What kind of chemical reaction is this?
Or this?
A chemical reaction is anything that causes a change
in the chemical composition of a substance.
There ARE signs you can look for, like a colour change,
or bubbles of gas released.
Some reactions are dramatic, like fires or explosions.
But other chemical changes take a long time, like how the
remains of plants and animals that lived millions of years ago
slowly change into fossil fuels.
These ARE different types of reactions, and it can be helpful
to know what TYPE of reaction it is, to have an idea
how long it’s going to take, or to predict what
the products of the reaction will be.
Chemists typically classify reactions into 5 basic types:
Synthesis,
Decomposition,
Single-displacement,
double displacement, and - everyone’s favourite:
combustion.
Some of these types of reactions can be split
into further categories - for instance,
precipitation reactions are a specific kind of
double-displacement reaction.
So are neutralization reactions, when an acid and base
react and cancel each other out.
Today we’ll learn how to classify reactions -
to recognize their general forms.
This also lets us predict the products of some reactions.
You’ll see.
We’ll pick a real-life example for each type of reaction.
If you learn these examples, you’ll be able to recognize
when a less familiar reaction is actually
a similar type of chemical change.
If you’re studying for a TEST about the types of
chemical reactions, we have a PRACTICE TEST for you,
complete with an answer key with explanations,
available for purchase on our website: socratica.com.
Type 1: Synthesis or Combination.
Synthesis Reactions are also known as Combination reactions
- they involve the combination of two or more simpler
chemical reactants into one larger, more complex product.
The general form of a synthesis reaction is A + B → AB
Here’s our rather surprising real life example: Rusting.
Well, it was surprising to me.
I don’t know about you, but I always thought of metal rusting
as being kind of a breaking down.
I was wrong.
It’s actually a kind of building up!
A metal reacts with oxygen and forms a new compound
that’s a combination of the two elements.
Our example reaction is Iron rusting -
Iron reacts with oxygen to form iron(III)oxide .
Just as a side note, this rusting process happens in the
presence of water, which allows for
easier transfer of electrons.
So one way to prevent rust is to keep your metal dry.
Do you recognize the general form of
a synthesis reaction here?
Iron is A, Oxygen is B,
and Iron (III) oxide {show formula Fe2O3} is AB.
The balanced chemical equation is
4Fe + 3O2 → 2Fe2O3.
Not sure how we balanced this equation?
We have a couple videos to help - one that teaches
the inspection method of balancing chemical equations,
and one that teaches the algebraic method.
You should watch them both, and see which
method works best for you.
You can find the links in the description below.
Type 2: Decomposition
The decomposition reaction type is the
reverse of the synthesis reaction -
a compound is broken into smaller pieces.
You may also hear this reaction type called
an “analysis reaction” because one way you can
ANALYZE a compound in the lab is by
breaking it up into its components.
Generally, since you’re breaking bonds,
decomposition reactions require an input of energy,
like heat or electricity, but there are some exceptions.
The general form of a decomposition reaction is AB → A + B.
An example of a decomposition reaction is the electrolysis
of water to form hydrogen gas and oxygen gas.
The word electrolysis should remind you that it
takes energy in the form of electricity to lyse, or cut,
the water into its components.
The balanced chemical equation is:
2H2O → 2H2 + O2.
Another real-world example you might have going on
in your medicine cabinet right now is the decomposition
of hydrogen peroxide into water and oxygen.
This happens slowly, WITHOUT the input of energy.
You can speed this decomposition process up if
you warm up the hydrogen peroxide or
leave it out in the light - that’s why hydrogen peroxide
comes in dark brown bottles.
When you buy a bottle of hydrogen peroxide at the store,
you’d like to be actually buying hydrogen peroxide,
and not just a bottle of water.
If your hydrogen peroxide has been sitting in
your medicine cabinet for a few years,
it’s probably time to replace it.
The balanced chemical equation for this
decomposition reaction is:
2H2O2 → 2H2O + O2.
Type 3: Single-displacement
Single displacement reactions can be recognized
by a kind of swapping that takes place.
One ion trades places with another.
They can be two cations (positively charged ions) or
two anions (negatively charged ions).
The general form of a single-displacement reaction is
A + BC → AC + B.
Here you can see that A and B swapped places.
It’s common to see single displacement reactions
when you put metals together in an aqueous solution.
So for instance, if you put a piece of copper into
a solution of silver nitrate, over the course of a few hours,
the copper and silver swap places.
The balanced chemical equation is:
2AgNO3 + Cu → Cu(NO3)2 + 2Ag.
But how do you know that they will do that?
What if we started with a copper (II) nitrate solution,
and stuck in a bar of silver.
Would the swapping still take place?
Cu(NO3)2 + 2Ag → ??
NO REACTION
We use a metal activity series table to predict
whether one metal will displace another.
This isn’t something anyone would expect you to reason out.
This is a table of data that was experimentally determined.
In general, a metal higher up in the activity series
will be able to replace a metal lower down.
So you can see copper will replace silver,
but not the other way around.
You can find this information in your textbook,
but to make it easier for you, we’ve made a free handout
that you can use when you solve these kinds of problems.
It also has a summary of the solubility rules,
which I’ll explain when we get to PRECIPITATION reactions.
Visit socratica.com to get your free handout -
we’ll include a link below.
Now we’ve been talking this whole time about metals,
and you should know that some single-displacement
reactions DON’T involve metals.
