Battle of the Acids

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

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 Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. 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 disposed of according to Flinn Suggested Disposal Method #24b.

Prelab Preparation

1. Hydrochloric acid and acetic acid are supplied in the kit as 2 M solutions. Dilute 1:1 with distilled or deionized water to prepare 1 M solutions needed for the demonstration. For example, add 200 mL of 2 M hydrochloric acid to water and dilute to 400 mL with additional distilled water.
2. 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 x 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 cylinders 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 H₃O⁺ 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?
8. Add 16 g of sodium acetate, followed by about 20 drops of “rainbow acid” universal indicator, to cylinder 4. Mix thoroughly 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.

Student Worksheet PDF

12042_Student1.pdf

Teacher Tips

• This kit 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 “rainbow 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

Sample Data

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

Answers to 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 common ion is added. Chloride is hydrochloric acid’s common ion, and acetate is acetic acid’s common ion. The addition of chloride ion to hydrochloric 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) + H₂O(l) → H₃O⁺ + Cl^–(aq)

b. Ionization of acetic acid in water

CH₃COOH(aq) + H₂O(l) → H₃O⁺ + CH₃COO^–(aq)

c. Calcium carbonate with H₃O⁺

2H₃O⁺(aq) + CaCO₃(s) → Ca²⁺(aq) + CO₂(g) + H₂O

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 H₃O⁺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.

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 (H₃O⁺) and their conjugate bases, chloride ion and acetate ion, respectively (Equations 1 and 2).

Comparing the pH of these two acids indicates that the amount of hydrogen ions produced in the two solutions is very different—there are more H₃O⁺ 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 H₃O⁺ ions. In acetic acid, however, only a few H₃O⁺ ions are produced (most of the CH₃COOH 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 H₃O⁺ ions is greater than in acetic acid. A “rainbow spectrum” of indicator color changes is observed as the pH changes and the reaction proceeds.

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—ionization 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 hydrogen ion concentration in the mixed acetic acid/sodium acetate solution is 100–1000X lower than in acetic acid itself. The reaction 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).

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.

References

Shakhashiri, B. Z. Chemical Demonstrations; The University of Wisconsin Press: Madison, 1989; Vol. 3, pp 155–157.