Chapter 4 Acids and Bases 16 November 27, 2016

Chapter 14:

Acids & Bases

14.1 The Nature of Acids and BasesWhen you finish this section you will be able to:• Define acids and bases using the BrØunsted-Lowey model• Identify conjugate acid-base pairs.• Write equilibrium expressions for acid dissociations.

The Arrhenius concept says that an acid supplies H+ to an aqueous solution. (limiting concept because it applies only to aqueous solutions and hydroxide ion bases).

The BrØunsted-Lowey model - an acid is a proton (H+) donor, a base is a proton acceptor.

HA + H2O H3O+ + A-

acid 1 base 2 conjugated base 1

conjugated acid 2

HA donates a proton to H2O giving A- and H3O+. HA and A- are conjugate pairs. H2O and H3O+ are also conjugated pairs.

Chapter 4 Acids and Bases 16 November 27, 2016

Write the dissociation reaction for each of the following acids in water, and identify the conjugated acid-base pairs:a. formic acid (HCOOH)b. perchloric acid (HClO4)

Write an equilibrium for each of the following acids in water:a. formic acid (HCOOH)b. perchloric acid (HClO4)

Example 14.1B Equilibrium Expression for Acid Dissociations

Equilibrium expressions for acid dissociations are written using the same concepts as for any other chemical equation. K is known as Ka

Example 14.1A Conjugated Pairs

14.2 Acids StrengthWhen you finish this section you will be able to:• Compare relative strengths of acids• Solve problems regarding the autoionization of water.

The strength of an acid is indicated by the equilibrium position of the dissociation reaction.

HA + H2O A- + H3O+

Weak acid if the equilibrium lies far to the left the acid does not dissociate very much.

Strong acid if the equilibrium lies far to the right the acid strongly dissociate.

Chapter 4 Acids and Bases 16 November 27, 2016

Common Strong Acids: Sulfuric acid, hydrochloric acid, nitric acid, and perchloric acid.

Monoporic acid - an acid having one acidic proton

Diporic acid - an acid having two acidic proton

* sulfuric acid is a diporic acid. It is a strong acid virtually 100% dissociation in water.H2SO4(aq) H+(aq) + HSO4-(aq)

The HSO4- ion is a weak acid

HSO4-(aq) H+(aq) + SO4-2(aq)

Example 14.2A Relative Acid and Base StrengthsUsing table 14.2 in your textbook, and knowing Ka for H2O = 1 x 10-14 at 25oC, arrange the following acids in order of their strength. Then arrange conjugate bases in order.

HOC6H5, H2O, HSO4-, NH4+, HNO3

Chapter 4 Acids and Bases 16 November 27, 2016

Chapter 4 Acids and Bases 16 November 27, 2016

14.3 The pH Scale• When you finish this section you will be able to convert among pH, pOH,

[OH-] and [H+].

pH = -log[H+] pOH = -log[OH-]

The pH scale provides a convenient way to represent solution acidity. The pH is a log scale based on 10, where

Significant Figures for logarithms: the number of decimal places in the log is equal to the number of significant figures in the original number.

[H+] = 1.0 x 10-9 M pH = 9.00

2 significant figures

2 decimal places

The operation "p" means "-log of the concentration of"

Since pH is a log scale based on 10, the pH changes by 1 for every power of 10 change in [H+]. For example a solution of pH 3 has an H+ concentration 10 times that of pH 4 and 100 times that of pH 5.

Chapter 4 Acids and Bases 16 November 27, 2016

In general:pX = -log[X][X] = 10-pX

Example 14.3B Converting Between p and ConcentrationCalculate the "p" or "[ ]" as necessary for each of the following . (Remember to use the proper number of sig. figs)a. Calculate [Cl-] if pCl = 7.32b. Calculate pAg if [Ag+] = 0.034 M.c. Calculate pNO3 if [NO3-] = 15 M.d. Calculate [NH4+] if the pNH4+ = 11.87

Example 14.3C Converting Among pH, pOH, [H+], and [OH-]Fill in the following table:

Recall that Kw = [H+][OH-], or pKw = pH + pOH. At 25 oC, Kw = 1.0 x 10-14 and pKw = 14. Therefore, at 25 oC, pH + pOH = 14.0

Chapter 4 Acids and Bases 16 November 27, 2016

15.4 Titrations an pH Curves (p.713-717)

Volumetric Analysis - is a technique for determining the amount of certain substance by doing a titration.

Titration - involves delivery (from a buret) of a measured volume of a solution of known concentration (titrant) into a solution containing the substance being analyzed (the analyte)

Example 15.4 A Strong Acid-Strong Base Titration

Strong Acid - Strong BaseThe titration of a HCl solution with NaOH. We say that the process is complete; however we can calculate the equilibrium constant.

