2021 P1 Q24 - Deduce Structure of Glycine at pH 13

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Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 24.

2021 p1 q24 question

We are required to assign the values for pKa1 and pKa2, and deduce the formula of glycine at pH 13.

From the equations given, we can tell the first dissociation for pKa1 is for the alpha acid group, and second dissociation for pKa2 is for the alpha amino group.

2021 p1 q24 assign pKa value

Hence the acid group should have a pKa1 value that is less than 7, while amine group should have a pKa2 value that is greater than 7.

We can then deduce that pKa1 should be 2.4 and pKa2 should be 9.8.

Therefore options C and D are eliminated.

2021 p1 q24 eliminate options C and D

Next we need to deduce the structure of glycine at pH 13.

Since pH 13 is greater than pKa1, solution is alkaline with respect to alpha acid hence acid will react to form conjugate base -COO-.

Since pH 13 is greater than pKa2, solution is alkaline with respect to alpha amine hence amine will not take part in any reaction.

The structure of glycine at pH 13 will therefore be H2N-CH2-COO-.

2021 p1 q24 deduce glycine at pH 13

Hence the answer to this question will be option B.

2021 p1 q24 answer

Topic: Nitrogen Compounds and Proteins, Organic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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