CH 101 Chapter 12 Notes

Acid-Base Chemistry

Svante Arrhenius

The man who created the first chemical definition of acids and bases

Arrhenius acid

a substance that produces H1+ ions when dissolved in
water

Arrhenius base

produces OH1- ions when dissolved in water

acid ionization constant.

The equilibrium constant for the ionization where an acid ionizes in water much as an ionic substance

What are the reactants and products of acids and bases?

The reaction of an acid with a base is called neutralization, and the products are water and a salt.

Lewis bases and acids

A Lewis base contains a lone pair, a Lewis acid contains an empty orbital that can overlap with the lone pair, and a Lewis acid-base reaction is the formation of a coordinate covalent bond (bonds in which both bonding electrons are supplied by the same atom) between a Lewis acid and a Lewis base

A Lewis base is readily identified by the presence of a lone pair. Bases are strengthened by negative charge. Lewis acids are often more difficult to identify. The following should help:

• A Lewis acid must be able to accommodate an additional electron region (the new bond), so, if it obeys the octet rule, a Lewis acidic atom must have less than four regions.
• Attack by a lone pair is facilitated by positive charge, so Lewis acidity is strengthened by
positive charge.

How to tell Lewis acids and bases apart:

Strong Lewis acids have low-energy empty orbitals, and strong Lewis bases have high-energy lone pairs.

What determines whether electrons are transferred or shared when a lone pair comes into contact
with an empty orbital?

As has been the case so often in our study of chemistry, the
answer lies in their relative energies: electrons do whatever is most efficient at increasing their electrical potential in order to lower their energy

Curved arrows are used to indicate the direction of electron pair attack in Lewis acid-base reactions

• A curved arrow from a lone pair on one atom to another atom indicates that the lone pair becomes a covalent bond between the atoms.
• A curved arrow from a bond to an atom indicates that the bonding electrons become a lone pair on the atom.

A very broad classification of Lewis is used to define acid-base reactions

A very large number of chemical reactions can be classified as either acid-base or redox reactions. In one, a base shares its electrons with an acid; in the other, a reductant transfers its electrons to an oxidant.

Brønsted acids

proton donors

Brønsted bases

proton acceptors

A Brønsted acid-base reaction

a proton transfer from the acid to the base

strong acids

acids that are strong electrolytes and dissociate 100% in water

What is the dissociation of an acid in water viewed as in Arrhenius theory?

The dissociation of an acid in water is viewed as
ionization in Arrhenius theory, but acids are not ionic compounds, and the term “ionization” is misleading

What is the difference between redox and Bronsted reactions?

Redox reactions involve the transfer of the basic unit of negative charge (the electron), while Brønsted acid-base reactions involve the transfer of the basic unit of positive charge (the proton).

weak acids

weak electrolytes

What is required for a hydrogen atom to be acidic?

a hydrogen atom must be covalently bound to a highly electronegative atom to be acidic

Why can placing the acidic proton first in the formula be misleading?

placing the acidic proton first in the formula can be misleading because it often places the proton next to an
atom to which it is not bound. However, in most if not all cases, having the hydrogen listed first means the compound is an acid

How are acids named?

The manner in which an acid is named depends on whether it is a binary acid (an acid that contains only two elements, such as HCl and H2S) or a polyatomic acid (an acid that contains more than two elements, such as HClO or H3PO4).

Binary Acids

Binary acids are derived from gases. When they are dissolved in water, the names are changed in the following manner:
1) replace “hydrogen” with “hydro”,
2) change the -ide ending to -ic and
3) add the word “acid”.

oxacids

Acids derived from polyatomic anions, or protonated oxoanions. The acidic proton is always attached to an oxygen atom. They are named as follows:
1) change the -ate ending of the polyatomic ion to -ic * or
2) change the -ite ending of the polyatomic ion to -ous* and
3) add the word “acid”.

What can we learn about naming acids?

In summary, an elementate ion becomes an elementic acid, and an elementite ion becomes an elementous acid. If the acid is also an ion, its name is unchanged

What representation is opposite to the direction of proton transfer?

The Lewis formalism that uses curved arrows to show the direction of electron pair attack, which is opposite to the direction of proton transfer.

What forms a conjugate acid-base pair?

When an acid and a base that differ by a single proton are conjugate with one another

What can we conclude from acid-base reactions and pairs?

The products of a Brønsted acid-base reaction are the conjugate base of the reacting acid and the conjugate acid of the reacting base. In other words, all Brønsted acid-base reactions consist of two conjugate acid-base pairs and nothing else.

amphiprotic

Substances that can function as either an acid or a base

the extent of proton transfer

The position of the equilibrium: if the concentration of at least one reactant in an acid-base reaction is much smaller than any of the product concentrations, then the reaction is an extensive proton transfer.

How does the strength of an acid increase?

the strength of an acid also increases with the
electronegativity of the atom to which the proton is attached.

The strength of the O-H bond depends upon the electron density in the bond, which, in turn, depends upon the electron withdrawing ability of X:

the more electron density X draws from the O-H bond, the
weaker the bond becomes, and the stronger the acid becomes.

