Battle of the Acids

Weak Acids and Bases

Introduction

Not all acids are created equal. This demonstration compares the “frothing and foaming” activity of different acids with

calcium carbonate and examines their behavior in the presence of their conjugate bases to distinguish strong versus weak

acids. The use of a “rainbow acid” universal indicator produces a rainbow spectrum of color changes as the mixtures react.

Concepts

• Strong acid • Weak acid

• Conjugate base • pH

Materials (for each demonstration)

Acetic acid, CH

COOH, 1 M, 400 mL Water, distilled or deionized

Calcium carbonate, CaCO

, 40 g Demonstration tray, large

Hydrochloric acid, HCl, 1 M, 400 mL Graduated cylinder, 250-mL

“Rainbow acid” universal indicator, 5 mL Hydrometer cylinders, 600-mL, 4

(includes accompanying color chart)

Pipet, Beral-type

Sodium acetate, NaCH

, 16 g

Weighing dishes or small beakers, 6

Sodium chloride, NaCl, 12 g

Safety Precautions

Hydrochloric acid and acetic acid solutions are toxic and corrosive. Avoid contact with skin and eyes. “Rainbow-acid” universal indicator

solution is an alcohol-based solution and is flammable. Avoid contact with flames or other ignition sources. Wear chemical splash goggles,

chemical-resistant gloves, and a chemical-resistant apron. Please review current Material Safety Data Sheets for additional safety,

handling, and disposal information.

Preparation

To save time in the presentation, pre-measure the amounts of solids needed for the demonstration. Weigh out 12 g of

sodium chloride, 16 g of sodium acetate, and 4 × 10-g samples of calcium carbonate in separate, labeled weighing dishes or

small beakers.

Procedure

1. Obtain 4 large hydrometer cylinders or tall-form beakers and place them on a large demonstration tray. Label the cyl-

inders #1–4.

2. Using a graduated cylinder, add 200 mL of 1 M hydrochloric acid to cylinders #1 and 2.

3. Using a graduated cylinder, add 200 mL of 1 M acetic acid to cylinders #3 and 4.

4. Add about 20 drops (1 mL) of “rainbow acid” universal indicator to cylinders #1 and 3. Compare the color and pH of

hydrochloric acid versus acetic acid.

5. Write equations for the ion-forming reactions of hydrochloric acid and acetic acid in water to give H

ions. Identify

the “common ion” or conjugate base of each acid (chloride ion and acetate ion, respectively).

6. Add 12 g of sodium chloride, followed by about 20 drops of “rainbow acid” universal indicator, to cylinder #2. Mix

thoroughly to dissolve.

7. Compare the color and pH of cylinder #2 with that in cylinder #1. What effect does adding chloride ion (its “common

ion” or conjugate base) have on the pH of hydrochloric acid?

Publication No. 91565

061616

SCIENTIFIC

Battle of the Acids continued

8. Add 16 g of sodium acetate, followed by about 20 drops of “rainbow acid” universal indicator, to cylinder #4. Mix thor-

oughly to dissolve.

9. Compare the color and pH of cylinder #4 with that in cylinder #3. What effect does adding acetate ion (its “common ion”

or conjugate base) have on the pH of acetic acid?

10. Add 10 g of calcium carbonate to each cylinder #1–4.

11. Compare the amount of frothing and foaming and observe the rainbow of indicator color changes in the four cylinders.

12. Relate the activity of the solutions to pH, the difference between strong and weak acids, and the “common ion” effect.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing

the disposal of laboratory waste. The waste solutions may be disposed down the drain with excess water according to Flinn

Suggested Disposal Method #26b. Excess hydrochloric acid and acetic acid may be saved for future use or neutralized and dis-

posed of according to Flinn Suggested Disposal Method #24b.

Tips

• Flinn Scientific offers a demonstration kit, Battle of the Acids—Strong versus Weak Acids Kit, (Catalog No. AP6287)

that contains enough chemicals to perform the demonstration as written seven times: 1.5 L each of 2 M hydrochloric

acid and acetic acid, 280 g of calcium carbonate, 120 g of sodium acetate, 100 g of sodium chloride, and 50 mL of “rain

bow acid” universal indicator.

• The use of a demonstration tray to catch any spillover (particularly from reactions #1 and 2) is strongly recommended.

The reaction mixtures bubble and froth and a solid wall of foam may erupt out of the hydrometer cylinders.

