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Acid-Base Properties, Behavior, and Structure

AP Notes 16.9-10
by

Katharine Higgins

on 10 December 2014

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Transcript of Acid-Base Properties, Behavior, and Structure

An Anion's Ability to React with Water
In general, and anion X in solution can be considered the conjugate base of an acid (ie Cl is the conjugate base of HCl).

Whether or not an anion reacts with water to produce hydroxide ions depends on the strength of the anion's conjugate acid.

To identify the acid and assess its strength, we add a proton to the anion's formula; if the acid HX determined in this way is one of the 7 strong acids (see p. 664), the anion has a negligible tendency to abstract protons from water and does not affect the pH of the solution.

If HX is not one of the 7 strong acids, it is a weak acid, and X is a weak base, which reacts to a small extent with water to produce OH (which will increase the pH of the solution, making it basic).
The situation is more complicated for salts containing anions that have ionizable protons, such as HSO .

These salts are amphiprotic, and how they behave in water is determined by the relative magnitudes of K and K for the ion.
Combined Effect of Cation and Anion in Solution
To determine whether a salt forms an acidic, a basic, or a neutral solution when dissolved in water, we must consider the action of both cation and anion. There are four possible combinations:
If the salt contains an anion that does not react with water and a cation that does not react with water, we expect the pH to be neutral (such is the case when the anion is a conjugate base of a strong acid and the cation is either from group 1A or one of the heavier members of group 2A).
If the salt contains an anion that reacts with water to produce hydroxide ions and a cation that does not react with water, we expect the pH to be basic (such is the case when the anion is a conjugate base of a weak acid and the cation is either from group 1A or one of the heavier members of group 2A).
If the salt contains a cation that reacts with water to produce hydronium ions and an anion that does not react with water, we expect the pH to be acidic (such is the case when the cation is the conjugate acid of a weak base or a small cation with a charge of +2 or greater).
If the salt contains an anion and cation both capable of reacting with water, both hydronium and hydroxide ions are produced (whether the solution is acidic, basic, or neutral depends on the relative abilities of the ions to react with water).
The mechanism by which metal ions produce acidic solutions is as follows:
Metal ions are positively charged, therefore they attract unshared electron pairs of water molecules and become hydrated.
The larger the charge on the metal ion, the stronger the interaction between the ion and the oxygen of its hydrating water molecules.
As the strength of this interaction increases, the O-H bonds in the hydrating water molecules become weaker.
This facilitates the trasfer of protons from the hydration water molecules to solvent water molecules.

Acid-Base Properties of Salt Solutions
Because nearly all salts are strong electrolytes, we can assume that any salt dissolved in water is completely dissociated; therefore, the acid-base properties of salt solution are due to the behavior of the cations and anions.

Many ions react with water to generate H (aq) or OH (aq) ions; this type of reaction is called hydrolysis.

The pH of an aqueous salt can be predicted quantitatively by considering the salt's cations and anions.
A Cation's Ability to React with Water
Polyatomic cations containing one or more protons can be considered the conjugate acids of weak bases, which will donate a proton to water, creating the hydronium ion (H O ) and lower the pH.

Many metal ions react with water to decrease the pH of an aqueous solution (acidity increases as ionic charge increases).
Acid-Base Properties, Behavior, and Structure
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If K > K , the ion causes the solution to be acidic.

If K > K , the solution is made basic by the ion.
a
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a
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The ions of alkali and alkaline earth metals (due to their low +1 and +2 charges) do not react with water to affect the pH.
Acid-Base Behavior
When a substance is dissolved in water, it may behave as an acid, behave as a base, or exhibit no acid-base properties.

A molecule containing H will act as a proton donor (an acid) only if the H-X bond is polarized in this way:

In ionic hydrides, such as NaH, the reverse is true: the H atom possesses a negative charge and behaves as a proton acceptor (a base).

Nonpolar H-X bonds (such as the H-C bond in CH , produce neither acidic nor basic aqueous solutions.

A second factor that helps determine whether a proton will be donated is the strength of the bond; very strong bonds are less easily broken than weaker ones.

A strong bond indicates a weak acid (a weak bond is easily broken, allowing the substance to ionize completely in solution).

Stability of the conjugate base (X ) will also help determine the ease of ionization; the greater the stability of the conjugate base, the stronger the acid.
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H X
Binary Acids
For a series of binary acids HX in which X represents members of the same
group
in the periodic table, the strength of the H-X bond is generally the most important factor determining acid strength (the strength tends to decrease as the element X increases in size).
Bond strength decreases and acidity increases as you move down a group.

Bond polarity is the major factor in determining acidity for binary acids when X represents members of the same
period
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Acidity increases as the electronegativity of the element X increases (increases moving left to right across a period).
Oxyacids
Many common acids contain one or more O-H bonds; acids in which OH groups and possibly additional oxygen atoms are bound to a central atom are called oxyacids.

If the atom bonded to the OH group is a metal, the substance will create OH in solution and be basic.
If the atom bonded to the OH group is a nonmetal, OH will NOT be created in solution, so the compound is either acidic or neutral.
Generally, as the electronegativity of the central atom increases, so does the acidity of the substance.

The strength of an acid increases as additional electronegative atoms bond to the central atom.

In a series of oxyacids, the acidity increases as the oxidation number of the central atom increases.
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Carboxylic Acids
A carboxyl group is often written as -COOH (acetic acid has a carboxyl group).
Only the H in the carboxyl group can be ionized.

Acids that contain a carboxyl group are called carboxylic acids, and the form the largest category of organic acids.

Two factors contribute to the behavior of carboxylic acids:
1: the additional oxygen atom attached to the carbon atom of the carboxyl group draws electron density from the O-H bond, increasing its polarity and helping to stabilize the conjugate base.
2: the conjugate base of a carboxylic acid (a carboxylate anion) can exhibit resonance (see section 8.6), which contributes to the stability of the anion by spreading the negative charge over several atoms.
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