The following two factors dictate the electron withdrawing abilities of X:

1. Oxidation state: The ability of X to withdraw electrons from the O-H bond increases with its oxidation state, so the strength of an oxoacid increases with the oxidation state of X. For example, HNO3 is a stronger acid than HNO2 because the oxidation state of the nitrogen atom is greater in HNO3 (+5) than in HNO2 (+3).
2. Electronegativity: The amount of electron density withdrawn from the O-H bond increases with the electronegativity of X, as does the acid strength of the oxoacid. HClO2 is a stronger acid than HBrO2 because chlorine is more electronegative than bromine.

How will we represent an oxoacid?

We will represent an oxoacid as XOH, where X is an atom that may have other atoms attached to it (often other oxygen atoms)

How do hydrogen bonds differ in strong and weak acids?

A strong acid is one that has a weakly bound hydrogen, but a strong base is one that forms a strong bond with hydrogen. Consequently, strong acids have weak conjugate
bases, and weak acids have strong conjugate bases. In other words, the strength of a base varies inversely with the strength of its conjugate acid.

How do we conclude that an acid-base reaction is extensive when the reacting acid is stronger than the produced acid?

If a proton transfer is extensive, then the forward reaction is more extensive than the reverse reaction, which means that the reacting acid gives up its proton more easily than the produced acid.

How do we predict the extent of an acid-base reaction?

In order to predict the extent of an acid-base reaction, we need know only the relative strengths of the reacting and produced acids. This is done by measuring how extensively each acid reacts with a reference base. The reference base is water, and the extent of the reaction is given by the value of the equilibrium constant for the reaction.

acid dissociation
acid ionization constant
K_a

The equilibrium constant for the reaction of an acid with water

Thus far we have used only the relative strengths of the reacting and produced acids to determine if a reaction is extensive, but the Ka values of the reacting and produced acids can be used to determine the value of the equilibrium constant for the reaction.

K = Ka reacting acid
---------------------------
Ka produced acid

(The equilibrium constant for an acid-base reaction equals the Ka of the reacting acid divided by the Ka of the produced acid)

hydrolysis

The reaction involves the breaking of an O-H bond of water

Stronger acids are located above weaker acids, so proton transfer is extensive when

the reacting acid is above (stronger than) the produced acid. Stated somewhat differently, the reaction between an acid and a base is extensive when the acid is above the base on the acid-base table

A solution of a strong acid is represented by the hydronium ion and its conjugate base, but a solution of a weak acid is written as

the unreacted (undissociated) acid.

ion product constant for water: Kw.

The equilibrium constant for the reaction of pure liquid water entering the equilibrium constant as unity
Kw = [H3O1+] [OH1-] = 1.0x10-14 at 25 oC

Equation 12.3: [H3O1+ ] = [OH1- ] = Kw = 1.0 ×10-7 M

Solutions for which Equation 12.3 is valid are called neutral; solutions in which [H3O1+] > [OH1-] are called acidic; and solutions in which [H3O1+] < [OH1-] are called basic or alkaline.

pH

-log[H3O1+]

What do high and low pH's mean?

Because of the negative sign, a high pH implies a low hydronium ion concentration, and a low pH implies
a high hydronium ion concentration. Solutions with low
hydronium ion concentrations have high hydroxide ion concentrations, so a high pH also implies a high hydroxide ion concentration and a low pH implies a low hydroxide ion concentration.

pKa = -log Ka

A high pKa indicates a weak acid

CHAPTER SUMMARY AND OBJECTIVES

A Lewis base is a substance with an electron pair that can be used to form a covalent bond. A Lewis acid is a substance with an empty orbital that can overlap with the electron pair on the base to form a bond. The Lewis definition is the most general acid-base definition. A Brønsted acid is a proton donor, and a Brønsted base is a proton acceptor. All Brønsted bases are Lewis bases and vice versa. The proton is a Lewis acid because it will readily accept a pair of electrons to share in a covalent bond, but the proton defines all Brønsted acids.
As the strength of the H-A bond increases the strength of HA as an acid decreases and the strength of its conjugate base, A1-, increases. Many acids contain the unit H-O-X. For these oxoacids, the acid strength increases as the electronegativity and/or the oxidation state of X increases. The relative strength of an acid is measured by its acid dissociation constant, Ka, which is the equilibrium constant for the reaction of the acid with water. If
Ka >> 1, the acid is a strong acid; if Ka << 1, the acid is a weak acid. The equilibrium of a Brønsted acid-base reaction is equal to the Ka of the reacting acid divided by the Ka of the produced acid.
The products of a Brønsted acid-base reaction are the conjugate base of the reacting acid and the conjugate acid of the reacting base. The reaction is extensive when the produced acid and base are weaker than the reacting acid and base, which is quantified by the expression K = Ka(reacting)/Ka(produced).
The pH of a solution is often used instead of the hydronium ion concentration to avoid the awkwardness of using large, negative exponentials. The pH is defined as -log[H3O1+]. Solutions with pH = 7 are said to be neutral, while solutions with pH > 7 are basic and those with pH < 7 are acidic. Exponentials can also be avoided when referring to the Ka of the acid by using pKa, which is defined as -log Ka. A high pKa implies a weak acid.