Double Displacement Reactions

7-1

Experiment 7

Pre-Lab Assignment

Before coming to lab:

• Read the lab thoroughly.

• Answer the pre-lab questions that appear at the end of this lab exercise.

Purpose

A set of double displacement reactions will be performed and their products both predicted

beforehand and observed afterward. The results will be expressed appropriately in molecular,

total, and net ionic equations.

Background

A double displacement, or metathesis, reaction involves two ionic compounds “switch partners”.

The cation of the first molecule exchanges for the anion of the second, and the cation of the

second for the anion of the first in the general form in Eqn. 1.

AX + BY → AY + BX where A,B = cations and X,Y = anions Eqn. 1

A specific example of a double displacement reaction is shown in Eqn. 2 between magnesium

nitrate and sodium carbonate.

Mg(NO

3

)

2

(aq) + K

2

CO

3

(aq) → MgCO

3

(s) + 2 KNO

3

(aq) Eqn. 2

Here, Mg

2+

and K

+

are the cations (A and B) and NO

3

-

and CO

3

2-

are the anions (X and Y). Since

MgCO

3

(s) is an insoluble precipitate in aqueous solution, the reaction is also said to be a

precipitation reaction.

There are three different ways that double displacement reactions can be expressed. The first,

called the molecular equation, has all species written in their undissociated or ionic forms

where their overall charges are neutral. Phases and stoichiometric coefficients are also clearly

indicated. This is seen in Eqn. 2. This equation expresses the chemicals in the form that they are

labeled with and used in the laboratory.

The second way is called the complete or total ionic equation. In this equation, all species

are written in the form in which they predominately exist in solution. Insoluble compounds, such

as solid precipitates, weak electrolytes, or pure gases and liquids, are written in their

undissociated forms. Soluble compounds, such as strong electrolytes, are written in their

dissociated or ionic forms. Charges and phases are clearly written. Notice that the stoichiometric

coefficients remain to keep the equation balanced. Rewriting Eqn. 2 as a complete ionic equation

is shown in Eqn. 3.

Mg

2+

(aq) + 2 NO

3

-

(aq) + 2 K

+

(aq) + CO

3

2-

(aq) → MgCO

3

(s) + 2 K

+

(aq) + 2 NO

3

-

(aq) Eqn. 3

7-2

Remember that strong acids and bases are considered to be strong electrolytes, so would be

shown in the complete ionic equation in their dissociated ionic form. Weak acids are weak

electrolytes so should be written in molecular form.

The third way to write a double displacement reaction is called the net ionic equation. In this

form, only reacting ions and their product(s) are written exactly how they appear in the complete

ionic equation. Spectator ions are any ions that do not change throughout the course of the

reaction. These are not shown in the net ionic equation as they were not involved in the actual

reaction. The net ionic equation can be seen in Eqn. 4.

Mg

2+

(aq) + CO

3

2-

(aq) → MgCO

3

(s) Eqn. 4

The net ionic equation shows clearly that a reaction has occurred but does not indicate any

context. All three equations are needed to express a single double displacement reaction as each

tells a different layer of information.

If either product (AX or BY) from a double displacement reaction forms an insoluble precipitate,

the reaction is additionally classified as a precipitation reaction. The solid is called the precipitate

and the solution is the supernatant. A clear solution is one that does not contain any precipitate.

A colorless solution is a solution that is absent of color. The color of the precipitate may be

different than the color of the supernatant. Eqn. 4 indicates that the reaction between Mg(NO

3

)

2

and K

2

CO

3

is a precipitation reaction since MgCO

3

(s) is formed. The solubility of some ionic

compounds in aqueous solution is expressed in Table 1.

If either product (AX or BY) is a gas, it is called a gas evolution reaction. The most common gases

produced in gas evolution and double displacement reactions are CO

2

(g), SO

2

(g), NH

3

(g), and

H

2

S(g). The first three are produced via the immediate breakdown of gas evolution intermediates,

or compounds that, when formed via a double displacement reaction, undergo an immediate

second decomposition reaction. These are seen in Eqns. 5-7. Hydrogen sulfide (H

2

S(g)) is

naturally a gas in normal conditions.

H

2

CO

3

(aq) → H

2

O(l) + CO

2

(g) Eqn. 5

H

2

SO

3

(aq) → H

2

O(l) + SO

2

(g) Eqn. 6

NH

4

OH(aq) → H

2

O(l) + NH

3

(g) Eqn. 7

If the reaction produces heat (is exothermic) and a weak electrolyte, it is an acid-base

neutralization. Acids are compounds that release H

+

in aqueous solution and bases release OH

-

.

When combined, H

+

and OH

-

neutralize one another to make H

2

O(l), as in Eqn. 8. Since the

production of H

2

O(l) in aqueous solution cannot be easily observed, an increase in solution

temperature indicating an exothermic reaction will instead be used as evidence for a successful

acid-base neutralization.

H

+

(aq) + OH

-

(aq) → H

2

O(l) Eqn. 8

If both products (AY and BX) are soluble, then no reaction is said to have occurred and no net

ionic equation can be written as all ions will be spectators.

