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Ch.16 - Acid-Base Equilibria
All textbooksBrown 14th EditionCh.16 - Acid-Base EquilibriaProblem 114
Chapter 16, Problem 114

The amino acid glycine H₂N¬CH₂¬COOH can participate in the following equilibria in water: H₂N¬CH₂¬COOH + H₂O ⇌ H₂N¬CH₂¬COO⁻ + H₃O⁺ with Ka = 4.3 × 10⁻³, and H₂N¬CH₂¬COOH + H₂O ⇌ +H₃N¬CH₂¬COOH + OH⁻ with Kb = 6.0 × 10⁻⁵. (c) What would be the predominant form of glycine in a solution with pH 13? With pH 1?

Verified step by step guidance
1
Step 1: Understand the chemical equilibria involved. Glycine can act as both an acid and a base. The first equilibrium shows glycine donating a proton to water, forming its conjugate base (H₂N¬CH₂¬COO⁻) and hydronium ion (H₃O⁺). The second equilibrium shows glycine accepting a proton from water, forming its conjugate acid (+H₃N¬CH₂¬COOH) and hydroxide ion (OH⁻).
Step 2: Consider the pH 13 scenario. At pH 13, the solution is highly basic. In a basic environment, the predominant form of glycine will be its deprotonated form, which is the conjugate base (H₂N¬CH₂¬COO⁻). This is because the high concentration of OH⁻ ions will drive the equilibrium towards the formation of the conjugate base.
Step 3: Consider the pH 1 scenario. At pH 1, the solution is highly acidic. In an acidic environment, the predominant form of glycine will be its protonated form, which is the conjugate acid (+H₃N¬CH₂¬COOH). This is because the high concentration of H₃O⁺ ions will drive the equilibrium towards the formation of the conjugate acid.
Step 4: Use the given Ka and Kb values to understand the strength of the acid and base forms. The Ka value indicates the tendency of glycine to donate a proton, while the Kb value indicates the tendency to accept a proton. The larger Ka compared to Kb suggests that glycine is more likely to exist in its deprotonated form in basic conditions and protonated form in acidic conditions.
Step 5: Summarize the findings. At pH 13, glycine predominantly exists as H₂N¬CH₂¬COO⁻ due to the basic environment. At pH 1, glycine predominantly exists as +H₃N¬CH₂¬COOH due to the acidic environment. This understanding is based on the equilibria and the influence of pH on the protonation state of glycine.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Equilibria

Acid-base equilibria describe the reversible reactions between acids and bases in solution. In the context of glycine, it can act as both an acid (donating a proton) and a base (accepting a proton), leading to different ionic forms depending on the pH of the solution. The equilibrium constants, Ka and Kb, quantify the strength of these acid-base reactions.
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Arrhenius Acids and Bases

pH and Its Relation to Protonation States

pH is a measure of the hydrogen ion concentration in a solution, influencing the protonation state of molecules. At low pH (acidic conditions), amino acids like glycine tend to be protonated, while at high pH (basic conditions), they are more likely to be deprotonated. Understanding how pH affects the protonation states is crucial for predicting the predominant form of glycine in different environments.
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Neutron-Proton Curve Example

Zwitterions

Zwitterions are molecules that contain both positive and negative charges but are overall electrically neutral. Glycine, as an amino acid, can exist as a zwitterion at physiological pH, where the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻). The presence of zwitterions is significant in determining the behavior of amino acids in various pH conditions.
Related Practice
Textbook Question
Ritalin is the trade name of a drug, methylphenidate, usedto treat attention-deficit/hyperactivity disorder in youngadults. The chemical structure of methylphenidate is

(a) Is Ritalin an acid or a base? An electrolyte or a nonelectrolyte?
Textbook Question

The following observations are made about a diprotic acid H2A: (i) A 0.10 M solution of H2A has pH = 3.30. (ii) A 0.10 M solution of the salt NaHA is acidic. Which of the following could be the value of pKa2 for H2A: (i) 3.22, (ii) 5.30, (iii) 7.47, or (iv) 9.82?

Textbook Question

The amino acid glycine (H2N–CH2–COOH) can participate in the following equilibria in water:

H2N–CH2–COOH + H2O ⇌ H2N–CH2–COO– + H3O+ Ka = 4.3 × 10-3

H2N–CH2–COOH + H2O⇌ +H3N–CH2–COOH + OH- Kb = 6.0 × 10-5

(a) Use the values of Ka and Kb to estimate the equilibrium constant for the intramolecular proton transfer to form a zwitterion: H2N–CH2–COOH ⇌ +H3N–CH2–COO–

Textbook Question

The amino acid glycine (H2N–CH2–COOH) can participate in the following equilibria in water:

H2N–CH2–COOH + H2O ⇌ H2N–CH2–COO– + H3O+ Ka = 4.3 × 10-3

H2N–CH2–COOH + H2O⇌ +H3N–CH2–COOH + OH- Kb = 6.0 × 10-5

(b) What is the pH of a 0.050 M aqueous solution of glycine?