acids and bases chapter 16. 2 copyright © by houghton mifflin company. all rights reserved. 16...

Acids and Bases

Chapter 16

Chapter 16 2Copyright © 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+).


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).


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.


16.2 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, HNO3 , and H2SO4.

An example is perchloric acid, HClO4.

)aq(ClO)aq(OH)l(OH)aq(HClO 4324


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.

)aq(OH)aq(Na)s(NaOH OH 2


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, NH4OH, is a weak base.

)aq(OH)aq(NH )aq(OHNH 44

(aq)OHC(aq)OH 2323

An example is acetic acid, HC2H3O2.

)l(OH)aq(OHHC 2232


Acid Base Pairs

Consider the reaction of NH3 and H2O.

• 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+

)aq(OH )aq(NH )l(OH )aq(NH 423

base acid

The species NH4+ and NH3 are a conjugate acid-base

pair.


Acid Base Pairs

Consider the reaction of NH3 and H2O.

)aq(OH )aq(NH )l(OH )aq(NH 423

base acid

Here NH4+ is the conjugate acid of NH3 and NH3 is

the conjugate base of NH4+.


Acid Base Pairs

Consider the equilibrium below.

(aq)OHC(aq)OH 2323 )l(OH)aq(OHHC 2232

acid acidbase base

conjugate acid-base pairs


problems

Consider the reaction HNO2(aq) + H2O(l) H3O+(aq) + NO2

-(aq). Which species is the conjugate base to the acid HNO2(aq) ?

NO2-(aq)

What is the conjugate base to the acid H3O+

(aq)?

H2O(l)


16.3 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).


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 NH3.

)aq(OH)aq(NH)l(OH)aq(NH 423

H+


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.

)aq(OH)aq(F)l(OH)aq(HF 32

H+


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.

)aq(OH)aq(OH )l(OH)l(OH 322


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).


Self-ionization of Water

In pure water

[H30+] [OH-] = 1.0 x 10 -14 = Kw

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

Kw = ion-product constant for water (any water soln at 25oC)

[H+] = [OH-] neutral

[H+] > [OH-] acid

[H+] < [OH-] base


problem

If you have 1.0 x 10-5 M OH- what is the concentration of H+?

[1.0 x 10-5] [H+] = 1.0 x 10-14

[H+] = 1.0 x 10-9


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).

)aq(Cl)aq(H)aq(HCl

Substituting [H+]=0.10 into the ion-product expression, we get:

]OH)[10.0(100.1 14


answer

MOH 13-14-

101.0 10.0101.0

][


problem

As an example, calculate the concentration of H+ ion in 0.010 M NaOH.

Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH-(aq).

)aq(OH)aq(Na)s(NaOH OH 2 Substituting [OH-]=0.010 into the ion-product expression, we get:

)010.0](H[100.1 14


answer

MH 12-14-

101.0 010.0101.0

][


16.4 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.

]Hlog[pH


The pH of a Solution

For a solution in which the hydrogen-ion concentration is 1.0 x 10-3, the pH is:

00.3)100.1log( 3 pH



In a neutral solution, whose hydrogen-ion concentration is 1.0 x 10-7, the pH = 7.00.

For acidic solutions, the hydrogen-ion concentration is greater than 1.0 x 10-7, so the pH is less than 7.00.

Similarly, a basic solution has a pH greater than 7.00.

Figure 16.6 The pH Scale


A Problem to Consider

A sample of orange juice has a hydrogen-ion concentration of 2.9 x 10-4 M. What is the pH?

]Hlog[pH

)109.2log(pH 4

54.3pH



The pH of human arterial blood is 7.40. What is the hydrogen-ion concentration?

)pHlog(anti]H[

)40.7log(anti]H[

M100.410]H[ 840.7



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.

]OHlog[pOH



A measurement of the hydroxide ion concentration, similar to pH, is the pOH.

Then because Kw = [H+][OH-] = 1.0 x 10-14 at 25 oC, you can show that

00.14pOHpH



An ammonia solution has a hydroxide-ion concentration of 1.9 x 10-3 M. What is the pH of the solution?

You first calculate the pOH:

72.2)109.1log(pOH 3

Then the pH is:

28.1172.200.14pH


16.5 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)

acids and bases chapter 16. 2 copyright © by houghton mifflin company. all rights reserved. 16...

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