Page 40 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry
Question 2

The acid dissociation constants, K a for benzoic acid and 4-nitrobenzoic acid are
given below:

Acid Benzoic acid 4-Nitrobenzoic acid
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K a /mol dm 6.4 × 10 –5 4.0 × 10 – 4
-3
With reference to the above, which of the following is correct?
A 4-Nitrobenzoic acid is a stronger acid than benzoic acid.
B 4-Nitrobenzoic acid is more stable than benzoic acid.
C The benzoate ion is more stable than the 4-nitrobenzoate ion.
D Benzoate ion is a weaker base compared to 4-nitrobenzoate ion.

Answer: A
The higher the K a value, the stronger is the acid, and the less stable is the acid
molecule.
The stronger the acid, the weaker is the conjugate base, and the more stable is
the conjugate base.
For example,
O 2 NC 6 H 4 COOH L O 2 NC 6 H 4 COO + H K a = 6.4 × 10 mol dm –3
–5
+
–
(Acid) (Conjugate base)
C 6 H 5 COOH L C 6 H 5 COO + H K a = 4.0 × 10 mol dm –3
+
–4
–
(Acid) (Conjugate base)
4-Nitrobenzoic acid is stronger than benzoic acid. That is, the degree of
dissociation of 4-nitrobenzoic acid is higher. This means that 4-nitrobenzoic
acid is less stable compared to benzoic acid.
On the other hand, the 4-nitrobenzoate ion is a weaker base than the benzoate
ion. This means that the tendency for 4-nitrobenzoate ion to combine with H
+
is less than that of benzoate ion. That is, the 4-nitrobenzoate ion is more stable
than the benzoate ion.
Section B Structured Questions

Question 3
Consider the four carboxylic acids below:

CH 3 COOH !COOH HO! !COOH O 2 N! !COOH
Term
3 I II III IV

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