A 0.2000 mol L −1 solution of dichloroacetic acid (CHCl 2 COOH) has a pH of 1.107. Dichloroacetic acid is monoprotic. Calculate the K a for dichloroacetic acid. The following data apply to the ionisation of acetic acid (CH 3 COOH) and trichloroacetic acid (CCl 3 COOH). | pK a | CH 3 COOH | CCl 3 COOH | |---------------|-----------------|-------------------| | ΔH ° (kJ mol −1 ) | -0.1 | 1.2 | | ΔS (J K −1 mol −1 ) | -91.6 | -5.8 | | -TΔS (kJ mol −1 ) | 27.3 | 1.7 | | ΔG ° (kJ mol −1 ) | 27.2 | 2.9 | Explain the relative strength of these acids with reference to the data.

SSCE HSC Chemistry Alcohols: A 0.2000 mol L−1 solu

A 0.2000 mol L−1 solution of dichloroacetic acid (CHCl2COOH) has a pH of 1.107 - HSC - SSCE Chemistry - Question 35 - 2023 - Paper 1

Question 35

A 0.2000 mol L−1 solution of dichloroacetic acid (CHCl2COOH) has a pH of 1.107. Dichloroacetic acid is monoprotic. Calculate the Ka... show full transcript

Worked Solution & Example Answer:A 0.2000 mol L−1 solution of dichloroacetic acid (CHCl2COOH) has a pH of 1.107 - HSC - SSCE Chemistry - Question 35 - 2023 - Paper 1

Step 1

Calculate Ka for dichloroacetic acid

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Answer

To find the ionization constant Ka for dichloroacetic acid, we first need to determine the concentration of hydrogen ions [H+] provided by the pH.

Using the formula:

$[H^+] = 10^{-pH} = 10^{-1.107} = 0.0782 \text{ mol L}^{-1}$

Next, we can set up the equilibrium expression for the dissociation of dichloroacetic acid:

[ \text{CHCl}_2\text{COOH}(aq) \rightleftharpoons \text{H}^+(aq) + \text{CHCl}_2\text{COO}^-(aq) ]

We can create a table for the solution:

Species	Initial (mol L<sup>−1</sup>)	Change (mol L<sup>−1</sup>)	Equilibrium (mol L<sup>−1</sup>)
CHCl<sub>2</sub>COOH	0.2000	-0.0782	0.1218
H<sup>+</sup>	0	+0.0782	0.0782
CHCl<sub>2</sub>COO<sup>−</sup>	0	+0.0782	0.0782

Now, we can substitute these values into the Ka equation:

$K_a = \frac{[H^+][CHCl_2COO^-]}{[CHCl_2COOH]} = \frac{(0.0782)(0.0782)}{0.1218}$

Calculating this gives:

$K_a = 0.0501$

Step 2

Explain the relative strength of CH3COOH and CCl3COOH

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Answer

The pKa of trichloroacetic acid is lower than the pKa of acetic acid, indicating that trichloroacetic acid is a stronger acid than acetic acid.

A significant factor that contributes to this is the ΔS° data: both acids have different entropy changes, which affects their Gibbs free energy (ΔG°). The ΔG° values show that trichloroacetic acid has a much more favorable dissociation compared to acetic acid.

Furthermore, the strong electron-withdrawing effect of the three chlorine atoms in trichloroacetic acid stabilizes the negative charge on the conjugate base (CCl3</sub)COO−, making it a much stronger acid compared to CH3COOH, which has a less effective electron-withdrawing structure.

A 0.2000 mol L<sup>−1</sup> solution of dichloroacetic acid (CHCl<sub>2</sub>COOH) has a pH of 1.107 - HSC - SSCE Chemistry - Question 35 - 2023 - Paper 1

Worked Solution & Example Answer:A 0.2000 mol L<sup>−1</sup> solution of dichloroacetic acid (CHCl<sub>2</sub>COOH) has a pH of 1.107 - HSC - SSCE Chemistry - Question 35 - 2023 - Paper 1

Calculate K<sub>a</sub> for dichloroacetic acid

Explain the relative strength of CH<sub>3</sub>COOH and CCl<sub>3</sub>COOH

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