Double Replacement Reactions: Trading Partners

There are many situations in life where items are traded or exchanged. Trades also occur in chemical reactions. In double replacement reactions, the positive ions exchange negative ion partners. A double replacement reaction is represented by the general equation

AB + CD → AD + CB

An example of a double replacement reaction is the reaction of iron sulfide with hydrogen chloride:

iron (II) sulfide + hydrogen chloride yields hydrogen sulfide + iron (II) chloride

FeS + 2HCl → H₂S + FeCl₂

In this activity you will compare buying popcorn at a concession stand to a double replacement reaction.

Materials

Five pink index cards; Five green index cards; One white index card

Activity

1. Prepare the index cards as follows:
   1. Write ''(A) Boy'' on one pink index card, ''(B) money'' on one pink card, ''(C) concession stand'' on one pink card, ''(D) popcorn'' on one pink card, and ''+'' on the last pink card. The pink cards represent the reactant side of the analogy.
   2. Write the exact same things on the five green cards. These are the product side of the analogy.
   3. Write → on the white index card.
2. Remember that the general equation for a double displacement reaction is AB + CD → AD + CB. With that in mind, here is your analogy:
   A boy at the movie theater takes his money to the concession stand to purchase popcorn. The boy leaves with his popcorn as the concession stand clerk places money in the register.
3. Using the analogy above, arrange the eleven index cards on your desk to show what happened.

Follow-Up Questions

1. How did you arrange the cards?
2. What were the positive ions in the analogy? What were the negative ions?

Answers

1. AB (pink) + (pink) CD (pink) → (white) AD (green) + (green) CB (green)
2. The positive ions were represented by pink A, pink C, green A, green C. The negative ions were pink B, pink D, green B, and green D.

Extension

Now that you see what happens in a double replacement reaction, predict the products of a double replacement reaction that has the following reactants:

copper (II) chloride + sodium sulfide → __________