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Context for question 7

Answer the following questions about the glycolate ion, C

_{2}

_{3}

_{3}^{-}

, which acts as a base in aqueous solution. A Lewis diagram for the ion is provided.

When the glycolate ion reacts with water, it forms glycolic acid, HC

_{2}

_{3}

_{3}

, according to the following equation. The

K_{b}

expression for the reaction is provided.

C_{2} H_{3} O_{3}^{-} (aq) + H_{2} O (l) ⇌ H C_{2} H_{3} O_{3} (aq) + O H^{-} (aq)

K_{b} = \frac{[ H C _{2} H _{3} O _{3} ] [ O H ^{-} ]}{[ C _{2} H _{3} O _{3}^{-} ]}

7(A)Acids, Bases and Salts: Properties and TheoriesEasy1 mark

On the Lewis diagram in part A, circle the atom that accepts the proton when the glycolate ion reacts with water.

Context for 7(B)

At 25

^{\circ}

C, a 2.5 M solution of glycolate is found to have [OH

^{-}

] =

1.3 \times 1 0^{- 5}

7(B)(i)pH, Weak Acids, Buffers and Titration CurvesEasy1 mark

Calculate the value of

K_{b}

for the glycolate ion.

7(B)(ii)pH, Weak Acids, Buffers and Titration CurvesEasy1 mark

Using your answer to part B(i), calculate the value of

K_{a}

for glycolic acid at 25

^{\circ}

Context for 7(C)

Glycolic acid can be produced from the hydrolysis of methyl glycolate, C

_{3}

_{6}

_{3}

. A proposed mechanism for the reaction is given.

Step 1:

C_{3} H_{6} O_{3} + H_{3} O^{+} ⇌ C_{2} H_{5} O_{3}^{+} + C H_{3} OH

Step 2:

C_{2} H_{5} O_{3}^{+} + H_{2} O ⇌ H C_{2} H_{3} O_{3} + H_{3} O^{+}

Overall:

C_{3} H_{6} O_{3} + H_{2} O ⇌ H C_{2} H_{3} O_{3} + C H_{3} OH

7(C)Catalysis, Reaction Profiles and Enzyme KineticsMedium1 mark

A student claims that H

_{3}

^{+}

is a catalyst for the reaction. Do you agree or disagree? Justify your answer based on the mechanism given.