7-3

Example Problem: Writing Molecular, Total, and Net Ionic Equations

Write the molecular, total, and net ionic equation for the reaction between barium

carbonate and nitric acid.

Step 1: Write the molecular equation

Balance all charges and the reaction and indicate all phases by checking the solubility

table. Note: acids are always (aq).

BaCO

3

(s) + 2 HNO

3

(aq) → H

2

CO

3

(aq) + Ba(NO

3

)

2

(aq)

Since H

2

CO

3

is a gas evolution intermediate, it cannot be shown as a product.

BaCO

3

(s) + 2 HNO

3

(aq) → H

2

O(l) + CO

2

(g) + Ba(NO

3

)

2

(aq)

Step 2: Write the total ionic equation

Split all (aq) species into separate ions. Leave (s), (l), and (g) intact.

BaCO

3

(s) + 2 H

+

(aq) + 2 NO

3

-

(aq) → H

2

O(l) + CO

2

(g) + Ba

2+

(aq) + 2 NO

3

-

(aq)

Step 3: Write the net ionic equation

Cancel out spectator ions (ions that are unchanged from reactants

→

products).

Write only what is left.

BaCO

3

(s) + 2 H

+

(aq) → H

2

O(l) + CO

2

(g) + Ba

2+

(aq)

7-4

Table 1: Solubility of Ions in Aqueous Solution

C

2

H

3

O

2

-

AsO

4

3-

Br

-

CO

3

2-

Cl

-

CrO

4

2-

OH

-

I

-

NO

3

-

C

2

O

4

2-

O

2-

PO

4

3-

SO

4

2-

S

2-

SO

3

2-

Al

3+

aq

I

aq

-

aq

-

I

aq

-

I

aq

d

-

NH

4

+

aq

-

aq

Ba

2+

aq

I

aq

I

aq

I

s

aq

I

s

I

d

I

Bi

3+

I

s

d

I

d

-

I

D

I

aq

d

I

-

Ca

2+

aq

I

aq

I

aq

I

aq

I

d

I

Co

2+

aq

I

aq

I

aq

I

aq

I

aq

I

Cu

2+

aq

I

aq

I

aq

I

-

aq

I

aq

I

-

Fe

2+

aq

I

aq

s

aq

-

I

aq

I

aq

I

aq

Fe

3+

I

aq

I

aq

-

I

-

aq

I

aq

I

-

Pb

2+

aq

I

aq

I

Mg

2+

aq

I

aq

I

aq

I

aq

d

aq

Hg

2+

aq

I

aq

s

I

aq

I

d

I

-

K

+

aq

s

aq

Ag

+

aq

I

-

I

aq

I

Na

+

aq

Zn

2+

aq

I

aq

I

aq

I

aq

I

7-5

aq = soluble in aqueous solution

s

= slightly soluble in aqueous solution

I = insoluble in aqueous solution d = decomposes in water - = compound does not form

7-6

Procedure

Part I: Observing Double Displacement Reactions

1. Using 10 clean, dried small test tubes, measure 20 drops of Solution #1 from Reaction Mixtures

1-5 in the first five test tubes, and then 20 drops of Solution #2 from Reaction Mixtures 1-5 in

the second five test tubes. Be sure to clearly label all ten test tubes.

2. Record the initial temperature for all 10 test tubes prepared in Step 1. Be sure to thoroughly

wash your thermometer with deionized water before using it in the next solution.

Note: Step 3 must be done quickly.

3. For Reaction Mixture 1, add Solution #1 to Solution #2 and shake. Note any precipitate,

evolution of a gas (formation of small bubbles), or color change in your data sheet. If a precipitate

does form, note the color. Record the final temperature of the reaction mixture.

4. Based on your observations, determine whether or not a reaction occurred. If it did, classify it

in as many ways as possible and write the molecular, total ionic, and net ionic equations. If no

reaction occurred, write “no reaction”. You do not have to write the equations.

4. Repeat Steps 3-4 for the remaining Reaction Mixtures 2-5.

5. Empty your test tubes in the appropriate waste container. Wash them with deionized water

and dry them.

6. Repeat Steps 1-5 for the solutions listed for Reaction Mixtures 6-10.

Reaction Mixture

Solution #1

Solution #2

1

1.0 M sodium carbonate

1.0 M hydrochloric acid

2

1.0 M potassium hydroxide

1.0 M hydrochloric acid

3

0.1 M sodium chloride

1.0 M sodium carbonate

4

0.1 M lead(II) nitrate

0.1 M sodium iodide

5

0.1 M sodium sulfate

0. 1 M barium chloride

6

1.0 M sodium carbonate

0.1 M calcium chloride

7

1.0 M potassium hydroxide

0.3 M magnesium nitrate

8

0.1 M sodium sulfate

0.1 M copper(II) nitrate

9

1.0 M hydrochloric acid

1.0 M potassium bicarbonate

10

0.3 M strontium nitrate

0.3 M potassium iodate

Part II: Writing Double Displacement Reactions

7-7

1. Complete the double displacement reactions provided on your data sheet, writing the

molecular, total ionic, and net ionic equation for each. If no reaction occurs, write “no reaction”.

