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A Level Chemistry 2017 Paper 1 Question 20 Solution - Exclusive

In this exclusive video we want to discuss the suggested solution for A Levels Chemistry (H2 Chemistry) 2017 Paper 1 Question 20.

Let's take a look at this question:

2017 H2 Chemistry Paper 1 Qn 20 question

The topic tested in this question is on the acidity of carboxylic acids.

From the pKa values we know that the acidity of the acids are in the following order:

(least acidic) CH3COOH < ClCH2COOH < FCH2COOH (most acidic)

Acidity of an organic compound is related to the stablity of its conjugate base.

The more stable the conjugate base, the more likely the dissociation of the weak acid to form the conjugate base, the more H+ is released hence the more acidic the weak acid.

For chloroethanoic acid, the electronegative Cl group is electron withdrawing which helps disperse the negative charge on oxygen of the carboxylate anion.

The conjugate base is more stable therefore chloroethanoic acid is more acidic and has a smaller pKa value than ethanoic acid.

2017 H2 Chemistry Paper 1 Qn 20 chloroethanoic acid

For fluoroethanoic acid, F is more electronegative than Cl which makes it a more electron withdrawing group.

The conjugate base will be stabilised to the greatest extent therefore fluoroethanoic acid is the most acidic and has the smallest pKa value.

2017 H2 Chemistry Paper 1 Qn 20 fluoroethanoic acid

Once we have the concept sorted out we can go through the 3 statements and decide that only statement 1 is valid. Hence the answer to this question is option C.

2017 H2 Chemistry Paper 1 Qn 20 compare statements and solution

Check out this video for the full solution and detailed explanation!

Topic: Carboxylic Acids, Organic Chemistry, A Level Chemistry, Singapore

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