Reaction Types 4/8/22

Chemistry with Richard Bleil

One of the more challenging exercises for students to learn in chemistry is probably figuring out reactions.  Fairly early in the course, they will be given partial equations and asked to finish them.  That is, they’ll be given the reactant (or reactants), and they have to provide the product (or products).  Here’s silver nitrate and copper, what will it form? 

The first gut reaction is, “how should I know?”  The truth is that if you know a little bit about reactions, you can figure it out.  It helps, though, to know the basic categories of chemical reactions.  There are some variations of these depending on the textbook you are using, but really there are only four major forms of chemical reactions (a fifth for organic chemistry).  Knowing them helps you predict what the reactions will produce.  It’s an old trick used by chemists that makes us look far more intelligent than we really are, as if we know every possible chemical reaction ever discovered when, in reality, we just deduce the results. 

The first one is the “addition reaction”.  I’ve heard other names for this, but addition is a simple term that explains it, so who really cares about a more scientificy sounding term?  In an addition reaction, there are two (or more) reactants, but only one product.  It’s easy to identify by just looking at the products.  For example, sodium metal in the presence of chlorine gas will form sodium chloride.  That’s a single product, but it started with multiple reactants, so it must be addition.  I used to explain it as “boy meets girl, and they form a couple”.  My dad used to explain things to me by breaking them down into humanistic terms to make them easy to understand, but, yes, I got into trouble for using “boy” and “girl”.

You know, relationships don’t always work out.  Couples will break up, leading to our second type of reaction, the “decomposition reaction”.  The decomposition reaction can be thought of as the opposite of addition.  There is always a single reactant, but two or more products.  The couple breaks up into the boy and the girl again.  For example, one of the more beautiful (but subtly dangerous) demonstration is heating mercury oxide (red), so it becomes mercury (silvery liquid) and oxygen.  One reactant, mercury oxide, and two products, mercury and oxygen. 

In my experience, though, relationships are a little more like a soap opera.  Sometimes couples just fall apart, but in my marriage, my wife wanted to leave me and go to another man.  This is called a single-replacement reaction, where one element simply replaces another in a reaction.  One of my favorite demonstrations is the silver nitrate and copper reaction.  In solution, silver nitrate is clear and colorless, looking suspiciously as innocent as water.  When you dip a clean copper (usually coiled) wire in the solution, the copper simply replaces the silver.  The result is a solution of copper (II) nitrate, a beautiful sky-blue color, as the copper simply replaces the silver.  The silver from the nitrate, in turn, becomes elemental silver, forming beautiful silver needles on the coil not unlike a Christmas tree. 

The double replacement reaction is more like a spouse swap, which even I was smart enough not to bring up in class.  The metal of one salt will swap with the metal in another.  Calcium nitrate will react with sodium hydroxide, for example, to form calcium hydroxide and sodium nitrate.  We’ll discuss this more in just a bit.

The organic reaction that is not really in this list is the “rearrangement” reaction.  Organic molecules are very complicated, and sometimes undergo internal rearrangement to become different.  Think about Lego’s, for example.  If you take something you’ve constructed, and rearrange the bricks, neither adding to them nor removing bricks, you’ll get something different. 

There are additional affectations of reactions.  For example, some books might talk about “precipitation reactions” as a category on their own.  Often these are just double-replacement reactions, like the one we discussed above.  A precipitate is simply a solid that forms from the reaction.  Sodium hydroxide is clear and colorless, as is calcium nitrate.  As it turns out, sodium nitrate is also clear and colorless and will remain in solution, but calcium hydroxide will form a white solid.  We say it will “drop out” of solution, meaning the solution will become cloudy, and eventually solid calcium hydroxide will settle on the bottom if you let it sit long enough.

Another affectation is called a “redox reaction”.  All of these reactions will fit into one of the original four categories as well.  When we say something is a redox reaction, that means we’re shuffling around electrons in the reaction.  Almost all single-replacement reactions are also redox reactions.  For example, in the silver nitrate with copper example, as part of the nitrate, silver has lost an electron.  It has a positive one charge making it a “cation”, or simply an “ion”.  It cannot have a charge to become elemental metal silver, so it takes an electron away from the copper that is replacing it.  The copper will actually lose two electrons (one to each of two silver ions), forming a plus two cations as a nitrate.  The electrons moved from the copper to the silver making it a redox reaction, but it’s still a single replacement.  With rare exception, addition and decomposition reactions are also redox, and double replacement reactions can be redox as well.  So “redox” is kind of an addition to the type of reaction.

A quick word on combustion.  I’ve seen this used, frequently, in chemistry textbooks as another type of reaction.  It’s really not.  Combustion reactions are really addition reactions (and redox reactions), but one of the reactants is specifically oxygen. 

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