7. Answer the following questions about the glycolate ion, C2H3O3-, which acts as a base in aqueous — Acid-Base Equilibria, Reaction Mechanisms Chemistry Question
Problem Context
- Answer the following questions about the glycolate ion, C2H3O3-, which acts as a base in aqueous solution. A Lewis diagram for the ion is provided.
A. On the Lewis diagram in part A, circle the atom that accepts the proton when the glycolate ion reacts with water.
Model Answer
Accept one of the following: The student must circle one of the two
oxygen atoms on the right side of the molecule (the scoring guidelines note:
"Because of resonance, the two C O− bonds on the right are equivalent.") [1].
When the glycolate ion reacts with water, it forms glycolic acid, HC2H3O3, according to the following equation. The Kb expression for the reaction is provided.
B. At 25°C, a 0.25 M solution of glycolate is found to have [OH-] = 1.3 × 10^-5 M.
i. Calculate the value of Kb for the glycolate ion.
Model Answer
Kb = [HC2H3O3][OH−] / [C2H3O3−] = (1.3 × 10−5)(1.3 × 10−5) / (2.5 −
1.3 × 10−5) ≈ (1.3 × 10−5)^2 / (2.5) = 6.8 × 10−11 [1].
ii. Using your answer to part B (i), calculate the value of Ka for glycolic acid at 25°C.
Model Answer
Ka = Kw / Kb = (1.0 × 10−14) / (6.8 × 10−11) = 1.5 × 10−4 [1].
Glycolic acid can be produced from the hydrolysis of methyl glycolate, C3H6O3. A proposed mechanism for the reaction is given.
Step 1: C3H6O3 + H3O+ ⇌ C2H5O3+ + CH3OH
Step 2: C2H5O3+ + H2O → HC2H3O3 + H3O+
Overall: C3H6O3 + H2O → HC2H3O3 + CH3OH
C. A student claims that H3O+ is a catalyst for the reaction. Do you agree or disagree? Justify your answer based on the mechanism given.
Model Answer
Agree. H3O+ is consumed in step 1 and regenerated in step 2, which is
consistent with the behavior of a catalyst [2].
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