H+ (aq) + OH- (aq) H2O(l)

The reaction is the reverse for the autoionization of water therefore

K = 1/Kw = 1 x 1014

Calculate the pH after the following total volumes of 0.2500 M HCl have been added to 50.00 mL of 0.1500 M NaOH

a. 0.00 mL c. 29.50 mL e. 30.50 mL

b. 4.00 mL d. 30.00 mL f. 40.00 mL

Chapter 4 Acids and Bases 16 November 27, 2016

Indicator - a substance added in the beginning of the titration that changes color at (or very close) the equivalence point.End point the point in the titration at which the indicator changes color.

Example 15.5 B Indicator ColorsA solution has a pH of 7.0. What is the color of the solution with each of the following indicators?

a. phenolphthalein

b. methyl orange

c. bromothymol blue

d. thymol blue

Equivalence point or the stoichiometric point the point in the titration where enough titrant has been added to react exactly with the analyte.

Titration Curve

Chapter 4 Acids and Bases 16 November 27, 2016

Example 4.8B Acid-Base TitrationYou wish to determine the molarity of sodium hydroxide. To do this, you titrate a 25.00 mL aliquot of your sample, which has 3 drops phenolphthalein indicator added so that it is pink, with 0.1067 M HCl. The sample turns clear (indicating that the NaOH(aq) has been precisely neutralized by the HCl solution) after the addition of 42.95 mL of the HCl. Calculate the molarity of your NaOH.

Does the Answer Makes Sense?You had 25.00 mL of NaOH solution. It took almost twice as much the volume of 0.1067 M HCl to neutralize. Because you have the same number of moles in about half the solution volume, it makes sense that the NaOH solution would be almost twice the concentration.

Example 4.8D Molar Mass of an AcidYou want to determine the molar mass of an acid. The acid contains one acidic hydrogen per molecule. You weigh out a 2.879 g sample of the pure acid and dissolve it, along with 3 drops of phenolphthalein indicator, in distilled water. You titrate the sample with 0.1704 M NaOH. The pink endpoint is reached after addition of 42.45 mL of the base. Calculate the molar mass of the acid.

Chapter 4 Acids and Bases 16 November 27, 2016

14.4 Calculating the pH of Strong Acid Solutions• When you finish this section you will be able to calculate the pH of

strong acid solutions.

Calculating the pH of strong acid is a in general fairly straightforward because the dissociation equilibrium lies far to the right, the acid completely dissociates.

The [H+] at equilibrium is ≈ [strong acid] except in very dilute solutions (<10-6 M).

Example 14.4A The pH of a Strong AcidCalculate the pH and [OH-] of a 5 x 10-3 M HClO4 solution.

Example 14.4B Practice with Strong AcidsA solution is prepared by adding 15.8 g of HCl to enough water to make a total volume of 400. mL. What is the pH of the solution? How much hydroxide ion is contributed by the autoionization of water?

Chapter 4 Acids and Bases 16 November 27, 2016

14.5 Calculating the pH of Weak Acid • When you finish this section you will be able to calculate the pH

of weak acidic solutions.

Example 14.5A The pH of a Weak AcidCalculate the pH of a 0.500 M aqueous solution of formic acid, HCOOH. Ka = 1.77 x 10-4

Use I.C.E charts to solve for the concentrations at equilibrium.

14.6 Bases• When you finish this section you will be able to calculate the pH of strong

base solutions.Bases work in the same way that acids do. Strong bases completely dissociate. Using LiOH in water as an example,

LiOH(s) Li+(aq) + OH-(aq)

Therefore, one can consider that [OH-] ≈ [LiOH]. Once you know [OH-], you can use Kw to calculate[H+] and pH

All alkali hydroxides are strongly basic. Alkaline earth hydroxide are also strongly basic, but are somewhat less soluble than alkali hydroxides.

Example 14.6A The pH of a Strong BaseCalculate the pH of a solution made by putting 4.63 g of LiOH into water and diluting to a total volume of 400. mL. How much hydrogen ion is contributed by the autoionization of water?

Chapter 4 Acids and Bases 16 November 27, 2016

14.6B Calculating the pH of Weak Base • When you finish this section you will be able to calculate the pH of

weak basic solutions.

Example 14.6B The pH of a Weak BaseCalculate the pH of a 0.350 M aqueous solution of methylamine, CH3NH2. Kb = 4.38 x 10-4

Use I.C.E charts to solve for the concentrations at equilibrium.