8.3 Classifying and Completing Single and Double Displacement Reactions

Indigenous Perspective: Natural Chemical Reactions

Chemical reactions related to Indigenous practices in Canada are used as a teaching tool, described within the document “Chemical Reactions: Background Information Science 10 [PDF]“.

The teaching tool was developed in collaboration with the Steward Resources Center, the Government of Saskatchewan and the Saskatchewan Teachers’ Federation.

Chemical reactions are involved in practices passed down from one generation to the next. This is important so that First Nations and Metis communities can make their own supplies by following the methods from previous generations. These practices include the process of making natural bleach, jellies, medicines, meals and household products.

 

 

Up to now, we have presented chemical reactions as a topic, but we have not discussed how the products of a chemical reaction can be predicted. Here we will begin our study of certain types of chemical reactions that allow us to predict what the products of the reaction will be.

A single displacement reaction is a chemical reaction in which one element is substituted for another element in a compound, generating a new element and a new compound as products. For example:

2 HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

is an example of a single displacement reaction. The hydrogen atoms in HCl are replaced by Zn atoms, and in the process a new element – hydrogen – is formed. Another example of a single displacement reaction is:

2 NaCl(aq) + F₂(g) → 2 NaF(s) + Cl₂(g)

Here the negatively charged ion changes from chloride to fluoride. A typical characteristic of a single displacement reaction is that there is one element as a reactant and another element as a product.

Not all proposed single displacement reactions will occur between two given reactants. This is most easily demonstrated with fluorine, chlorine, bromine, and iodine. Collectively, these elements are called the halogens and are in the next-to-last column on the periodic table (see Figure 8.3b). The elements on top of the column will replace the elements below them on the periodic table but not the other way around. Thus, the reaction represented by:

CaI₂(s) + Cl₂(g) → CaCl₂(s) + I₂(s)

This reaction will occur, but the reaction

CaF₂(s) + Br₂(ℓ) → CaBr₂(s) + F₂(g)

will not because bromine is below fluorine on the periodic table. This is just one of many ways the periodic table helps us understand chemistry.

Chemical reactivity trends are easy to predict when replacing anions in simple ionic compounds – simply use their relative positions on the periodic table. However, when replacing the cations, the trends are not as straightforward. This is partly because there are so many elements that can form cations; an element in one column on the periodic table may replace another element nearby, or it may not. A list called the activity series does the same thing the periodic table does for halogens: it lists the elements that will replace elements below them in single-replacement reactions. A simple activity series is shown below.

Using the activity series is similar to using the positions of the halogens on the periodic table. An element on top will replace an element below it in compounds undergoing a single displacement reaction. Elements will not replace elements above them in compounds.

A double displacement reaction occurs when parts of two ionic compounds are exchanged, making two new compounds. A characteristic of a double displacement equation is that there are two compounds as reactants and two different compounds as products.

An example of a double displacement reaction is:

CuCl₂(aq) + 2 AgNO₃(aq) → Cu(NO₃)₂(aq) + 2 AgCl(s)

There are two equivalent ways of considering a double displacement equation: either the cations are swapped, or the anions are swapped. (You cannot swap both; you would end up with the same substances you started with.) Either perspective should allow you to predict the proper products, as long as you pair a cation with an anion and not a cation with a cation or an anion with an anion.

Predicting whether a double displacement reaction occurs is somewhat more difficult than predicting a single displacement reaction. However, there is one type of double displacement reaction that we can predict: the precipitation reaction. A precipitation reaction occurs when two ionic compounds are dissolved in water and form a new ionic compound that does not dissolve; this new compound falls out of solution as a solid precipitate. The formation of a solid precipitate is the driving force that makes the reaction proceed.

For example, consider the double displacement reaction between Na₂SO₄ and SrCl₂.  The double displacement reaction products are NaCl and SrSO₄.  The balanced chemical equation is:

Na₂SO₄(aq) + SrCl₂(aq) → 2 NaCl(aq) + SrSO₄(s)

You would expect to see a visual change corresponding to SrSO₄ precipitating out of solution as noted by the (s) in the product formed within the chemical equation (Figure 8.3c).

Except where otherwise noted, this page is adapted by Adrienne Richards from “Chapter 4: Chemical Reactions and Equations: Types of Chemical Reactions: Single and Double-Displacement Reactions” In Introductory Chemistry: 1st Canadian Edition by David W. Ball and Jessica A. Key, licensed under CC BY-NC-SA 4.0.