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Replacement Reactions

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Replacement Reactions

What a job! The man hanging from the burning helicopter in the photo above is a stunt man. He fills in for the lead actor in the movie in dangerous scenes like this one. Like other stunt workers, he has special training to do such dangerous work as safely as possible. Replacing actors with stunt workers is common in action movies. Some chemical reactions also involve replacements. More reactive elements replace less reactive elements in compounds. These reactions are called replacement reactions.

What Is a Replacement Reaction?

A replacement reaction occurs when elements switch places in compounds. This type of reaction involves ions (electrically charged versions of atoms) and ionic compounds. These are compounds in which positive ions of a metal and negative ions of a nonmetal are held together by ionic bonds. Generally, a more reactive element replaces an element that is less reactive, and the less reactive element is set free from the compound. There are two types of replacement reactions: single and double. Both types are described below.

Q: Can you predict how single and double replacement reactions differ?

A: One way they differ is that a single replacement reaction involves one reactant compound, whereas a double replacement reaction involves two reactant compounds. Keep reading to learn more about these two types of reactions.

Single Replacement Reactions

A single replacement reaction occurs when one element replaces another in a single compound. This type of reaction has the general equation:

A + BC → B + AC

In this equation, A represents a more reactive element and BC represents the original compound. During the reaction, A replaces B, forming the product compound AC and releasing the less reactive element B.

An example of a single replacement reaction occurs when potassium (K) reacts with water (H 2 O). A colorless solid compound named potassium hydroxide (KOH) forms, and hydrogen gas (H 2 ) is set free. The equation for the reaction is:

2K + 2H 2 O → 2KOH + H 2

In this reaction, a potassium ion replaces one of the hydrogen atoms in each molecule of water. Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. You can watch the reaction occurring at this URL: http://commons.wikimedia.org/wiki/File:Potassium_water_20.theora.ogv

Q: Find potassium in the periodic table of the elements. What other element might replace hydrogen in water in a similar replacement reaction?

A: Another group 1 element, such as lithium or sodium, might be involved in a similar replacement reaction with water.

Double Replacement Reactions

A double replacement reaction occurs when two ionic compounds exchange ions. This produces two new ionic compounds. A double replacement reaction can be represented by the general equation:

AB + CD → AD + CB

AB and CD are the two reactant compounds, and AD and CB are the two product compounds that result from the reaction. During the reaction, the ions B and D change places.

Q: Could the product compounds be DA and BC?

A: No, they could not. In an ionic compound, the positive metal ion is always written first, followed by the negative nonmetal ion. Therefore, A and C must always come first, followed by D or B.

An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation:

NaCl + AgF → NaF + AgCl

During the reaction, chloride and fluoride ions change places, so two new compounds are formed in the products: sodium fluoride (NaF) and silver chloride (AgCl). To better understand how and why double replacement reactions occur, watch the video at this URL: http://www.brightstorm.com/science/chemistry/chemical-reactions/double-replacement-reactions/

Q: When iron sulfide (FeS) and hydrogen chloride (HCl) react together, a double replacement reaction occurs. What are the products of this reaction? What is the chemical equation for this reaction?

A: The products of the reaction are iron chloride (FeCl 2 ) and hydrogen sulfide (H 2 S). The chemical equation for this reaction is:

FeS + 2HCl → H 2 S + FeCl 2

Summary

  • A replacement reaction occurs when elements switch places in compounds. This type of reaction involves ions. Generally, more reactive elements replace less reactive elements.
  • A single replacement reaction occurs when one element replaces another element in one compound. This type of reaction is represented by: A + BC → B + AC.
  • A double replacement reaction occurs when two ionic compounds exchange ions, producing two new ionic compounds. This type of reaction is represented by: AB + CD → AD + CB.

Vocabulary

  • replacement reaction : Chemical reaction in which ions switch places in one compound (single replacement) or in two compounds (double replacement).

Practice

Examine the double replacement reactions at the following URL. Watch the movies of the reactions, and relate the molecular diagrams to the slides and still photos. Select one of the reactions, and write a paragraph describing what occurs during the reaction. Explain how you can tell that a chemical change has occurred, and identify which compound is the precipitate.

http://www.jce.divched.org/JCESoft/CCA/CCA1/R1MAIN/CD1R2360.HTM

Review

  1. What is a replacement reaction?
  2. Compare and contrast single and double replacement reactions, and give the general equation for each type of reaction.
  3. Which of the following reactions is not a replacement reaction?
    1. Fe + CuSO 4 → FeSO 4 + Cu
    2. 2Na + 2H 2 O → 2NaOH + H 2
    3. Zn + 2HCl → ZnCl 2 + H 2
    4. 2Na + Cl 2 → 2NaCl

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