4.3: Single and Double Replacement Reactions

Timothy Minger, Ph.D.

4.3: Single and Double Replacement Reactions

Learning Objectives

Recognize certain 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.

Before we begin, remember that we often write chemical equations showing the phases of matter of the reactants and products: (s) for solid, (l) for liquid, (g) for gas, and (aq) for a compound that is dissolved in aqueous solution (i.e. dissolved in water). Keep these notations in mind as you go through the following examples.

Single Replacement Reactions and the Activity Series

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. Here is an example of a single replacement reaction:

$2 HCl (aq) + Zn (s) \to {ZnCl}_{2}^{} (aq) + H_{2}^{} (g)$

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:

$2 NaCl (aq) + F_{2}^{} (g) \to 2 NaF (s) + {Cl}_{2}^{} (g)$

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. The elements that are higher in the column will replace the elements below them on the periodic table, but not the other way around. Thus, this reaction

${CaI}_{2}^{} (s) + {Cl}_{2}^{} (g) \to {CaCl}_{2}^{} (s) + I_{2}^{} (s)$

will occur, because chlorine is above iodine on the periodic table, but this reaction

${CaF}_{2}^{} (s) + {Br}_{2}^{} (ℓ) \to {CaBr}_{2}^{} (s) + F_{2}^{} (g)$

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

Example 4.3.1

Will a single replacement reaction occur? If so, identify the products.

a. MgCl₂ + I₂ → ?
b. CaBr₂ + F₂ → ?

Solution

a. Because iodine is below chlorine on the periodic table, a single replacement reaction will not occur.
b. 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 4.3.1

Will a single replacement reaction occur? If so, identify the products.

FeI₂ + Cl₂ → ?

Answer

Yes. Chlorine is above iodine on the periodic table and thus can replace it in a compound. The products are FeCl₂ and I₂.

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 higher up on the list will replace any element below it in compounds undergoing a single-replacement reaction. Elements will not replace elements above them in compounds.

Table 4.3.1: Activity Series for Element Replacement in Single-Replacement Reactions

Li
K
Ba
Sr
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H₂
Cu
Hg
Ag
Pd
Pt
Au

Example 4.3.2

Use the activity series to predict the products, if any, of each equation.

a. FeCl₂ + Zn → ?
b. HNO₃ + Au → ?

Solution

a. Because zinc is above iron in the activity series, it will replace iron in the compound. The products of this single replacement reaction are ZnCl₂ and Fe.
b. Gold is below hydrogen in the activity series. As such, it will not replace hydrogen in a compound with the nitrate ion. No reaction is predicted.

Exercise 4.3.2

Use the activity series to predict the products, if any, of this equation.

${AlPO}_{4}^{} + Mg \to$

Answer

Mg₃(PO₄)₂ and Al

Double Replacement Reactions and Solubility Rules

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:

${CuCl}_{2}^{} (aq) + 2 {AgNO}_{3}^{} (aq) \to Cu {({NO}_{3}^{})}_{2}^{} (aq) + 2 AgCl (s)$

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. Remember that when you recombine ions, you must do so in a way that creates a realistic chemical formula for an ionic compound – one where the positive and negative charges cancel each other out. You’ll adjust the exact numbers of each type of atom later when you balance the equation.

Example 4.3.3

Predict the products of this double replacement equation, then balance the equation:

${BaCl}_{2}^{} + {Na}_{2}^{} {SO}_{4}^{} \to$

Solution

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

${BaCl}_{2}^{} + {Na}_{2}^{} {SO}_{4}^{} \to$

which, when balanced, becomes

${BaCl}_{2}^{} + {Na}_{2}^{} {SO}_{4}^{} \to$

Exercise 4.3.3

Predict the products of this double replacement equation:

$KBr + {AgNO}_{3}^{} \to$

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.

