Often, we describe weak acids and bases as not fully dissociating (also known as

Question

Often, we describe weak acids and bases as not fully dissociating (also known as ionizing) in water and strong acids and bases as fu dissociating in water. What is actually happening on a molecular level? Note: the answer options only mention strong and weak acid for simplicity In the case of strong acids, the salt that the hydrogen is bound to is always very soluble in water (ex. HCL, Cl-is very soluble). Because of this high solubility, the hydrogens are free to dissociate away from the salt and interact with the water. In the case of weak acids, the salts formed when the hydrogens dissociate are much less soluble. Because of this low solubility, many of the hydrogens do not dissociate in water, and remain attached to the acid A In the case of strong acids, the ion that is created when the hydrogen leaves is very stable. Because of this stability, it is favorable for the hydrogen to leave, since breaking that bond will release energy into the system. Weak acids, however, form a very unstable ion when the hydrogen leaves. This instability negates the energy gain that would happen from the breaking of the bond with the hydrogen, so it is less favorable to brealk. In the qpse of strong acids, the hydrogens are very mobile and able to interact with the water easily. Because of how accessible the hydrogen is to the water, the water can easily pull off the hydrogen, and will fully ionize the acid. In the case of weak acids, the water has less access to the hydrogen (the hydrogen is less mobile and somewhat inaccessible to the water). Because of this inaccessibility, the water cannot pull off many of the hydrogens, leading to incomplete ionization of the acid. c In the case of a strong acids, the ion that is created when the hydrogen leaves is very stable in water. Because this ion is so stable, the hydrogen is free to interact and bond with the water molecules instead of re-bonding to reform the acid. In the case of a weak acid, the ion that is created when the hydrogen leaves is not very stable in solution. This instability means that the hydrogen is more likely to associate with the ion and reform the acid instead of interacting and bonding with the water molecules. O D In the case of strong acids, the dissociation of the hydrogen is extremely favorable because the bond that will formed between the hydrogen and water is much more stable than the bond that will break between th hydrogen and the acids. For weak acids, the hydrogen is bound to the acid with a much stronger bond, making it harder for the water to pullitoff Because of this diffrence in bond strength, the strong acid will befully ionized be O E while the weak acid will be only partially ionized

Explanation / Answer

Ans. Correct option E. It is the ease and extent of H+ donation that differentiates an acid into strong and weak acid. Consider an example of HCl and HF acid strength. The proton is more strongly associated with F- than Cl- because F is more electronegative than Cl as well as a smaller atomic radius. Due to stronger bond, HF can NOT easily donate its proton. Comparatively, HCl can instantly release its proton due to relatively weaker association between Cl and H-atoms. Therefore, HCl is a strong acid whereas HF is a weak acid.

# Option A- Incorrect. H-atom is NOT associated to a salt (Cl- is NOT a salt).

# Option B. Incorrect. Weak acids are NOT very unstable. They are stable like strong acids, too.

# Option C- Incorrect. A proton, once released, is always very mobile (proton hopping) irrespective of its origin from strong or weak acid.

# Option D- Incorrect. Once released by the acid, the proton acts as an independent entity. It need not seek a re-union with the parent conjugate base molecule for any reason unless the equilibrium drives so. The equilibrium is affected by relative strength of the acid and [H+] in the acid.

For example, both HCl and HClO4 are strong acid. However, HClO4 is much stronger than HCl. So, when a small quantity of HClO4 is added to a solution of HCl (HCl is in 100% dissociated form initially in solution), some of Cl- bind to H+ to form HCl. That is, addition of HClO4 drives the equilibrium of HCl dissociation to the left side.

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