For example, the halogens are known to
take part in single-replacements.
Like the metal activity series, there is also
a halogen activity series -
and what makes it easy to remember is it’s the same order
as the halogens appear in the periodic table.
Elements higher up in the column will
replace elements below them.
So for instance, chlorine will replace bromine in this reaction:
Cl2 + 2KBr → 2KCl + Br2.
Type 4: Double-displacement
We just saw how in single displacement reactions,
one set of ions swap places.
In double displacement reactions,
TWO sets of components swap places.
You have two reactants swapping parts,
to form two new compounds.
These kinds of reactions follow the general form
AB + CD → AD + CB.
We commonly see this kind of reaction when you have
two ionic compounds in an aqueous solution.
VERY often, this results in the formation of a precipitate.
So let’s look at precipitation reactions as
a specific KIND of double-displacement reaction.
Precipitation is what happens when two ions join
together to form a compound that is insoluble in water.
This new compound precipitates out of solution.
How do you KNOW if a precipitate is going to form?
That’s when you’ll pull out your table of solubility rules.
Again, you can find this in your textbook, but
we’ve summarized the rules, along with
the metal activity series, on a free handout
you can find on our website, socratica.com.
So let’s look at a double-displacement reaction
where a precipitate forms:
Sodium hydroxide and calcium chloride react
to form sodium chloride and calcium hydroxide.
The balanced chemical equation is
2NaOH + CaCl2 → 2NaCl + Ca(OH)2
You can see how it matches the general form
of a double-displacement reaction:
AB + CD → AD + CB.
Are either of these products going to be solid in water?
Let’s look at the solubility rules.
Off the top of your head, you probably know that NaCl
is soluble in water, but to practice using the solubility rules,
here we see that chloride salts are usually soluble,
except for compounds with silver and
some compounds with mercury and lead.
So NaCl is not a precipitate - it’s in solution.
Calcium hydroxide, on the other hand, is insoluble.
From the solubility rules, we see that most hydroxides
are insoluble, with the exceptions including compounds
of alkali metals and ammonia.
We can add a downward arrow ↓ to calcium hydroxide
to show a precipitate is formed -
or we could write an s in parentheses, to stand for solid.
If you use that kind of label, then you’d go through and
label the other chemical species with (aq) in parentheses -
for aqueous, to show they’re all in solution.
Now remember, all precipitation reactions are
double displacement reactions, but not all
double-displacement reactions are precipitation reactions.
In other words, there are other kinds of
double-displacement reactions that don’t result in
a precipitate being formed.
Sometimes instead of a solid precipitate,
a gas is formed instead.
For instance, an old home remedy for
too much stomach acid is to have a little baking soda,
also known as sodium bicarbonate.
If you combine sodium bicarbonate with HCl,
it produces salt and carbonic acid (H2CO),
and then the carbonic acid quickly breaks down into
water and Carbon Dioxide.
The final balanced chemical equation is
NaHCO3 + HCl → NaCl + H2O + CO2↑
(notice the upwards arrow to show a gas is produced -
you can also write it as (g) in parentheses.
Acid-base reactions are another kind of
double-displacement reaction that doesn’t form a precipitate.
Let’s look specifically at neutralization reactions,
when an acid and base react to form water and a salt.
So the general form is acid + base → H2O + salt.
Writing it this way makes it hard to see
the double-displacement aspect, so here’s the
more general form of an acid-base reaction:
HA + BOH → H2O + BA.
If you write water as H-OH, that should help
make it very clear which parts are swapping.
The classic example is
HCl (acid) + NaOH (base) → H2O + NaCl.
I want to caution you to not oversimplify this in your mind.
In regular life outside the lab, we call NaCl “salt,” or
sometimes “table salt,” but it’s not the only salt
in the world of chemistry.
As an exercise, try writing out the reaction between
a different acid and base, like the acid
hydrogen bromide (HBr) reacting with
the base potassium hydroxide (KOH) .
What’s the salt formed by this reaction?
Type 5: Combustion
Combustion reactions involve a substance reacting
with oxygen gas, usually releasing
carbon dioxide and water.
Human beings have a long history of
being fascinated with fire, so we’re especially familiar
with things that CAN burn - wood, witches (just kidding),
and especially fuels like gasoline or propane
that are hydrocarbons.
We still use combustible materials in transportation
and cooking.
You might have a propane grill.
When you cook on the grill,
propane takes part in a combustion reaction -
here’s the balanced chemical equation:
C3H8 + 5O2 → 3CO2 + 4H2O.
The general form of this kind of combustion reaction is
hydrocarbon + O2 → CO2 + H2O.
Combustion reactions don’t strictly have to
have hydrocarbon as one of the reactants -
it’s just that they usually do.
The Hindenburg accident is an example of
Hydrogen gas combusting:
2H2 + O2 → 2H2O
notice, no carbon dioxide is produced here,
because hydrogen gas isn’t a hydrocarbon -
it doesn’t contain carbon.
Hydrogen gas is very flammable.
When the Hindenburg exploded, the entire airship
burned up in less than a minute.
This is why we’re not flying around
in hydrogen zeppelins anymore.
It’s very sad.
Socratica Friends, if you’re preparing to take a test
on this subject, my best advice is
to study what you DON’T KNOW.
And you can figure out what you don’t know by
taking a practice test.
We’ve made a practice test complete with an answer key
available for a reasonable price on our website, socratica.com.
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