Predicting reaction products

So, now that you’ve learned all about chemical equations, it’s time to figure out how to use them in the real world.  As you might have guessed, real chemists don’t usually get magical packages of equations whenever they need to do something.

1-chemistpackage

In the real world, it’s the baby that gives you the equations.

Fortunately, you’ve got this tutorial.  And fortunately, it’ll tell you what you need to know to predict the products of a chemical reaction.


Why we need to predict reaction products

As with many chemistry-oriented things, the skill of predicting reaction products is useless if you don’t actually use it for something.  Actually, that’s true for just about everything, whether chemistry-oriented or not.

2-uselesssoccer

And some things are just plain useless.

The reason chemists actually use this stuff is because they want to figure out how to make some particular chemical.  It’s not that we want to see what we’ll make if we put two chemicals together – that’s a stupid way to get things done.  Instead, we start with what we want to make and work our way backwards using the same idea.  This process is known as retrosynthesis.

Yep.  Predicting reaction products isn’t ever done.  However, the idea behind it is the basis for making a large percentage of the chemicals you use every day.  And that’s not useless.


What type of reaction is it?

When you learned the types of reaction in an early tutorial, this is again an example of information that doesn’t have any intrinsic value.  Knowing it has no value until you use it for something, and predicting the products of a reaction is this very important use.  See, you thought that was all a waste of time, didn’t you?

I’ll recap the types of chemical reaction, for those of you who don’t want to look it up:

  1. Combustion:  [some compound with C and H] + O₂→ CO₂ + H₂O.  This reaction is exothermic and the water is in the form of water vapor.
  2. Synthesis:  A + B → C.  Put another way, one compound is made from several others.
  3. Decomposition:  C → A + B.  Put another way, one compound breaks apart to form several others.
  4. Single displacement:  A + BC → B + AC.  A pure element switches places with an element in a chemical compound.  This is usually (but not always) a metal.
  5. Double displacement:  AB + CD → AD + CB.  The cations on two ionic compounds switch places.
  6. Acid-base:  HA + BOH → BA + H₂O.  This is a double displacement reaction that makes water.
3-bootsy

Like the last reaction, Bootsy Collins can always be depended upon to bring the bass.

Now that we know what all the reaction types are, let’s figure out how to figure out the products of each.

Before we start, however, I have to give you this very important warning:  If the hypothetical products of your chemical compounds aren’t valid (something like CaN or the like), then your answer will be wrong.  Make sure when you’re done that anything you write down is a valid and reasonable formula.


Combustion reactions:

Combustion reactions are pretty simple to identify because they start with an organic compound (something with C and H) and react with oxygen.  For example, if you were asked what type of reaction this is:

C₂H₂ + O₂ → 

You should expect that this is a combustion reaction.  And, like all combustion reactions, the products are carbon dioxide and water.  Once you’ve written those as products, finishing the process consists of balancing the equation:

2 C₂H₂ + 5 O₂ →4 CO₂ + 2 H₂O 

3-fire

It was a terrible tragedy, but it gives me peace to know the chemical reaction that killed my entire family.


Synthesis reactions:

If you see two very simple compounds followed by an arrow, it’s a synthesis reaction.  These compounds will either be pure elements, or they will be simple compounds like water, methane, ammonia, or any of the other little ones.

CO₂ + H₂O → ?

Your product, in this case, can be guessed in one of the following ways:

  • If two pure elements are combining, it’s some valid combination of the two.  When carbon and oxygen combine, it’s reasonable to assume that CO₂ is the product.  These products will often be easily-recognizable compounds like ammonia, carbon dioxide, hydrochloric acid, sodium chloride, and so on.
  • If other things are combining, just add the formulas together and see what you end up with.  For example, if you look at the example I gave, putting them together gives you the equation:

CO₂ + H₂O → H₂CO₃

Though you may not recognize this product as carbonic acid, you can tell from the formula that it’s a valid acid because an anion with a -2 charge is bonded to two H atoms with +1 charge.


Decomposition reactions:

This one is simple enough to identify that I won’t even give you a specific example.  If you see any single chemical formula in front of the arrow, it’s a decomposition reaction.  After all, what else could it be?¹

As for the products, you can go one of two ways:

  • Break it into its constituent elements.  If the compound on the reagent side of the equation is ionic, this is the way to go.
  • Break it into smaller, familiar molecules.  For example, if I were to reproduce the example above in decomposition form:

H₂CO₃ 

You could either assume it breaks into hydrogen gas, carbon, and oxygen gas (which is a messy combination of things) or you could look for small covalent molecules you know such as carbon dioxide and water:

H₂CO₃ CO₂ + H₂O

4-decomposition reaction

In rare occasions, it may be a decomposing cow. If you don’t know the answer to a decomposition problem, try writing “cow.”


Single displacement reactions:

The dead giveaway that something is a single displacement reaction is that you have a single element reacting with some other chemical compound.  In just about every case, this will be a pure metal reacting with an ionic compound:

Li + Fe(NO₃)

You might think the product would be lithium nitrate and iron metal, but single displacement reactions don’t always take place when you put two compounds together.  In fact:

In order for a single displacement reaction to occur, the pure metal has to be higher on the activity series than the metal already in a compound.

In this example, this means that the reaction won’t do anything unless lithium is higher on the activity series than iron.

