Grade 9

Intermediate

7 min

Lesson 8 of 12

Double Replacement Reactions

Not Started

Study reactions where ions 'swap partners' to form new compounds, often resulting in a precipitate.

What if you could perform a 'magic trick' where mixing two clear liquids instantly creates a solid 'cloud' in a jar? This isn't magic—it's a molecular dance where atoms trade partners to create something entirely new.

Learning Objectives

1.Identify the general pattern of a double replacement reaction ( $AB + CD \rightarrow AD + CB$ )
2.Define a precipitate and explain how it forms when ions become 'stuck' together
3.Recognize neutralization reactions as a special type of swap involving acids and bases

The Molecular Dance

A double replacement reaction (also called a double displacement reaction) is like a dance where two couples switch partners. In these reactions, the positive ions (cations) and negative ions (anions) of two different ionic compounds trade places. The general formula is:

AB + CD \rightarrow AD + CB

In this formula,

A

and

C

are the positive ions, while

B

and

D

are the negative ions. For this reaction to occur, the reactants are usually dissolved in water, labeled as aqueous

(aq)

. The 'swap' only happens if it results in the formation of a solid, a gas, or a molecular compound like water. If everything stays dissolved, no real reaction has occurred!

Quick Check

In the reaction $XY + ZW \rightarrow XW + ZY$ , which letters represent the cations (positive ions) if we follow the standard pattern?

Cations are always written first in a chemical formula.

Answer

X and Z are the cations because they are written first in their respective compounds.

When Solids Appear: Precipitates

When two aqueous solutions are mixed, sometimes the new ion combination is insoluble, meaning it cannot stay dissolved in water. These ions stick together so strongly that they form a solid called a precipitate. You will see this in a lab as a cloudy substance or 'flakes' appearing in the liquid. We mark a precipitate in a chemical equation with the symbol $(s)$ for solid. If you mix two clear liquids and they turn cloudy, you are witnessing the birth of a precipitate! This happens because the attraction between the two new ions is stronger than the attraction of the water molecules trying to pull them apart.

Let's look at mixing Silver Nitrate and Sodium Chloride: 1. Identify Reactants: $AgNO_3(aq) + NaCl(aq)$ 2. Identify Partners: $Ag^+$ is with $NO_3^-$ ; $Na^+$ is with $Cl^-$ . 3. The Swap: $Ag^+$ joins $Cl^-$ and $Na^+$ joins $NO_3^-$ . 4. Write Products: $AgCl(s) + NaNO_3(aq)$ .

Because $AgCl$ is insoluble, it forms a white precipitate.

Neutralization: The Invisible Swap

A special type of double replacement is a neutralization reaction. This happens when an acid (which starts with

H^+

) reacts with a base (which usually ends with

OH^-

). Instead of a solid precipitate, the

H^+

and

OH^-

ions find each other and form liquid water (

H_2O

). The remaining ions form an ionic compound called a salt. The general form is:

Acid + Base \rightarrow Water + Salt

Because water is a very stable molecule, this reaction 'drives' the swap forward even if you don't see a solid forming. The 'cloudiness' isn't there, but the chemical change is powerful!

When you take an antacid, you are performing a neutralization reaction: 1. Reactants: Stomach acid ( $HCl$ ) and Sodium Hydroxide ( $NaOH$ ). 2. The Swap: $H^+$ pairs with $OH^-$ to make $H_2O$ . $Na^+$ pairs with $Cl^-$ to make $NaCl$ . 3. The Equation: $HCl(aq) + NaOH(aq) \rightarrow H_2O(l) + NaCl(aq)$ . 4. Result: The 'burning' acid and 'caustic' base are neutralized into harmless salt water.

Quick Check

What are the two products always formed in a neutralization reaction?

Answer

Water ( $H_2O$ ) and a salt (an ionic compound).

Predict the products for Lead(II) Nitrate and Potassium Iodide: 1. Reactants: $Pb(NO_3)_2(aq) + KI(aq)$ . 2. Identify Ions: $Pb^{2+}$ , $NO_3^-$ , $K^+$ , $I^-$ . 3. Swap and Balance Charges: $Pb^{2+}$ needs two $I^-$ to be neutral, forming $PbI_2$ . $K^+$ and $NO_3^-$ form $KNO_3$ . 4. Final Balanced Equation: $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$ .

*Note: $PbI_2$ is a brilliant yellow solid!*

Key Takeaways

1Double replacement reactions follow the pattern $AB + CD \rightarrow AD + CB$ .
2A precipitate is an insoluble solid $(s)$ that forms when certain ions bond together in solution.
3Neutralization is a specific double replacement between an acid and a base that produces water and a salt.
4The reaction only 'counts' if a product is a solid, a gas, or a molecular compound like water.

Quick Check

Multiple Choice

Which of the following represents the general form of a double replacement reaction?

Multiple Choice

What do we call the solid that sometimes forms during these reactions?

True/False

In a neutralization reaction, the products are always an acid and a base.

What's Next?

Review Tomorrow

In 24 hours, try to write down the general formula for a double replacement reaction and explain the difference between $(aq)$ and $(s)$ to a friend.

Practice Activity

Look at the ingredients on a bottle of Milk of Magnesia. It contains Magnesium Hydroxide. Can you figure out what salt it would form if it reacted with stomach acid ( $HCl$ )?

Browse

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Double Replacement Reactions

Learning Objectives

The Molecular Dance

When Solids Appear: Precipitates

Neutralization: The Invisible Swap

Key Takeaways

Quick Check

What's Next?

Double Replacement Reactions

Learning Objectives

The Molecular Dance

When Solids Appear: Precipitates

Neutralization: The Invisible Swap

Key Takeaways

Quick Check

What's Next?