Weak Acids and Bases

From MyMCAT

Introduction

Where strong acids and bases are completely ionized when dissolved in water, weak acids and bases only partially ionize in solution and thus form equilibriums. These concepts have in fact already been discussed in terms of K_a, K_b, pK_a and pK_b, however here we will take a more detailed look at them and try to understand their unique properties.

Acid and Base Strength

If acidity is based on the amount of free hydronium ions, then acids that dissociate more than others must be more acidic. This can be seen in K_a values as K_a values represent the equilibrium constant. Thus, the larger the K_a, the greater the concentration of the products and the greater the concentration of free hydronium ions.

All acids not considered to be strong (like HCl, HBr, etc) have a K_a and are considered weak acids (even if some are slightly stronger than others). The exact same principles apply to weak bases.

Some examples of weak acids include: formic adic (HCOOH), acetic acid (CH₃COOH), CCl₃COOH, HF (Note this acid is an exception to all the other halogen acids which are strong!), HCN, H₂S, and H₂O.

Some examples of weak bases include: ammonia (NH₃), N(CH₃)₃, NH₄OH, the HS- ion, and H₂O. (Note how water can act as both a weak acid and weak base.)

Acetic Acid, K_a = 5.5 x 10⁴

Benzoic Acid, K_a = 1.6 x 10⁴

Cyanic Acid, K_a = 2.9x 10³

Hypochlorous Acid, K_a = 3.5 x 10⁷

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To identify the strongest of the weak acids, all we need to do is to find the acid with the strongest Ka. Hypochlorous Acid fits this requirement, note that we do not need to know anything about their structures, the Ka is enough.

Hydrofluoric Acid

Hydrochloric Acid

Phenol

Benzoic Acid

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In asking which one does not have a Ka value, we are really asking which one is either a strong acid (because they do not have Ka values) or not an acid at all. In this case, they can ALL act as acids (Phenol and Benzoic acid are both beneze derivates with an alcohol and carboxylic group respectively, both of which have acidic character). Because Phenol and Benzoic acid are aromatic organic molecules they will most likely be weak, most organics are. Thus if we look at hydrofluoric acid and hydrochloric acid we are comparing HF and HCl. HCl is a strong acid and thus this is our answer, HF however while in the same group of halogen acids is an exception to the rule and is not strong!

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Conjugate Acids and Bases

A natural consequence of the Brønsted-Lowry definitions of acids and bases, is the relationship between acids and bases. If an equilibrium exists in the acid dissociation reaction, then we can consider the HA molecule to be the acid and the A^- ion to be its conjugate base. This base, is free to accept H⁺ to turn back into the acid, because the whole system is in an equilibrium. Thus any acid will always have an associated conjugate base along with it, and vice versa for bases having a conjugate acid.

If we are considering a strong acid, then the ability of its conjugate base to regain a H⁺ to reform the acid is very limited and we say the conjugate base is weak. Thus the stronger the acid, the weaker the conjugate base. Similarly, the stronger the base, the weaker the conjugate acid!

If we extend this all the way to the "official" strong acids (like HCl, HBr, etc), then we can consider their conjugate bases (Cl^-, Br^-, etc) to be so weak that they cannot react back again, this is why we get 100% dissociation of these acids.

Acid and Base Character

For a molecule with a H-X bond to be an acid, the hydrogen must have a positive oxidation number so it can ionize to form a positive +1 ion. For instance, in sodium hydride (NaH) the hydrogen has a -1 charge so it is not an acid but it is actually a base. Molecules like CH4 with nonpolar bonds also cannot be acids because the H does not ionize. Molecules with strong bonds (large electronegativity differences), are less likely to be strong acids because they do not ionize very well. For a molecule with an X-O-H bond (also called an oxyacid) to be an acid, the hydrogen must again ionize to form H+. To be a base, the O-H must break off to form the hydroxide ion (OH-). Both of these happen when dealing with oxyacids.

Salts as Acids and Bases

When a salt is placed in water it dissolves. The ions of this salt are then free to react with whatever is in the solution (in this case, the water itself). These reactions involve either the H+ or OH- of water and thus form weak acid/base equilibriums, often altering the pH of the solution.

Lets look at the different possibilities more closely.

Hydrolysis of a Strong Acid-Strong Base Salt

What would happen if KBr were placed in water. First, it is a soluble salt, therefore it will dissolve into K+ and Br-. From our examinations of acids and bases, we can identify that K+ is the conjugate acid of KOH and Br- is the conjugate base of HBr. KOH however is a strong base, and as we know, the formation of this would never occur, thus the K+ does not combine with any of the free OH present in the solution. Similarly, HBr is a strong acid, so again it would be unlikely that any Br- would be able to react with free H+ in the solution to form HBr.

In summary, neither species from the ionic salt was able to react with H+ or OH- and thus the pH does not change.

Hydrolysis of a Weak Acid-Strong Base Salt

What would happen if KCN were placed in water. First, it would dissolve in K+ and CN-. We already know that K+ does not react with anything and so does not affect the pH. CN-, on the other hand, is a weak base, and thus can exist in an equilibrium with HCN. This equilibrium requires the absoption of free H+ ions, thus the salt solution is now short on H+ in comparison to OH- and so the final pH becomes slightly basic.

Does NaF alter the pH of water and if so, is the solution acidic or basic? (basic)

Summary

In summary, there are only four combinations and we can predict whether a solution will be acidic basic or neutral by qualitatively finding the pH:

1. Salts from a strong acid and a strong base :

no hydrolysis of cation or anion
pH = 7.00

2. Salts from a strong base and a weak acid :

hydrolysis of anion from the weak acid ( to produce OH- ions )
pH > 7.00 ( basic )

3. Salts from a weak base and a strong acid :

hydrolysis of cation from weak base ( to produce H+ ions )
pH < 7.00 ( acidic )

4. Salts from a weak base and a weak acid :

both cation and anion hydrolyze
if Kb of hydrolysis of an anion is greater than Ka of hydrolysis of a cation, the solution is basic (pH > 7.00), and vice versa.