CR_Types of Chemical Reactions Lesson

Types of Chemical Reactions

In first year chemistry you probably learned the five basic types of chemical reactions:

• Synthesis (Combination)
• Decomposition
• Single Displacement
• Double Displacement (Metathesis)
• Combustion

The reason for knowing the types of reactions is so that you can predict the products that will be formed.

Synthesis (Combination)

A synthesis reaction is when two or more simple compounds combine to form a more complicated one. The general equation is:

$\color{blue}\text{A+B}\rightarrow \text{AB}$

In this case, A and B do not have to represent single elements (although they can). A and B can represent simple compounds or molecules as well.

One example of a synthesis reaction is the combination of iron and sulfur to form iron (II) sulfide:

$\text{8Fe+S}_8 \rightarrow \text{8FeS}$

Decomposition

A decomposition reaction is the opposite of a synthesis reaction. Here, a complex compound or molecule breaks down to make simpler ones. The general equation is:

$\color{blue}\text{AB}\rightarrow \text{A+B}$

One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:

$\text{2H₂O} \rightarrow \text{2H₂ + O₂}$

Decomposition can also split one compound into two simpler compounds (or compound and an element) as in these examples:

$\text{CaCO}_3 \rightarrow \text{CaO+CO}_2 \\ \text{Na}_2\text{CO}_3 \rightarrow \text{Na}_2 \text{O+CO}_2 \\ \text{KClO}_3 \rightarrow \text{KCl+O}_2 \\ \text{Ba(ClO}_3)_2 \rightarrow \text{BaCl}_2 \text{+ O}_2 \\$

In every case so far, there is only one substance on the left-hand (reactant) side. This is always the case in a decomposition reaction.

Single Displacement

In a single displacement (also called single replacement) reaction, an element trades places with another element in a compound. The general equation is:

$\color{blue}\text{A+BC}\rightarrow \text{AC+B}$

You can think of this as an old-fashioned dance. Visualize a couple dancing, and a single guy comes in a taps the guy dancing on the shoulder. He says, "May I cut in?" If he cuts in, a new dancing couple is formed, and now the dancing man is alone. This is what happens in a single displacement reaction! When a single displacement reaction occurs, only a metal can "cut in" or displace another metal. Only a non-metal can displace another non-metal.

One example of a single displacement reaction is when zinc metal replaces copper in copper II chloride to make magnesium copper metal and zinc chloride:

$\text{Zn+CuCl}_2 \rightarrow \text{ZnCl}_2\text{Cu}$

In this example, a metal (zinc) replaced another metal (copper).

 

Another example is the reaction of chlorine gas, which is diatomic, with sodium bromide:

$\text{Cl}_2\text{NaBr} \rightarrow \text{NaCl+Br}_2$

In this example, a non-metal (chlorine) replaces another non-metal (bromine).

Double Displacement (Metathesis)

Double displacement (also called double replacement) is when the anions and cations of two different compounds switch places, forming two entirely different compounds. This type of reaction can also be called a metathesis reaction. The general equation is:

$\color{blue}\text{AB+CD}\rightarrow \text{AD+BC}$

Think of the dance analogy we mentioned earlier. Now, visualize two couples dancing then they switch dance partners! When compounds switch partners, the switch is very specific. Only metals can switch with metals. Only non-metals can switch with non-metals.

One example of a double displacement reaction is the reaction of lead (II) nitrate with potassium iodide to form lead (II) iodide and potassium nitrate:

$\text{Pb(NO}_3)_2 \text{+2KI} \rightarrow \text{PbI}_2 \text{2KNO}_3$

Combustion

A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat.   Most commonly we talk about the combustion of a hydrocarbon, a molecule containing hydrogen and carbon in varying amounts.   When a hydrocarbon is burned with sufficient oxygen supply, the products are always carbon dioxide and water vapor. The general equation is:

$\color{blue} \text{C}_x \text{O}_2(g) \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2 \text{O}_(g)$

An example of this kind of reaction is the burning of napthalene:  

$C_{10}H_8+120_2 \rightarrow 10CO_2+4H_2O$

If the supply of oxygen is low or restricted, then carbon monoxide will be produced. This is why it is so dangerous to have an automobile engine running inside a closed garage or to use a charcoal grill indoors. This is often referred to as incomplete combustion.

When you see a statement that says something is burned, you should assume that complete combustion occurs unless told otherwise. This could be indicated by something like "insufficient oxygen".

Chemicals other than hydrocarbons can undergo combustion as well. This still involves the addition of oxygen, but the products will be different.

For example, the combustion of magnesium is:  

$Mg+O_2 \rightarrow 2MgO$

This particular example could also be classified as a synthesis reaction.

Remember to work on the module practice problems as you complete each section of content.

[CC BY-NC-SA 4.0 Links to an external site.] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.