Chemistry

Chemistry

Problem

Carbonic acid (H2CO3\text{H}_2\text{CO}_3) is a diprotic acid with pK_a1 = 6.37 and pK_a2 = 10.25. A closed aquatic system maintains three distinct regions with the following measured pH values: Region I: pH = 5.4 Region II: pH = 7.4 Region III: pH = 9.8 Assume that only the species H₂CO₃, HCO₃⁻, and CO₃²⁻ are present in appreciable concentration. a. For each region, identify the carbonic-acid–derived species that constitutes more than 50 % of the total carbonate species present. Provide reasoning for each region. b. Write balanced net ionic equations showing the acid–base transformations that interconvert the predominant species between Region I and Region II, and between Region II and Region III. c. Predict the sign (positive, zero, or negative) of the net charge on carbonic-acid–derived species that predominates in each region, and relate your prediction to the molecular formula of each species. d. Explain how the values of pK_a1 and pK_a2 enable the carbonate system to buffer the pH of natural waters, particularly near pH 7.4.
Intuitive explanation

Carbonic acid's two ionization steps create a pH-dependent equilibrium where different species dominate in different regions—this is the essence of a buffer system with distinct speciation zones. Your task is to use the Henderson-Hasselbalch equation with each pKapK_a value as a boundary to determine which form (H2CO3\text{H}_2\text{CO}_3, HCO3−\text{HCO}_3^-, or CO32−\text{CO}_3^{2-}) predominates in each region. The key insight: when pH is well below a pKapK_a, the protonated form wins; when pH is well above it, the deprotonated form wins; near the pKapK_a, both coexist significantly.

Intuitive explanation

Carbonic acid's two ionization steps create a pH-dependent equilibrium where different species dominate in different regions—this is the essence of a buffer system with distinct speciation zones. Your task is to use the Henderson-Hasselbalch equation with each pKapK_a value as a boundary to determine which form (H2CO3\text{H}_2\text{CO}_3, HCO3−\text{HCO}_3^-, or CO32−\text{CO}_3^{2-}) predominates in each region. The key insight: when pH is well below a pKapK_a, the protonated form wins; when pH is well above it, the deprotonated form wins; near the pKapK_a, both coexist significantly.

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