Acids and Bases Chapter 16

2 Copyright © by Houghton Mifflin Company. All rights reserved. 16 Concept of Acids and Bases According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydrogen ion (H + ).

Chapter 163 Copyright © by Houghton Mifflin Company. All rights reserved. Concept of Acids and Bases A base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH - (aq).

Chapter 164 Copyright © by Houghton Mifflin Company. All rights reserved. Concept of Acids and Bases A base is the species accepting the proton in a proton-transfer reaction. According to the Brønsted-Lowry concept, an acid is the species donating the proton in a proton-transfer reaction.

Chapter 165 Copyright © by Houghton Mifflin Company. All rights reserved Strong Acids and Bases In the Arrhenius concept, a strong acid is a substance that ionizes completely in aqueous solution to give H + (aq) and an anion. Strong acids include HCl, HNO 3, and H 2 SO 4. An example is perchloric acid, HClO 4.

Chapter 166 Copyright © by Houghton Mifflin Company. All rights reserved. Strong Acids and Bases In the Arrhenius concept, a strong base is a substance that ionizes completely in aqueous solution to give OH - (aq) and a cation. Other strong bases include, Ca(OH) 2, and Ba(OH) 2. An example is sodium hydroxide, NaOH.

Chapter 167 Copyright © by Houghton Mifflin Company. All rights reserved. Weak Acids and Bases Most other acids and bases that you encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions. Ammonium hydroxide, NH 4 OH, is a weak base. An example is acetic acid, HC 2 H 3 O 2.

Chapter 168 Copyright © by Houghton Mifflin Company. All rights reserved.

Chapter 169 Copyright © by Houghton Mifflin Company. All rights reserved. Acid Base Pairs Consider the reaction of NH 3 and H 2 O. A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton. Acid gains H +, base loses H + baseacid The species NH 4 + and NH 3 are a conjugate acid- base pair.

Chapter 1610 Copyright © by Houghton Mifflin Company. All rights reserved. Acid Base Pairs Consider the reaction of NH 3 and H 2 O. baseacid Here NH 4 + is the conjugate acid of NH 3 and NH 3 is the conjugate base of NH 4 +.

Chapter 1611 Copyright © by Houghton Mifflin Company. All rights reserved. Acid Base Pairs Consider the equilibrium below. acid base conjugate acid-base pairs

Chapter 1612 Copyright © by Houghton Mifflin Company. All rights reserved. problems Consider the reaction HNO 2 (aq) + H 2 O(l)  H 3 O + (aq) + NO 2 - (aq). Which species is the conjugate base to the acid HNO 2 (aq) ? NO 2 - (aq) What is the conjugate base to the acid H 3 O + (aq)? H 2 O(l)

Chapter 1613 Copyright © by Houghton Mifflin Company. All rights reserved Water as an acid and base Some species can act as an acid or a base. An amphoteric species is a species that can act either as an acid or a base (it can gain or lose a proton).

Chapter 1614 Copyright © by Houghton Mifflin Company. All rights reserved. Water as an acid The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions. Water reacts as an acid with the base NH 3. H+H+

Chapter 1615 Copyright © by Houghton Mifflin Company. All rights reserved. Water as a base The amphoteric characteristic of water is important in the acid-base properties of aqueous solutions. Water can also react as a base with the acid HF. H+H+

Chapter 1616 Copyright © by Houghton Mifflin Company. All rights reserved. Self-ionization of Water Self-ionization is a reaction in which two like molecules react to give ions. In the case of water, the following equilibrium is established.

Chapter 1617 Copyright © by Houghton Mifflin Company. All rights reserved. Solutions of Strong Acid or Base By dissolving substances in water, you can alter the concentrations of H + (aq) and OH - (aq). In a neutral solution, the concentrations of H + (aq) and OH - (aq) are equal, as they are in pure water. In an acidic solution, the concentration of H + (aq) is greater than that of OH - (aq). In a basic solution, the concentration of OH - (aq) is greater than that of H + (aq).

Chapter 1618 Copyright © by Houghton Mifflin Company. All rights reserved. Self-ionization of Water In pure water [H ] [OH - ] = 1.0 x = K w [H + ] [OH - ] = 1.0 x = K w K w = ion-product constant for water (any water soln at 25 o C) [H + ] = [OH - ] neutral [H + ] > [OH - ] acid [H + ] < [OH - ] base

Chapter 1619 Copyright © by Houghton Mifflin Company. All rights reserved. problem If you have 1.0 x M OH - what is the concentration of H + ? [1.0 x ] [H + ] = 1.0 x [H + ] = 1.0 x 10 -9

Chapter 1620 Copyright © by Houghton Mifflin Company. All rights reserved. problem As an example, calculate the concentration of OH - ion in 0.10 M HCl. Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H + (aq). Substituting [H + ]=0.10 into the ion-product expression, we get:

Chapter 1621 Copyright © by Houghton Mifflin Company. All rights reserved. answer

Chapter 1622 Copyright © by Houghton Mifflin Company. All rights reserved. problem As an example, calculate the concentration of H + ion in M NaOH. Because you started with M NaOH (a strong base) the reaction will produce M OH - (aq). Substituting [OH - ]=0.010 into the ion-product expression, we get:

Chapter 1623 Copyright © by Houghton Mifflin Company. All rights reserved. answer

Chapter 1624 Copyright © by Houghton Mifflin Company. All rights reserved The pH scale Although you can quantitatively describe the acidity of a solution by its [H + ], it is often more convenient to give acidity in terms of pH. The pH of a solution is defined as the negative logarithm of the molar hydrogen-ion concentration.

Chapter 1625 Copyright © by Houghton Mifflin Company. All rights reserved. The pH of a Solution For a solution in which the hydrogen-ion concentration is 1.0 x 10 -3, the pH is:

Chapter 1626 Copyright © by Houghton Mifflin Company. All rights reserved. The pH of a Solution In a neutral solution, whose hydrogen-ion concentration is 1.0 x 10 -7, the pH = For acidic solutions, the hydrogen-ion concentration is greater than 1.0 x 10 -7, so the pH is less than Similarly, a basic solution has a pH greater than 7.00.

Figure 16.6 The pH Scale

Chapter 1628 Copyright © by Houghton Mifflin Company. All rights reserved. A Problem to Consider A sample of orange juice has a hydrogen-ion concentration of 2.9 x M. What is the pH?

Chapter 1629 Copyright © by Houghton Mifflin Company. All rights reserved. A Problem to Consider The pH of human arterial blood is What is the hydrogen-ion concentration?

Chapter 1630 Copyright © by Houghton Mifflin Company. All rights reserved. The pH of a Solution A measurement of the hydroxide ion concentration, similar to pH, is the pOH. The pOH of a solution is defined as the negative logarithm of the molar hydroxide-ion concentration.

Chapter 1631 Copyright © by Houghton Mifflin Company. All rights reserved. The pH of a Solution A measurement of the hydroxide ion concentration, similar to pH, is the pOH. Then because K w = [H + ][OH - ] = 1.0 x at 25 o C, you can show that

Chapter 1632 Copyright © by Houghton Mifflin Company. All rights reserved. A Problem to Consider An ammonia solution has a hydroxide-ion concentration of 1.9 x M. What is the pH of the solution? You first calculate the pOH: Then the pH is:

Chapter 1633 Copyright © by Houghton Mifflin Company. All rights reserved measuring pH using indicators The pH of a solution can accurately be measured using a pH meter which is electronic and very sensitive to [H + ]. Indicators = substance that change colors in acidic and basic solution. (litmus paper, purple cabbage)