8.3 Classifying and Completing Single- and Double-Displacement Reactions
8.3 Classifying and Completing Single- and Double-Displacement Reactions
Learning Objectives
By the end of this section, you will be able to:
- Identify chemical reactions as single-replacement reactions and double-replacement reactions.
- Use the periodic table, an activity series, or solubility rules to predict whether single-replacement reactions or double-replacement reactions will occur.
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-replacement 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:
2HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)
is an example of a single-replacement 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-replacement reaction is:
2NaCl(aq) + F2(g) → 2NaF(s) + Cl2(g)
Here the negatively charged ion changes from chloride to fluoride. A typical characteristic of a single-replacement reaction is that there is one element as a reactant and another element as a product.
Not all proposed single-replacement 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 8.3a “Halogens on the Periodic Table”). 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.
Example 8.3a
Problems
Will a single-replacement reaction occur? If so, identify the products.
- MgCl2 + I2 → ?
- CaBr2 + F2 → ?
Solutions
- Because iodine is below chlorine on the periodic table, a single-replacement reaction will not occur.
- Because fluorine is above bromine on the periodic table, a single-replacement reaction will occur, and the products of the reaction will be CaF2 and Br2.
Exercise 8.3a
Will a single-replacement reaction occur? If so, identify the products.
FeI2 + Cl2 → ?
Check Your Answer[1]
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 near