Of course, the question that’s probably on your mind is “What the heck is he talking about?”  What the heck I’m talking about is the activity series, which is basically just a list of elements placed in order of how much they want to react with other things to be like the nearest noble gas.²  For example, if lithium were higher on the activity series than iron, this would mean that lithium wants to react a lot more than iron does, which would cause it to displace the iron in this compound.

Let’s take a look at a partial activity series:³

  K  > Na > Li > Ba > Ca > Mg > Al > Zn > Cr > Fe > Ni > Sn > Pb > Cu > Ag

Most active                                                                                                Least active

As you can see, lithium is higher on the activity series than iron, which tells us the reaction in our example will occur.  As a result, we’ll see the following reaction take place:

2 Li + Fe(NO₃) →Fe + 2 LiNO₃

Note that we had to write a valid formula for lithium nitrate – if we’d written Li(NO₃)₂, we’d be wrong because there’s no such compound.

Another example:  If you were to see the reaction:

Ni + CaO →

You should correctly assume that a single displacement reaction would occur.  However, because nickel is lower on the activity series than calcium, this reaction will not take place because calcium is far more stable as it is.  For this example, the right answer is no answer.


Double displacement:

If you see two ionic compounds sitting before you, your first thought should be to switch the cations to make two more ionic compounds.  Remember, however, that these compounds must be valid!

LiCl + Pb(NO₃)

should look to you like a double displacement reaction, where lead (II) chloride and lithium nitrate are the products:

2 LiCl + Pb(NO₃)→2 LiNO₃ + PbCl

This is a good instinct, but for one thing:  Just like single displacement reactions, double displacement reactions only occur under some circumstances.  In order for a double displacement reaction to occur, the following conditions must be met:

  • Both of the reagents must be soluble in water:  If you can’t dissolve the reagents, they won’t be able to react with each other.
  • One (and only one) of the products must be insoluble (a solid) in water.  If both are solids, there is a reaction but it consists of a bunch of gunk all mixed together, giving us unusable product.  If both are aqueous, then you just have a big ionic soup with no real compound at all.  For the reaction to be successful, one compound must be easily separated from the other, as is the case when one has dissolved and the other has not.

How do we tell if something dissolves in water?  We use a solubility chart!  There’s one right at the end of this sentence at this link!  And if we look at this chart, we can see that lithium chloride, lead (II) nitrate, and lithium nitrate are all soluble in water.  And lead (II) chloride is not!  As a result, the equation we predicted above will take place.

However, let’s say that we tried to combine sodium chloride with lithium fluoride:

NaCl + LiF →

Because these are two ionic compounds, you’d be right to assume that this is a double displacement reaction with the following equation:

NaCl + LiF → NaF + LiCl

Unfortunately, all of the compounds in this equation are soluble in water, so nothing happens.

Now, you might be asking yourself how it is that nothing happens.  After all, you put compounds together – shouldn’t they be doing something?

Nope.  When you dissolve one compound in water, the following process occurs:

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And the other compounds in water do the same thing:

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In other words, if you put the reagents into water, they’ll just create a bunch of ions that will never actually form new compounds.  Sure, they’ll bump into each other, but they don’t really want to form new compounds any more than they wanted to be in their old compounds.  No reaction will occur.


Acid-base reactions:

Finally some good news:  If you’ve got an acid and put it next to a base, they’ll undergo a reaction. It doesn’t really matter whether this one or that one is dissolved – you’ll just see a reaction.  Better yet, you’ll already know that one of the products is water!

But why does this happen when you can’t count on a reaction in a double-displacement reaction:

  • When an acid reacts with a base, the reaction forms water which flakes the base apart.  It doesn’t really matter whether the base is dissolved or not!
  • You don’t need one solid product and one dissolved product because one of the products will always be water!  If you want to make a compound and it’s a solid, just strain away the water. If you want to make a compound and it has dissolved in water, just boil the water away.  Either way, your product awaits.

So, let’s have a look at an acid base reaction.  As you might have guessed, if you have an equation that contains an acid and base as reagents, you’ll have an acid-base reaction:

HBr + NaOH → NaBr + H₂O

Don’t worry about anything else – just put it together and have a delicious Fresca soft drink to celebrate!

I think they might want to find a new ad agency.

I think they might want to find a new ad agency.

 


Footnotes:

  1. There are examples of reactions that have only one chemical formula, such as dimerizations and polymerizations (both synthesis reactions) and rearrangements.  However, you’re unlikely to bump into these much in a first-year chemistry class, so we won’t worry about them.
  2. It’s also called the “reactivity series”, so don’t panic if you hear this instead.
  3. This activity series doesn’t list everything – just the elements that you’ll probably run into.  You can find more complete activity series information on Wikipedia.

Image credits:

  • Baby chemist:  Image courtesy of luigi diamanti at FreeDigitalPhotos.net
  • Useless soccer:  Image courtesy of pal2iyawit at FreeDigitalPhotos.net
  • The legendary Bootsy Collins:  By Juanbobadilla (Own work) [Public domain], via Wikimedia Commons
  • My house burning down:  Image courtesy of Praisaeng at FreeDigitalPhotos.net
  • Decomposing cow:  By Jon Sullivan [Public domain], via Wikimedia Commons
  • Fresca ad:  Image of shrugging woman courtesy of David Castillo Dominici at FreeDigitalPhotos.net;  Fresca bottle by User:Cokewww [Public domain], via Wikimedia Commons
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