You do not need to perform these reactions.

7-8

Experiment 7—Data Sheet

Name: ________________________________________

Part I: Observing Double Displacement Reactions

Write the ionic formulas for the chemicals you will be using today.

barium chloride ___________

calcium chloride ___________

copper(II) nitrate ___________

hydrochloric acid ___________

lead(II) nitrate ___________

magnesium nitrate ___________

potassium bicarbonate ___________

potassium hydroxide ___________

potassium iodate ___________

sodium carbonate ___________

sodium chloride ___________

sodium iodide ___________

sodium sulfate ___________

strontium nitrate ___________

7-9

6-10

Reaction Mixture #1

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

Reaction Mixture #2

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

6-11

6-12

Reaction Mixture #3

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

Reaction Mixture #4

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

6-13

6-14

Reaction Mixture #5

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

Reaction Mixture #6

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

6-15

6-16

Reaction Mixture #7

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

Reaction Mixture #8

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

6-17

6-18

Reaction Mixture #9

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

Reaction Mixture #10

Initial Observations (include T)

Solution #1:

Solution #2:

Did a reaction occur? List your observations to support your conclusion.

If so, what type of reaction?

ME

:

TIE

:

NIE

:

6-19

6-20

Part II: Writing Double Displacement Reactions

1. AlCl

3

(aq) + NH

4

OH(aq) →

2. Ca(HCO

3

)

2

(aq)

+ HI(aq) →

3. Na

2

CO

3

(aq) + CuSO

4

(aq) →

4. CaCO

3

(s) + HCl(aq) →

5. K

3

PO

4

(aq) + AgNO

3

(aq) →

6. HNO

3

(aq) + Ba(OH)

2

(aq) →

6-21

6-22

7. NaOH(aq) + NH

4

Cl(aq) →

8. MgSO

3

(aq) + HCl(aq) →

9. BaCl

2

(aq) + Na

3

PO

4

(aq) →

10. Fe

2

(SO

4

)

3

(aq) + NaOH(aq) →

11. MgSO

4

(aq) + Cu(NO

3

)

2

(aq) →

12. Na

3

AsO

4

(aq) + FeCl

3

(aq) →

6-23

6-24

13. NaNO

3

(aq) + FeCl

3

(aq) →

14. HCl(aq) + KC

2

H

3

O

2

(aq) →

15. Mg(OH)

2

(s) + H

2

SO

4

(aq) →

6-25

6-26

Experiment 7—Post-Lab Assignment

1. Scientists often use their knowledge of precipitation reactions to help them identify unknown

liquids or to separate particular ions out of mixtures. A scientist receives two test tubes, each of

which contains two aqueous solutions. What new third solution could be added to each test tube

that would react with one solution but not the other? In each case, describe any observations the

scientist should expect to see and write the molecular, total, and net ionic equations for any

reaction that occurs.

a. potassium nitrate and barium nitrate

b. sodium chloride and hydrochloric acid

2. A student was given five known solutions and asked to observe their interactions in double

displacement reactions. Based on your experiment, fill in the table below with the expected

observations for each pair of solutions. Use “NR” to indicate no reaction, “ppt” for precipitate,

and “gas” for any bubbles observed.

NaCl

HCl

Na

2

CO

3

Na

2

SO

4

Ba(NO

3

)

2

Ba(NO

3

)

2

Na

2

SO

4

Na

2

CO

3

HCl

NaCl

6-27

6-28

The student was then given a set of the same five solutions as the table above, but the labels for

each had been replaced with “A, B, C, D, and E” and their order had been changed. When the

five solutions were mixed, the student recorded the following observations.

A

B

C

D

E

NR

white ppt

gas

D

NR

gas

C

white ppt

NR

white ppt

B

NR

A

NR

white ppt

NR

Identify the solution in each test tube.

Solution A ____________________________________

Solution B ____________________________________

Solution C ____________________________________

Solution D ____________________________________

Solution E ____________________________________

Write the molecular, total ionic, and net ionic equation for each unique pair of solutions from the

table above that resulted in a successful double displacement reaction.

6-29

6-30

Experiment 7—Pre-Lab Assignment

Name: ________________________________________

For all calculations, show all work and draw a box around the final answers.

1. Classify the following solutions as a clear solution, a colorless solution, both, or neither.

a. The supernatant of a reaction mixture has no color and contains a blue precipitate.

b. The supernatant of a reaction mixture is orange and contains a white precipitate.

c. The supernatant of a reaction mixture has no color and contains no precipitate.

d. The supernatant of a reaction mixture is blue and contains no precipitate.

2. When a blue copper(II) nitrate solution is mixed with a colorless sodium sulfide solution, a

black precipitate is formed.

a. What is the chemical formula of the black precipitate? _______________________

b. Write the molecular equation for this reaction.

c. Write the total ionic equation for this reaction.

d. Write the net ionic equation for this reaction.

3. Will a successful double displacement reaction occur if a potassium hydroxide solution is mixed

with lead(II) nitrate solution? Why or why not? If a reaction does occur, write the net ionic

equation for the reaction.