Chapter 10 Acids and Bases

Acids produce H+ ions in waterH2O

HCl(g) H+(aq) + Cl−(aq)

they are electrolyteshave a sour taste turn litmus redneutralize bases

Some acids like sulfuric and phosphoric release more than 1 H+ in water; other like acetic acid (vinegar) release far less than 1 H+ per molecule

Bases produce OH− ions in waterare electrolytesfeel soapy and slipperyneutralize acids

NaOH sodium hydroxideKOH potassium hydroxide

sodium and potassium hydroxide release 1 OH- /molecule

other bases such as ammonium hydroxide (NH4OH) release far fewer OH-

fewer

Milk of Magnesia (Mg(OH)2

Tums Ca(CO3

vinegar C2H4O2

Cola H3PO4

Strong acids completely ionize (100%) in aqueous solutions.HCl(g) + H2O(l) H3O+(aq) + Cl−(aq)

Amount of acid added

Weak acids dissociate only slightly in water to form a solution of mostly molecules and a few ions.

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3−(aq)

NH3(g) + H2O(l) NH4+(aq) + OH−(aq) Windex weak base

H2CO3 + OH- HCO3- + H2O CO3

= + H3O+ Baking Soda weak base

NaOH Na+ + OH- Drano strong base

Water reacts with itself in the following manner:

H+ is transferred from one H2O molecule to another ;one water molecule acts as an acid, while another acts as a base

H2O + H2O H3O+ + OH−

.. .. .. ..H:O: + H:O: H:O:H+ + :O:H−

.. .. .. ..H H H

water water hydronium hydroxideion(+) ion(-)

The concentration of

H3O+ = OH- = 10-7 mols/L

pHThe pH of a solution is used to indicate

the acidity of a solution;it has values that usually range

from 0 to 14;the solution is acidic when the

values are less than 7;the solution is neutral with

a pH of 7;the solution is basic when the

values are greater than 7

How is the numerical value of pH determined?

pH = - log[H3O+ concentration]; pOH = -log [OH- concentration]

when the H3O+ concentration is

expressed in mols/L

pH + pOH = 14

What is the log of 4*108?

log[4]+log[108]

log[4] = 0.6

log[108] = 8

log[4*108] = 8.6

What is the log of 4*10-8?

log[10-8] = -8

log[4*10-8] = -7.4

pH = 7.4

Reactions of acids and bases

Acid + Base = Salt + Water

Mg(OH)2 + HCl (gastric juice) = MgCl2 + H2O

Mg(OH)2 + 2HCl (gastric juice) = MgCl2 + 2 H2O

CaCO3 + HCl = CaCl2 + H2CO3 = CaCl2 + H2O + CO2

CaCO3 + 2HCl = CaCl2 + 2H2CO3 = CaCl2 + 2H2O + 2CO2+ burp

O H - ( e q u i v a l e n t s )

0 .0 0 0 .0 2 0 .0 4 0 .0 6 0 .0 8 0 .1 0 0 .1 2

pH

0

1

2

3

4

5

6

7

How does the pH vary if we add NaOH (0.1 mol/L) dropwise to a solution of HCl (0.1 mol/L)?

HCl + NaOH = H2O + NaCl

pH of resulting solution

buffered in this region

0.001 M H+

0.1

0.01

Moles of NaOH added

How does the pH vary is we add NaOH (0.1 mol/L) dropwise to a solution of the weak

acid acetic acid (0.1 mol/L)?

OH - (equivalents)

0.00 0.02 0.04 0.06 0.08 0.10

pH

2

3

4

5

6

7

8

9

HOAc + NaOH = H2O + NaOAc

pH of resulting solution

buffered in this entire region

Moles of NaOH added

• The most important weak acid in blood is NaHCO3-

• H2CO3 is a very weak acid; however both hydrogens can be removed in the presence of strong base; the pH of a solution of NaHCO3 is very close to physiological pH; in the presence of an acid the HCO3- ion tends to pick up the proton, thus buffering the solution and preventing the solution to become too acidic.

• H+ + HCO3- H2CO3 CO2 + H2O

In the presence of a base, the HCO3- ion can

lose its proton as H+ and thus neutralize the strong base; thus the HCO3

- ion can buffer the solution in both directions

HCO3- + OH- CO3

-2 + H2O

The pH in living systems is very important. For example the pH of blood is kept at 7.4 and must be maintained within ±0.5 pH units. How is this done?

At a pH of 7.4, most CO2 is in the form of HCO3-

HCO3- can react with either acid or base

HCO3- + H3O+ H2CO3 CO2 + H2O

HCO3- + OH- H2O + CO3-2

In this manner, HCO3-

stabilizes the pH and does not allow it to become too acidic or to basic; it acts as a buffer

What is the H+ concentration in blood? pH = -log[H+]

[H+] = 10-7.4

10-7.4 = 100.6*10-8

100.6 = 4

10-7.4= 4*10-8 mols/L

CO2 + H2O = H2CO3 = H+ + HCO3-