Question

8. (4 pts. total; 2 pts. each) in the space provided below, explain the following atomic properties. Be succinct. Electronegativity (X) Polarizability 9. (4 pts) Trifluoroacetic acid (CF3CO2H; pka = 0.23) and acetic acid (CH3CO2H;pka = 4.75) are monoprotic acids (meaning they can dissociate only one H ion/molecule of acid). Using your knowledge of structure, explain why the acid dissociation (K) of acetic acid is greater than that for trifluoroacetic acid. Place your answer in the space provided.

Answer 1

8)

a) Electronegativity is the ability of an atom to attract electrons towards itself in a bond. Eg- Fluorine and Oxygen are highly electronegative

b) Polarasizability is the tendency of an atom to release the electron.

9)

K_a is given as 10^-pKa

For acetic acid, pKa = 4.76. Thus, K_a= 10^-4.76 = 1.74 x 10^-5

For Tri Fluoro Acetic Acid, pKa= 0.23. Thus K_a = 10^-0.23 = 0.58

Thus, K_a of Tri Fluoro Acetic Acid is higher than the acetic acid.

Note that the question is slightly incorrect. The pK_a of acetic acid is higher but the K_a of Tri Fluoro acetic acid is higher.

Reason:

Tri Fluoro Acetic acid has a higher K_a than the Acetic Acid because Tri Fluoro Acetic Acid is a stronger acid. This is because the electronegative Fluorine atoms withdraws the electron away from the -O-H bond and weakens the bond, hence, release of H⁺ becomes easier. Also, the withdrawal of electron stabilizes the negative charge on the conjugate base formed.

Figure with arrow showing movement of electron in Tri Fluoro Acetic Acid.-

о о F F +-с +- С +о +н -F - C +- с +- о Н F F