LessonI

What is a replacement reaction?

KEY TERMreplacement reaction: reaction in which one kind of matter replaces another kind

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LESSON16

What is a replacementreaction?

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Imagine that three children are playing.

Two are holding hands. The other is alone.

The child that was alone now joins the others.He takes the place of one of the children.

Now a different child is alone.

We have the same children that we started with. But now they arearranged in a different way.

Some chemical reactions work like this. A free element takes the place ofor replaces another element of a compound.

The element that was replaced is now "free." ,

A BC + BAC+free element compound new compound new free element

Let's study an actual replacement reaction-one between zinc (Zn) andhydrochloric acid (HCI).

The zinc is the" free" element. The hydrochloric acid is in the compound.

Zn 2HCl ZnCl2 + H2The hydrogen is set free.

+The zinc replaces the hydrogen.

The reaction produces a new compound, zinc chloride (ZnCI2),and freehydrogen (H2).Notice that the elements we started with are the elementswe ended with. They are just arranged in a different way.

This kind of reaction is called a single replacement reaction. In a singlereplacement reaction, a free element replaces a different element of acompound.

UNDERSTANDING SINGLE REPLACEMENT REACTIONS

What You Need (Materials)iron nail

copper sulfatesolution

Figure A

Figure B

iron nailcopper sulfate solutionbeaker

How To Do The Experiment(Procedure)

Place an iron nail in copper sulfatesolution.

Remove the nail in a few minutes.

What You Saw (Observations)

The nail is coated with copper.

This is the equation for the reaction.

I ,Fe + CuS04

-+ FeS04 + CuIron Copper sulfate Iron sulfate Copper

I •1. Name the free element we started with. _

2. Name the compound we started with. --------

3. Name the free element we ended with. _

4. Name the compound we ended with. _

5. a) Which element did the iron replace? _

b) What happened to this element? _

6. What do we call this kind of chemical reaction? _

7. What happens during a single replacement reaction? _

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IDENTIFYING SINGLE REPLACEMENT REACTIONS

Six equations are listed below. Some are single replacement reactions. Some are not. Mark a check(V') in the correct box next to each equation.

Single Not a singleEquation replacement replacement

reaction reaction

1. C + 2S ---+ CS2

2. H202---+ H2+02

3. 2AI + 6HCI ---+ 2AICl3 + 3H2

4. 2K + Cl2---+ 2KCl

- 5. Zn + PbO ---+ ZnO + Pb

6. Fe + CuS04---+ FeS04 + Cu

DOUBLE REPLACEMENT REACTIONS

AC

A single replacement reaction takes place between an element and a compound. The freeelement replaces one of the elements of the compound. This produces a new compoundand a new free element.

A BC+free element compound

+ Bnew compound new free element

A double replacement reaction takes place between two compounds. A part of onecompound changes place with a part of the other compound.

Let us use playing children as models again to see what happens.

Children A and B stand for compound AB.Children C and D stand for compound CD.

Child A changes place with child C.

What do we have now? Instead of compoundsAB and CD, we have two new compounds-CB and AD.When there are two change overs, a doublereplacement has taken place.

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Figure C

!

AA~IAIt]l~ c"1n(

Figure D

Figure E

Now let us study an actual double replacement reaction-the reaction between sodiumhydroxide (NaOH), and hydrochloric acid (HCl).

I •NaOH + HCI

+ I:> NaCI + HOH

• The sodium and hydrogen change places.

• Two new compounds form-NaCl (common table salt) and HOH (water).

Now you try. Read each equation carefully. Then answer the questions or fill in the blankswith each.

Equation I BaC12 +Barium chloride

Na2S04Sodium sulfate

BaS04 + 2NaCIBarium sulfate Sodium chloride

1. Name the reactants.

2. The reactants are _both elements, both compounds, an element and a compound

3. The barium changed places with the _sulfate, chlorine, sodium

4. Name the products. _

5. The products are ~~--_____:--:__--::__--_____:----both elements, both compounds, an element and a compound

6. What kind of chemical reaction is this? _

7. Double replacement is the reaction of two to form two new

Equation II AgN03

Silver nitrate+ NaBr ---+ AgBr +

Sodium bromide Silver bromideNaN03

Sodium nitrate

8. Name the reactants.

9. The reactants are _both elements, both compounds, an element and a compound

10. The silver changed places with thesulfate, chlorine, sodium

11. Name the products.

12. The products are _both elements, both compounds, an element and a compound

13. What kind of chemical reaction is this? _

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IDENTIFYING DOUBLE REPLACEMENT REACTIONS

Eight equations are listed below. Some are double replacement reactions. Some are not. Mark acheck (11') in the correct box next to each equation.

Double Not a doubleEquation replacement replacement

reaction reaction

1. Mg(OH)2 + 2HCl -+ MgCl2+2HOH

2. C6H100S + H2O -+ C6H1206

3. Na2S04 + BaCl2 -+ 2NaCl + BaS04

4. 3Mg + N2 -+ Mg3N2

5. H2S04 + BaCl2 -+ 2HCl + BaS04

6. ZnC03 -+ ZnO + CO2

7. CuS04 + H2S -+ H2S04 + CuS

8. NH4N03 -+ 2H20 + N20

IDENTIFYING CHEMICAL REACTIONS

Ten chemical equations are listed below. Identify each kind of reaction: synthesis, decomposition,single replacement, or double replacement.

Equation Kind of reaction

1. N2 + 3H2 -+ 2NH3

2. 2Br2 + 2H2O -+ 4HBr + 02

3. Mg + 2HCl -+ MgCl2 + H2

4. 2KBr + H2SO4-+ KzS04 + 2HBr

5. H2S03 -+ H20 + S02

6. Na2S + 2HCl -+ 2NaCl + H2S

7. 2Na + Iz -+ 2NaI

8. NaCl + AgN03 -+ NaN03 + AgCl

9. H2 + Cl2 -+ 2HCl

10. H2C03 -+ H20 + CO2

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