• “Rainbow acid” universal indicator is a new indicator solution that uses a combination of indicators to obtain a rainbow

spectrum of colors for acid solutions having pH values between 1 and 7. Use the color chart that accompanies the

indicator to estimate the pH of acidic solutions.

Discussion

The difference between strong and weak acids is confusing to many students. Some students assume that the difference arises

solely due to pH. Others attribute the difference to concentration. This demonstration compares the pH, activity, and “common

ion” effect in both hydrochloric and acetic acid solutions to illustrate the properties of strong versus weak acids.

Hydrochloric acid and acetic acid are Bronsted acids—they ionize in water to produce hydrogen ions (H

) and their conju-

gate bases, chloride ion and acetate ion, respectively (Equations 1 and 2).

HCl(aq) + H

O(l) → H

(aq) + Cl

–

(aq) Equation 1

COOH(aq) + H

O(l) → H

(aq) + CH

COO

–

(aq) Equation 2

Comparing the pH of these two acids indicates that the amount of hydrogen ions produced in the two solutions is very differ-

ent—there are more H

ions present in the hydrochloric acid solution than in acetic acid. In the hydrochloric acid solution,

all of the HCl molecules undergo ionization to form H

ions. In acetic acid, however, only a few H

ions are produced

(most of the CH

COOH molecules are not ionized). Note that this comparison—the relationship between pH and the degree

of ionization of hydrochloric acid versus acetic acid—is only valid because their initial concentrations are the same (1 M).

The activity of the two acids with calcium carbonate, a strong base, reinforces the pH comparison. The rate of reaction of

calcium carbonate with an acid (Equation 3) depends on the concentration of hydrogen ions in solution. The foam produced

in this reaction is due to carbon dioxide gas mixing with water and calcium carbonate powder. The amount of foaming and

the rate at which the foam rises are dramatically different for 1 M hydrochloric acid versus 1 M acetic acid. The reaction with

hydrochloric acid is significantly faster, suggesting again that the concentration of H

ions is greater than in acetic acid. A

“rainbow spectrum” of indicator color changes is observed as the pH changes and the reaction proceeds.

(aq) + CaCO

(s) → Ca

(aq) + CO

(g) + H

O Equation 3

Battle of the Acids continued

The effect of chloride ion and acetate ion on the pH and reactivity of hydrochloric acid and acetic acid, respectively, further

distinguishes the behavior of strong versus weak acids. Adding chloride ion to hydrochloric acid does not change either the pH

or the activity of the acid solution. This suggests that the reaction shown in Equation 1 takes place in one direction only—ion-

ization of hydrochloric acid is irreversible. Adding acetate ion to acetic acid, however, increases the pH of the solution from 2

to almost 5 and drastically slows down its reaction with calcium carbonate. Both of these observations suggest that the hydro-

gen ion concentration in the mixed acetic acid/sodium acetate solution is 100–1000× lower than in acetic acid itself. The reac-

tion shown in Equation 2 is thus effectively reversed in the presence of acetate, the “common ion.” Ionization of acetic acid is

reversible (Equation 4) and the equilibrium constant for this reaction is very small (approx. 10

–5

COOH(aq) + H

O(l)

←

→

(aq) + CH

COO

–

(aq) Equation 4

The following table summarizes the observations and conclusions in this demonstration. The concentration of the acid and

conjugate base components is 1 M in all cases.

Strong Acid Weak Acid

Example

Hydrochloric acid Acetic acid

“Rainbow Acid” Color

Red Yellow-Orange

≤ 1 2–3

Rate of Reaction with CaCO

Extremely fast Moderate

Ionization Reaction

Irreversible Reversible

Effect of Common Ion

≤ 1 (No change) 4–5

Rate of Reaction with CaCO

Extremely fast Slow

Connecting to the National Standards

This laboratory activity relates to the following National Science Education Standards (1996):

Unifying Concepts and Processes: Grades K–12

Systems, order, and organization

Constancy, change, and measurement

Content Standards: Grades 9–12

Content Standard B: Physical Science, structure and properties of matter, chemical reactions

Answers to Worksheet Data and Questions

Results Table. Properties of a Strong Acid (Hydrochloric Acid) and a Weak Acid (Acetic Acid)

Cylinder #1 Cylinder #2 Cylinder #3 Cylinder #4

Contents

1 M hydrochloric acid

1 M HCl + 2 g of

sodium chloride

1 M acetic acid

1 M acetic acid + 16 g

of sodium acetate

Color (pH)

Red (≤ 1) Red (≤ 1) Yellow-orange (2–3) Light green (4–5)

Reaction Rate

with CaCO

Fast Fast Moderate Slow

Reversibility of

Ionization Reaction

N/A Irreversible N/A Reversible

Battle of the Acids continued

Discussion Questions

1. Why was sodium chloride added to the hydrochloric acid in cylinders #1 and 2 and sodium acetate added to the acetic acid

in cylinders #3 and 4?

