Biological systems o They help control acidity of our blood since deviations can result in illness or death. Industry o For 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 (H 2 O). Note the proton is transferred from the HCl molecule to the water molecule to form H 3 O +, which is called the hydronium ion.

The general reaction that occurs when an acid is dissolved in water can be represented as HA(aq) + H 2 O(l) ⇌ H 3 O + (aq) + A - (aq) AcidBase Conjugate Acid Conjugate 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 - H 2 O and H 3 O +

Important!! HA(aq) + H 2 O(l) ⇌ H 3 O + (aq) + A - (aq) In the reaction there is a competition between the two bases, H 2 O and A -, for the proton. If H 2 O 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 H 2 O, the equilibrium lies far to the left (most of HA at equilibrium still HA).

HA(aq) + H 2 O(l) ⇌ H 3 O + (aq) + A - (aq) The equilibrium expression for the reaction can be written as: K a is called the acid dissociation constant. Note H 3 O + or H + can be used to represent the hydrated proton (in water). We will use H + henceforth. Note H 2 O (l) is omitted in the equilibrium expression; therefore, we can write: HA(aq) ⇌ H + (aq) + A - (aq)

Even though we omit water don’t forget that water plays an important role in causing the acid to ionize. Note that K a is the equilibrium constant for the reaction in which a proton is removed from HA to form the conjugate base A -. We use K a to represent only this type of reaction. Knowing this, you can write the K a 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) + H 2 O(l) ⇌ H 3 O + (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 [H 2 SO 4 (aq)], hydrochloric acid [HCl(aq)], nitric acid [HNO 3 (aq)], perchloric acid [HClO 4 (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 K a values. Note the strong acids are not listed. Their equilibrium lies so far to the right K a 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 (HC 2 H 3 O 2 ) and benzoic acid (HC 7 H 5 O 2 ). 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. H 2 O + H 2 O ⇌ H 3 O + + OH - acid(1) base(1) acid(2) base(2) This reaction gives the following equilibrium expression: K w = [H 3 O + ][OH - ] = [H + ][OH - ] K w = ion-product constant or dissociation constant for water.

Experiment shows that at 25 o C in pure water, [H + ] = [OH - ] = 1.0 x M which means that at 25 o C K w = [H + ][OH - ] = (1.0 x )(1.0 x ) = 1.0 x In any aqueous solution at 25 o C, no matter what is contains, the product of [H + ] and [OH - ] must always equal 1.0 x

K w = [H + ][OH - ] = (1.0 x )(1.0 x ) = 1.0 x 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