Question

The energy of an electrostatic interaction between two charged atoms is dependent on the charges on the atoms, the distance between them, and the dielectric constant of the solvent. For example, the strength of a weak acid (Ka, acid dissociation constant) depends on the strength of the electrostatic interaction between a negatively charged carboxylic acid group and a proton. The solvent dielectric constant has a large influence on the pKa for weak acids. Select the statements that correctly explain the influence of two solvents, water and hexane, on the pKa of acetic acid.

In water, the association of CH3COO– and H will be favored because charge-charge interactions are stronger in water.

Nonpolar solvents like hexane will weaken charge-charge interactions resulting in more dissociation of H .

The pKa will be higher in hexane.

Hexane cannot affect the pKa of acetic acid since hexane cannot donate or accept H .

More H will dissociate in water because the dielectric constant is higher.

Answer 1

Concepts and reason

This problem is based on the concept of acidity. Acidity of an acid depends upon the dissociation constant of that acid, upto which extent the acid on dissolving in a solvent releases the proton. This acidity mainly depends upon the dissociation and the solvent in which it is dissolved.

Fundamentals

The electrostatic force of attraction between any charged atoms mainly depends upon the charges on the atom and the distance between them, and most important factor which is generally not considered is the effect of solvent.

Solvent will promote the ionization of an acid due to several factors:

(a) When the solvent is protic, which is capable to form hydrogen bonds.

(b) When the solvent has a high dielectric constant (relative permittivity), makes them better solvent for the ionic species.

(c) Dissociation and solvent effect are interrelated.

Although the ionization of weak acids is very less, hence their acidity can be checked by the solvent. Two types of solvent mainly affect the acidity of weak acids: Protic solvent & Aprotic solvent.

In water, the association of ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CO}}{{\rm{O}}^ - }$ and ${{\rm{H}}^ + }$ will not be favored because the charge-charge interactions are not stronger in water. Similarly, non-polar solvents like hexane will not weaken charge-charge interactions resulting in more dissociation of H. And it can be seen that hexane can affect the ${\rm{p}}{K_a}$ of acetic acid as it cannot donate or accept H.

In case of other solvent, when hexane acts as nonpolar solvent. When the acid gets dissolved in this solvent, there is no such ion dipole interaction between the solvent molecule and the conjugate pairs. The relative permittivity of hexane at room temperature is 1.89 which is also very less in comparison to water. Hence the force of attraction between the conjugate pairs ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CO}}{{\rm{O}}^ - }$ and ${{\rm{H}}^ + }$ increases resulting the back formation of acid, ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}}$, which decreases the effective concentration of ${{\rm{H}}^ + }$ ions. So, the equilibrium will shift backwards. ${\rm{p}}{K_a}$ increases in case of hexane as the solvent, so the decrease in ${K_a}$ (equilibrium constant or ionization constant of an acid), decreases the acidity of an acid.

In case of water, the water acts as the protic solvent. As the acid get dissolve in it, these water molecules surround the respective ions of conjugate and the acid and maintains the ion dipole interaction in between water molecule and the ion. This newly formed ion dipole interaction decreases the electrostatic interaction between ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CO}}{{\rm{O}}^ - }$ and ${{\rm{H}}^ + }$ions. This increases the concentration of the conjugate base and the proton in the solvent, thus increases the acidity of the weak acid.

This ion dipole interaction of water is known dielectric nature of solvent, and promotes the increase in the acidity of acid.

Ans:

The ${\rm{p}}{K_a}$ will be higher in hexane.

More H will dissociate in water because the dielectric constant is higher.