•Biological systemsoThey help control acidity of our blood since deviations can result in illness or death.

•IndustryoFor example, the vast quantity of sulfuric acid produced in the US is needed to produce fertilizers, polymers, steel, and many other materials.

•Acids: taste sour•Citric acid is responsible for the sour taste of a lemon.

•Bases (sometimes called alkalis): taste bitter and feel slippery•Commercial preparations for unclogging drains are highly basic.

•Based on experimentation, Svante Arrhenius postulated that acids produce hydrogen ions in aqueous solution, while bases produce hydroxide ions.•This is known as the Arrhenius concept of acids and bases.•A more general concept was proposed by Johannes Brønsted and Thomas Lowry.•In the Brønsted-Lowry model, an acid is a proton (H+) donor, and a base is a proton (H+) acceptor.

•For example, when gaseous HCl dissolves in water, each HCl molecule donates a proton to a water molecule and so qualifies as a Brønsted-Lowry acid.•The molecule that accepts the proton, is a Brønsted-Lowry base (H2O).•Note the proton is transferred from the HCl molecule to the water molecule to form H3O+, which is called the hydronium ion.

•The general reaction that occurs when an acid is dissolved in water can be represented as

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)Acid Base Conjugate

AcidConjugate Base

•A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.•Above there are two conjugate acid-base pairs:

HA and A- H2O and H3O+

•Important!! HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

•In the reaction there is a competition between the two bases, H2O and A-, for the proton. •If H2O is a stronger base than A-, the equilibrium lies far to the right (most of HA will be ionized at equilibrium).•If A- is a stronger base than H2O, the equilibrium lies far to the left (most of HA at equilibrium still HA).

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

•The equilibrium expression for the reaction can be written as:

•Ka is called the acid dissociation constant.

•Note H3O+ or H+ can be used to represent the hydrated proton (in water). We will use H+ henceforth.•Note H2O (l) is omitted in the equilibrium expression; therefore, we can write:

HA(aq) ⇌ H+(aq) + A-(aq)

]HA[]A][H[

]HA[]A][OH[ 3

aK

•Even though we omit water don’t forget that water plays an important role in causing the acid to ionize.

•Note that Ka is the equilibrium constant for the reaction in which a proton is removed from HA to form the conjugate base A-. •We use Ka to represent only this type of reaction. •Knowing this, you can write the Ka expression for any acid, even one that is totally unfamiliar to you.

•The strength of an acid is determined by the equilibrium position of its dissociation (ionization) reaction:

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)•Strong Acid = equilibrium lies far to the right.•Weak Acid = equilibrium lies far to the left.

•A strong acid yields a weak conjugate base – one that has a low affinity for a proton.

•A weak acid yields a strong conjugate base – one that has a high affinity for a proton.

•The common strong acids are sulfuric acid [H2SO4(aq)], hydrochloric acid [HCl(aq)], nitric acid [HNO3(aq)], perchloric acid [HClO4(aq)], hydrobromic acid [HBr (aq)], and hydroiodic acid [HI (aq)].•Sulfuric acid is a diprotic acid – has two acidic protons.•The table below lists common monoprotic acids (one acidic proton) and their Ka values.•Note the strong acids are not listed. Their equilibrium lies so far to the right Ka cannot be correctly determined.

•Most acids are oxyacids, in which the acidic proton is attached to an oxygen atom.

•Organic acids, those with a carbon atom backbone, commonly contain the carboxyl group.

•Acids of this type are usually weak.•Examples are acetic acid (HC2H3O2) and benzoic acid (HC7 H5O2).•The acidic proton is written in the front. •The remainder of the hydrogens are not acidic – they do not form H+ in water.

•A substance is amphoteric is it can behave either as an acid or as a base.•Water is the most common amphoteric substance.•This is seen in the autoionization of water below.

H2O + H2O ⇌ H3O+ + OH-

acid(1) base(1) acid(2) base(2)

•This reaction gives the following equilibrium expression:

Kw = [H3O+][OH-] = [H+][OH-]Kw = ion-product constant or dissociation constant for water.

Experiment shows that at 25oC in pure water,

[H+] = [OH-] = 1.0 x 10-7 M

which means that at 25oC

Kw = [H+][OH-] = (1.0 x 10-7)(1.0 x 10-7) = 1.0 x 10-14

In any aqueous solution at 25oC, no matter what is contains, the product of [H+] and [OH-] must always equal 1.0 x 10-14.

Kw = [H+][OH-] = (1.0 x 10-7)(1.0 x 10-7) = 1.0 x 10-14

•This leads to three possible situations:

1.A neutral solution, where [H+] = [OH-].2.An acidic solution, where [H+] > [OH-].3.A basic solution, where [OH-] > [H+].

Remember the product of [H+][OH-] must equal 1.0 x 10-14.