6.5: Types of Chemical Reactions - Single and Double Replacement Reactions

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Learning Objectives

Recognize 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 until 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. Presented below:

\[\ce{2HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)}\nonumber \]

is an example of a single-replacement reaction. The hydrogen atoms in \(\ce{HCl}\) are replaced by \(\ce{Zn}\) atoms, and in the process a new element—hydrogen—is formed. Another example of a single-replacement reaction is

\[\ce{2NaCl(aq) + F2(g) → 2NaF(s) + Cl2(g)}\nonumber \]

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.

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

\[\ce{CuCl2(aq) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2AgCl(s)}\nonumber \]

There are two equivalent ways of considering a double-replacement 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.

Example \(\PageIndex{3}\)

Predict the products of this double-replacement equation:

\[\ce{BaCl2 + Na2SO4 → }\nonumber \]

Solution

Thinking about the reaction as either switching the cations or switching the anions, we would expect the products to be BaSO₄ and NaCl.

Exercise \(\PageIndex{3}\)

Predict the products of this double-replacement equation:

\[\ce{KBr + AgNO3 → }\nonumber \]

Answer: KNO₃ and AgBr

Predicting whether a double-replacement reaction occurs is somewhat more difficult than predicting a single-replacement reaction. However, there is one type of double-replacement 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.

Key Takeaways

A single-replacement reaction replaces one element for another in a compound.
A double-replacement reaction exchanges the cations (or the anions) of two ionic compounds.
A precipitation reaction is a double-replacement reaction in which one product is a solid precipitate.