Sodium chloride and sodium acetate were added to hydrochloric acid and acetic acid, respectively, to show the effect on an acid when its com-

mon ion is added. Chloride is hydrochloric acid’s common ion, and acetate is acetic acid’s common ion. The addition of chloride ion to hydro-

chloric acid does not change the pH or reaction rate with calcium carbonate. The addition of acetate ion to acetic acid, on the other hand,

raises the pH and slows the reaction with calcium carbonate. This shows that the ionization reaction of a weak acid such as acetic acid is

reversible, while the ionization of a strong acid is not.

2. Write a balanced chemical equation for each of the following reactions:

a. Ionization of hydrochloric acid in water

HCl(aq) + H

O(l) → H

+ Cl

–

(aq)

b. Ionization of acetic acid in water

COOH(aq) + H

O(l) → H

+ CH

COO

–

(aq)

c. Calcium carbonate with H

(aq) + CaCO

(s) → Ca

(aq) + CO

(g) + H

3. Name two factors that distinguish a strong acid from a weak acid based on what you observed in this demonstration.

The ionization reaction of a strong acid takes place in one direction only, and is therefore irreversible. The ionization reaction of a weak

acid, however, is reversible. Thus, if the concentrations of a strong acid and a weak acid are equal, fewer H

are produced when a

weak acid is dissolved in water than when a strong acid is. This results in a higher pH for the weak acid.

4. Would this comparison between the strength of hydrochloric acid and acetic acid be valid if the hydrochloric acid was 2 M

and the acetic acid was 1 M? Explain.

No, because the initial concentration of the acid is a variable that affects the concentration of H

ions produced in water. Therefore, comparing

the pH of the solutions would reflect the concentration of the acids, rather than the strength of the acids.

Flinn Scientific—Teaching Chemistry

™

eLearning Video Series

A video of the Battle of the Acids activity, presented by Irene Cesa, is available in Weak Acids and Bases, part of the Flinn

Scientific—Teaching Chemistry eLearning Video Series.

Materials for Battle of the Acids are available from Flinn Scientific, Inc.

Materials required to perform this activity are available in the Battle of the Acids—Strong versus Weak Acids Kit available from

Flinn Scientific. Materials may also be purchased separately.

Catalog No. Description

AP6287 Battle of the Acids—Strong versus Weak Acids Kit

AP8599 Hydrometer Cylinder, 600-mL

AP5429 Demonstration Tray, Large

A0095 Acetic Acid Solution, 1 M, 1 L

C0347 Calcium Carbonate, Lab Grade, 100 g

H0057 Hydrochloric Acid Solution, 1 M, 1 L

U0012 Rainbow Acid Universal Indicator Solution, 100 mL

S0036 Sodium Acetate, Reagent, 100 g

S0061 Sodium Chloride, Reagent, 500 g

Consult your Flinn Scientific Catalog/Reference Manual for current prices.

Battle of the Acids continued

Battle of the Acids Worksheet

Results Table

Cylinder #1 Cylinder #2 Cylinder #3 Cylinder #4

Contents

Color (pH)

Reaction Rate

with CaCO

Reversibility of

Ionization Reaction

N/A N/A

Discussion Questions

1. Why was sodium chloride added to the hydrochloric acid in cylinders #1 and 2 and sodium acetate added to the acetic acid

in cylinders #3 and 4?

2. Write a balanced chemical equation for each of the following reactions:

a. Ionization of hydrochloric acid in water

b. Ionization of acetic acid in water

c. Calcium carbonate with H

3. Name two factors that distinguish a strong acid from a weak acid based on what you observed in this demonstration.

4. Would this comparison between the strength of hydrochloric acid and acetic acid be valid if the hydrochloric acid was 2 M

and the acetic acid was 1 M? Explain.