At first glance these chemical reactions might seem like a mess of information, but there
are patterns among them.
It’s helpful for chemists to sort all the chemical reactions into categories so that
they can make predictions about how chemicals might react.
There are five main types of chemical reactions: combination which is also known as synthesis,
single replacement, double replacement, decomposition and combustion.
Now, not every reaction falls perfectly into one category, sometimes a reaction falls under
two categories.
But there are some key features to each category.
Let’s start with combination reactions.
Combination reactions combine the two or more substances to form a new single substance.
This example is elemental magnesium and oxygen combining to form magnesium oxide.
The two elements combined and formed one ionic compound.
Even though it made two units of magnesium oxide, they are the same substance.
So here we have the balanced chemical equation, a particulate drawing of that same reaction
and now here’s a general equation for combination reactions.
A + B yields substance AB.
The reverse version of this kind of reaction is called decomposition.
In this reaction, things fall apart instead of coming together.
A single substance breaks down into two or more simpler products.
Here is an example of hydrogen peroxide breaking down into water and oxygen.
Usually, decomposition reactions need heat, or fire or electricity to get started.
This reaction can happen quickly with a catalyst like yeast or manganese dioxide.
So we have the equation, a drawing and now a simple formula for decomposition, AB yields
A plus B.
The resulting compounds are smaller and simpler than the starting compound.
Let’s look at the next type of reaction Single Replacement.
In this reaction, one element replaces a second element in a compound.
We’ll use the example of zinc reacting with iron chloride to make zinc chloride and elemental
iron.
The two metals in this case, have swapped places.
So here we have an equation, drawing and now a simple formula for single replacement reactions,
A plus BC yields AC + B.
But not all single replacement scenarios actually work.
In order to see if the swap will occur, we need to reference a chart the activity series
of metals.
The more active metals are toward the top and the metals decrease in their activity
as you move to the bottom.
In the previous reaction, zinc was the individual element and was higher than iron on the activity
series.
So this reaction occurred.
But if we tried to react elemental iron with zinc chloride, nothing would happen, because
iron has a lower activity than zinc.
Now let’s look at the next type of reaction Double Replacement.
This reaction is similar to single replacement except we’re starting with two compounds.
the reaction has an exchange of positive ions between the two compounds.
In This example copper(II) chloride and hydrosulfuric acid swap their positive ions to make copper
sulfide and hydrochloric acid.
So here we have an equation, drawing and now a simple formula for double replacement reactions,
AB plus CD yields AD + CB.
You can see from this equation that A and C have switched places and those represent
the positive ions.
Usually a double replacement reaction creates one of three products a precipitate, molecular
substance or a gas.In this double replacement reaction, we started with two aqueous solutions
and end with aqueous hydrochloric acid but solid copper sulfate.
This will sink, or precipitate to the bottom of the container.
Another possible product would be a molecule.
In this example, we start with aqueous substances, but one of the products is a liquid, the water
and the other product is still aqueous.
Lastly, another possible product from a double replacement reaction is a gas.
Again, we start with aqueous substances, sodium cyanide and sulfuric acid, but form a gas
as a product: poisonous hydrogen cyanide gas.
Don’t try this one at home, folks.
The last type of chemical reaction is a fun one, combustion.
In combustion an element or compound, typically a hydrocarbon, reacts with oxygen, often producing
heat or light.
So here is the combustion of a hydrocarbon, called methane.
The combustion with a hydrocarbon will always make water and carbon dioxide as products.
This makes predicting the products really easy in this case!
So here we have an equation, drawing and now a formula for the combustion of a hydrocarbon.
The x and y variables here provide a shortcut for balancing the equation as well.
But not all combustion reactions happen with hydrocarbons.The combustion of magnesium in
oxygen is both a combustion reaction AND a combination reaction.
Remember, there are some reactions that can fit more than one type of reaction.Thanks
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