To judge whether double replacement reactions will occur, we need to know what kinds of ionic compounds form precipitates. For this, we use solubility rules, which are general statements that predict which ionic compounds dissolve in water (are soluble) and which do not (are insoluble). (We will look at solubility in more detail in the next section.) Tables $4.3. 1$ list some general solubility rules. We need to consider each ionic compound (both the reactants and the possible products) in light of the solubility rules. If a compound is soluble, we use the (aq) label with it, indicating that it dissolves. If a compound is not soluble, we use the (s) label with it and assume that it will precipitate out of solution. If everything is soluble, then no reaction will be expected.

Table 4.3.2: Solubility Rules: Compounds That Are Soluble in Water

These compounds generally dissolve in water (are soluble):

Exceptions:

All compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and NH₄⁺

None

All compounds of NO₃⁻ and C₂H₃O₂⁻

None

Compounds of Cl⁻, Br⁻, I⁻

Ag⁺, Hg₂²⁺, Pb²⁺

Compounds of SO₄²

Hg₂²⁺, Pb²⁺, Sr²⁺, Ba²⁺

Table 4.3.3: Solubility Rules: Compounds That Are Insoluble in Water

These compounds generally do not dissolve in water (are insoluble):

Exceptions:

Compounds of CO₃²⁻ and PO₄³⁻

Compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and NH₄⁺ are soluble

Compounds of OH⁻

Compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, Sr²⁺, and Ba²⁺are soluble

For example, consider the possible double replacement reaction between Na₂SO₄ and SrCl₂. The solubility rules say that all ionic sodium compounds are soluble and all ionic chloride compounds are soluble, except for Ag⁺, Hg₂²⁺, and Pb²⁺, which are not being considered here. Therefore, Na₂SO₄ and SrCl₂ are both soluble. The possible double-replacement reaction products are NaCl and SrSO₄. Are these soluble? NaCl is (by the same rule we just quoted), but what about SrSO₄? Compounds of the sulfate ion are generally soluble, but Sr²⁺ is an exception: we expect it to be insoluble—a precipitate. Therefore, we expect a reaction to occur, and the balanced chemical equation would be:

${Na}_{2}^{} {SO}_{4}^{} (aq) + {SrCl}_{2}^{} (aq) \to 2 NaCl (aq) + {SrSO}_{4}^{} (s)$

You would expect to see a visual change corresponding to SrSO₄ precipitating out of solution (Figure 4.3.1).

A glass beaker with a blue solution contained within, precipitate has formed in the solution — Figure 4.3.1: Double-Replacement Reactions. Some double-replacement reactions are obvious because you can see a solid precipitate (the white solid at the bottom of the container) coming out of solution (the blue liquid). Source: Photo courtesy of Choij, http://commons.wikimedia.org/wiki/File:Copper_solution.jpg

Example 4.3.4

Will a double replacement reaction occur? If so, identify the products.

a. Ca(NO₃)₂ + KBr → ?
b. NaOH + FeCl₂ → ?

Solution

a. According to the solubility rules, both Ca(NO₃)₂ and KBr are soluble. Now we consider what the double-replacement products would be by switching the cations (or the anions)—namely, CaBr₂ and KNO₃. However, the solubility rules predict that these two substances would also be soluble, so no precipitate would form. Thus, we predict no reaction in this case.

b. According to the solubility rules, both NaOH and FeCl₂ are expected to be soluble. If we assume that a double-replacement reaction may occur, we need to consider the possible products, which would be NaCl and Fe(OH)₂. NaCl is soluble, but, according to the solubility rules , Fe(OH)₂ is not. Therefore, a reaction would occur, and Fe(OH)₂(s) would precipitate out of solution. The balanced chemical equation is:

$2 NaOH (aq) + {FeCl}_{2}^{} (aq) \to 2 NaCl (aq) + Fe {(OH)}_{2}^{} (s)$

Exercise 4.3.4

Will a double replacement reaction occur? If so, identify the products.

$Sr {({NO}_{3}^{})}_{2}^{} + KCl \to$

Answer

No; all possible products are soluble.

Key Takeaways

A single replacement reaction replaces one element for another in a compound.
The periodic table or an activity series can help predict whether single replacement reactions occur.
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.
Solubility rules are used to predict whether some double-replacement reactions will occur.

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Single Replacement Reactions and the Activity Series

Double Replacement Reactions and Solubility Rules

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