMStudy film: "Acid-Base Indicators"

[1-b] This film from the historic CHEMStudy curriculum has some nice demos, including an animation of an indicator ion donating and accepting a proton (at 4:00), and how a universal indicator can be understood as a mixture of multiple indicators (at 14:30).


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Build a Battery

[2-a] The transfer of an electron (redox reactions). The chemical reaction in a battery involves electrons being transferred. Since there's an external path for the electrons -- around the circuit through the load -- the energy released by the reaction can be transferred out.

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Indicating Electrolysis

[2-b] A battery can also be used to provide electrons and energy to split or "electrolyze" chemicals into component parts. In this activity, a 9V battery splits water into hydrogen and oxygen. Adding an acid-base indicator (like cabbage juice) into the water reveals that where oxygen bubbles form at one electrode the water is left more acidic (more hydrogen), and hydrogen bubbles at the other electrode leave the water more basic.

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Electroplating

[2-c] Electroplating is similar to electrolysis in that metal atoms from one electrode are losing electrons and turning into ions. But those ions in the cell migrate over to the other electrode where they "plate" onto the second metal as they regain electrons.

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Chlorine Microscale Gas Experiments: Experiment 7 - Hydrogen/Chlorine Rockets

[3-a] The sharing of electrons (radical reactions). There don't seem to be too many experiments that deal with radicals, perhaps because radicals are so reactive. Scroll down to Experiment 7 to see how to make micropipette rockets that explode hydrogen and chlorine gas -- a radical reaction.


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Hydrogen and chlorine - a photosensitive, free radical reaction

[3-b] Chlorine gas can be split by sunlight, and this variation shows how the sun can initiate the hydrogen/chlorine reaction. It also gets at why CFC (chlorofluorocarbon) molecules were banned. They were great for refrigerators and spray cans, but when they escaped into the atmosphere, sunlight would split off the chlorine atoms. These radicals would then destroy ozone molecules, which is bad for humans since ozone absorbs the sun's ultraviolet light (instead of our skin).


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Lewis theory of acids and bases

[4-a] The sharing of electron pairs (Lewis acid-base reactions). This approach to acids and bases is not taught much in traditional school chemistry education, though it is more general than Arrhenius hydrogen ion theory and Bronsted-Lowery proton transfer. This diagram shows the relationship.


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The Lewis Acid/Base Interaction Matrix

[4-b] To end this tour of "Skeletal chemistry," here is a page that claims a "staggering" range of chemistry is "encompassed and described" as Lewis Acid/Base interactions. It is beyond my current knowledge to know the validity of this approach, but given how complicated chemistry is, perhaps it is a lead towards a clearer way of thinking.


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"Skeletal chemistry" article

This is one of the nine central ideas that Peter Atkins lays out in his 2005 article, "Skeletal chemistry." Each of the other ideas has its own list in SMILE.