Classifying Chemical Reactions

By learning how to classify reactions, you will begin to recognize patterns that will make it easier to write chemical reaction equations.



B11.2 Examples of chemical reactions
The ability to classify reactions will give you the foundational skills needed to balance chemical reaction equations. As well, it is a stepping stone to predict products for chemical reactions when given the identity of the reactants.

You will be responsible for classifying five common types of reactions: formation, decomposition, single replacement, double replacement, and hydrocarbon combustion.

Simple formation reactions have elements reacting to produce one product.

Element + Element → Compound


For example, the formation of table salt from its constituent elements:

2Na(s) + Cl2(g) → 2NaCl(s)

A generalized model can be written as

A + B → AB

Or an alternate model is





Examples of more complex formation reactions can include one or more compounds reacting to form a single product, such as

2NO(g) + O2(g) → 2NO2(g)
This reaction has one reactant—a compound—that is broken down into elements.

Compound → Element + Element


For example, the decomposition of water into its constituent elements:

2H2O(l) → 2H2(g) + O 2(g)

A generalized model can be written as

AB → A + B

Or an alternate model is





Examples of more complex decomposition reactions can include one or more compounds being produced from a single reactant, such as

CaCO3(s) → CaO(s) + CO2(g)
This reaction has an element reacting with a compound. During the reaction, the commonly charged ions (the two positive ions or the two negative ions) swap out to produce a new element and a new compound.

Element + Compound → Element + Compound


For example, the reaction of copper metal with silver nitrate solution, as shown below. In this reaction, the two metal atoms swap places.

Cu(s) + 2AgNO3(aq) → 2Ag(s) + Cu(NO3)2(aq)

A generalized model can be written as

A + BC → B + AC

Or an alternate model is





This reaction has two compounds reacting to produce two new compounds.

Compound + Compound → Compound + Compound


Neutralization reactions are usually examples of double replacement.

HCl(aq) + NaOH(aq) → NaCl(aq) + HOH(l)

And usually precipitation reactions are examples of these as well.

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

A generalized model can be written as

AB + CD → AD + CB

Or an alternate model is




  Did You Know?

B11.3 water molecule

The chemical formula for water can be written as H2O(l) or HOH(l). When balancing a single replacement or double replacement reaction, it can be easier to write the chemical formula as HOH; that way, you can balance the hydroxide and the hydrogen as separate entities. You will learn more about this in Lesson 12.
You should recall from previous science courses that a fire triangle needs fuel, oxygen, and a source of ignition. In a hydrocarbon combustion reaction, a hydrocarbon—a compound composed of the elements hydrogen and carbon—reacts with oxygen to produce carbon dioxide and water. The water produced is usually a vapour.

For example, the combustion of propane (C3H8(g)) in a barbeque:

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)

A generalized model can be written as
CxHy(g) + O2(g) → CO2(g) + H2O(g)

hydrocarbon + oxygen → carbon dioxide + water


One key to being able to classify reactions is it to identify the reactants and products as elements or compounds. You also need to recognize that a hydrocarbon combustion reaction will always produce carbon dioxide and water vapour.

  Did You Know?

B11.4 Carbon monoxide detector

If there is a lack of oxygen when a hydrocarbon is undergoing combustion, deadly carbon monoxide can be produced in addition to carbon dioxide. Many homes are now equipped with CO detectors.


Examples


Classify each of the following chemical reaction equations as formation, simple decomposition, hydrocarbon combustion, single replacement, or double replacement. Each example has a video to go with it. To play the video, click on the play icon next to the example.

Compound + Element → Compound + Element
This is a single replacement reaction.  https://adlc.wistia.com/medias/7ldgb8ls6k
 



Hydrocarbon + Oxygen → Carbon Dioxide + Water Vapour
This is a hydrocarbon combustion reaction.  https://adlc.wistia.com/medias/ajkz8uyoyj
 

Compound → Element + Element
This is a simple decomposition reaction.  https://adlc.wistia.com/medias/084w9fzczt
 

Compound + Compound → Compound + Compound
This is a double replacement reaction.  https://adlc.wistia.com/medias/am2hbujy31
 

Element + Element → Compound
This is a formation reaction.  https://adlc.wistia.com/medias/eqd4t1xuoo
 


  Read This

Please read pages 91 to 101 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on recognizing and classifying types of chemical reactions. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice question to check your understanding of the concept you just learned. Make sure you write a complete answer to the practice question in your notes. After you have checked your answer, make corrections to your response (where necessary) to study from.

  1. Classify each of the following chemical reaction equations as formation, decomposition, hydrocarbon combustion, single replacement, or double replacement.

    1. P4(s) + 5O2(g) → P4O10(g)

      Element + Element → Compound
      This is a formation reaction.
    2. 2Na(s) + 2H2O(l)→ H2(g) + 2NaOH(aq)

      Element + Compound → Element + Compound
      This is a single replacement reaction.
    3. CH4(l) + 2O2(g) → CO2(g) + 2H2O(g)

      Hydrocarbon + Oxygen → Carbon Dioxide + Water
      This is a hydrocarbon combustion reaction.
    4. 2AgCl(s) → 2Ag(s) + Cl2(g)

      Compound → Element + Element
      This is a simple decomposition reaction.
    5. H2SO4(aq) + KOH(aq) → K2SO4(aq) + HOH(l)

      Compound + Compound → Compound + Compound
      This is a double replacement reaction.

  2. Classify each of the following diagrams as representing a formation, decomposition, single replacement, or double replacement chemical reaction.

    a.

    double replacement
    b.

    decomposition

    c.

    formation

    d.

    single replacement