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Acids - Base Equilibria Part I: pH and Acidity

Dr. C. Yau

Spring 2014

Jespersen Chap. 17 Sec 1 & 2

22

Different Definitions of Acid/Base

Arrhenius Definition:

An acid is a substance that increases the H3O+ concentration when placed in water.

A base is a substance that increases the OH- concentration when placed in water.

NaOH (s) Na+(aq) + OH-(aq)

HCl (g) + H2O (l) H3O+(aq) + Cl-(aq)

33

Different Definitions of Acid/Base

Brønsted-Lowry Definition:

An acid is a proton donor.

A base is a proton acceptor.

O H

H Cl O H

H....

..

....

: : :: :+ .. + Cl_

..

_

proton donor

proton acceptor

Typically a Brønsted base has a lone pair to attract the proton.

H O H H Cl H O H

H

..

....

....

: : :+ .. + Cl_

..

+

water acting as a base

444

Autoionization of Water

H2O + H2O H3O

+ + OH

2 H2O H3O+ + OH

+ -3

c 22

[H O ][OH ]K =

[H O]

With H2O as a solvent, [H2O] becomes a

constant. We rearrange the equation to give

Kc [H2O]2 = [H3O+][OH]

Kw = [H3O+][OH]

For simplicity, we write Kw = [H+][OH]

At 25°C, Kw = 1.0x10-14 = [H+][OH] Know well! Note: 2 sig. fig. for 1.0

5

Kw = 1.0x10-14 = [H+][OH]

As usual, changes in concentration does not change Kw. They only cause shifts in the equilibrium. Only T can change Kw.

Table 17.1 p. 775

6

Kw = 1.0x10-14 = [H+][OH]

Neutral solutions: [H+]=[OH]

Acidic solutions: [H+]>[OH]

Basic solutions: [H+]<[OH]

Note how small Kw is.

This means water is a VERY weak acid.

This is why it is considered nonelectrolytic.

It does not conduct electricity unless it is contaminated with ionic compounds.

Tap water IS electrolytic because it is not pure water and contains salts.

H2O auto-ionization occurs in any solution

• When other ions present– [H+] [OH]

– But Kw = [H+]·[OH] = 1.0 x 1014

• In aqueous solution, • Product of [H+] and [OH] equals Kw

• [H+] and [OH] may not actually equal each other

7

8

In a sample of blood at 25°C, [H+] is 4.6x10-8 M. Find the molar concentration of OH-, and decide if the sample is acidic, basic, or neutral.

[H+][OH]= 1.0x10-14

-14-

+

-14

-8

-7

1.0x10[OH ]

[H ]

1.0x10 =

4.6x10

=2.2x10

Ans. [OH-] = 2.2x10-7 M OH-

(very slightly basic, more OH- than H+)

Pract Exer 17.1 & 17.2 p. 776

Compare

9

pH Scale

When we have a dilute solution of acid, the H+ concentration is very small, such as 0.0001M, and it becomes cumbersome to be talking about H+ concentration of 0.0001 M or 1x10-4M.

The pH scale was invented to make it easier to communicate the acidity of a solution.

KNOW THIS WELL: pH = - log [H+]

or pH = - log [H3O+]

10

e.g. [H+] = 2.43x104 M, what is the pH? pH = log [H+] = log 2.43x104 (3 sig.fig.)

= (3.614) (3 decimal places)

= 3.614e.g. pH = 8.71, what is the [H+]? pH = log [H+] 8.71= log [H+] 8.71 = log [H+]Antilog(-8.71) = antilog log [H+] 10-8.71 = [H+] (2 decimal places)

Ans. [H+] = 1.9x10-9 M (2 sig. fig.)

REMEMBER: 10-pH = [H+]

11

We see that the pH and H+ concentration has an inverse relationship:

As H+ concentration increased, the pH decreases.

The smaller the pH, the more acidic it is.

The larger the pH, the more basic it is.

So, if the swimming pool water has a pH that is too high, that means it is too basic.

12

pOH = log [OH-]e.g. If [OH] = 4.2x103M, what is the pOH? pOH = log 4.2x103 (2 sig.fig.)

= 2.38 = +2.38 (2 decimal places)

e.g. If pOH = 12.3, what is the hydroxide conc? [OH] = 1012.3 (1 decimal place) = 5x1013 M (1 sig. fig.)

THIS IS HOW pH AND pOH ARE RELATED:Remember that [H+] [OH] = 1.0x1014

Finding the log of both sides of the equation gives us…log [H+] + log [OH] = log 1.0x1014

Multiply both sides of equation by 1.( log [H+] + log [OH]) = ( log 1.0x1014)( log [H+] )+( log [OH]) = (14.00) pH + pOH = 14.00 REMEMBER THIS!

13

Practice Exer. 17.4 p. 778Because rain washes pollutants out of the

air, the lakes in many parts of the world have undergone pH changes. In a New England state, the water in one lake was found to have a hydrogen ion conc of 3.2x10-5 mol L-1. What are the calculated pH and pOH? Is it acidic or basic?

Ans. pH = 4.49 (acidic)

pOH = 9.50

14

What is the pH of a NaOH soln at 25 oC in which the hydroxide ion concentration equals 0.0026 M?

Ans. pH = 11.41 (basic, as expected).Practice Exercises 17.3 & 17.5 p. 778

15

Calculate the values of pH, pOH and [OH-] for the following solutions:

a) 0.020M HClb) 0.00035 M Ba(OH)2.You must first note that HCl is a strong acid. Only

because this is so can you assume 0.020M HCl means 0.020 M H+.

Ans. a) pH = 1.70, pOH = 12.30, [OH-]=5.0x10-13M

b) 0.00035M Ba(OH)2 means [OH-] = ?Ans. [OH-] = 0.00070M, pOH = 3.15, pH = 10.85

16

Acid or basic solutes suppress the ionization of water.

H2O + H2O H3O+ + OH

2 H2O H3O+ + OH

Remember Le Chatelier’s Principle:

If an acidic solute (e.g. HCl) is added to water, it shifts the equilibrium to the left, thus suppressing the ionization of water.

This means that in an acidic solution the only source of H+ is from the acid and essentially no contribution from water.

In the same way, in a basic solution the only source of OH- is from the base, none from the dissociation of water.

17

Calculating without a calculator

Often we do not need a calculator to calculate pH or [H+].

[H+] = 108 What is the pH?

[OH] = 106 What is the pH?

pH = 4 What is the pOH?

pOH = 3 What is the pH?

18

A soln was made by dissolving 0.837 g Ba(OH)2 in 100 mL final volume. What is the molar conc of OH- in the soln? What are the pOH and the pH? What is the hydrogen ion concentration in the solution? MM(Ba(OH)2) = 171.3 g mol-1

Ans. [OH-] = 0.0977 M, pOH = 1.010pH = 12.990, [H+] = 1.02x10-13MPractice Exer 6, 7, & 8 p. 779

19

What is the pH of 0.00000001 M HCl?

[H+] =1x108M

pH = 8 (basic)

How can a solution of HCl be basic??

Answer: Its pH will NOT be 8.

At very dilute solutions, the H+ will NOT suppress the dissociation of water.

Water will contribute to the H+ concentration and keep it from becoming basic!

Ans. pH = 7 (stays neutral)