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 Acids/Bases: Intro
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The Equilibrium of Acids and Bases

Exercises
  1. A Brief Review

Acids

Bases

Operational Definitions: If you recall, operational definitions are based on what you can actually observe in the lab.

  • Acids taste sour.
  • They conduct electricity.
  • They destroy the properties of bases.
  • They turn blue litmus red.
  • They release H2 gas when added to some metals.

Operational Definitions: If you recall, operational definitions are based on what you can actually observe in the lab.

  • Bases taste bitter.
  • They conduct electricity.
  • They destroy the properties of acids.
  • They turn red litmus blue.
  • They feel slippery.
  • They turn fats into soaps.

Conceptual Definitions: The Arrhenius definition of an acid: a substance that releases H+1.

Example: HCl(aq) --> H+1(aq) + Cl-1(aq)

Very important: Always remember that when considering acids and bases, the H+1 ion is aqueous, in other words, it is dissolved in water.

 Conceptual Definitions: The Arrhenius definition of an acid: a substance that releases OH-1.

Example: NaOH(aq) --> Na+1(aq) + OH-1(aq).

Very important: Always remember that when considering acids and bases, the OH-1 ion is aqueous, in other words, it is dissolved in water.

Everyday substances that are acidic (pH <7) include fruits (contain citric and/or malic and/or tartaric acids) soda drinks(carbonic acid and/or phosphoric acid), toilet bowl cleaner (HCl), muriatic acid brick cleaner (HCl).

Everyday substances that are alkaline (basic) (pH >7) include baking soda (NaHCO3), soap, milk of magnesia (Mg(OH)2),ashes (contain KOH) and household ammonia (NH4OH).

 

2.Connection Between Kequilibrium and Acids/Bases.

a. Kw and pH Formula

A small amount of water constantly dissociates into H+1 and OH-1 ions. Of course these reassociate into water at the same rate to create the following equilibrium:

H2O(l) = H+1(aq) + OH-1(aq).

Kw = [H+1] [OH-1]

Notice we give water's equilibrium constant a special name: " Kw".

Experimentally Kw = 1.0 X 10-14, so you can see from the small value for K that there are very few H+1 and OH-1 ions in water.

Let's sidetrack for a minute: In chemistry "p" is an abbreviation for -log. It was introduced by a Danish biochemist, Sorenson in 1909— p is the initial letter of the German word potenz = power.

So pH = - log [H+1] or rearranging it mathematically, [H+1] =10-pH

 

Example 1

If [H+1] = 1.0 X 10-2 M in your stomach, what is its pH?

Solution

pH = - log [H+1]

pH = -log (1.0 X 10-2) = - (-2) = 2.

 

Example 2

If the pH of tomato juice = 3.5, what is its [H+1]?

Solution

[H+1] =10-pH

[H+1] =10-3.5 = 0.000316 mole/L or 0.000316 M

 

b. Why Is Neutral Stuff at a pH=7 ?

In pure water, or in any neutral solution, [H+1] = [OH-1], so by substituting, the expression

Kw = [H+1] [OH-1] becomes

Kw = [H+1] [H+1] or

  1. X 10-14 = [H+1]2. Square rooting both sides:
  1. X 10-7 = [H+1]. But since pH = - log [H+1],

pH = 7.

 

c. Why Is Acidic Stuff at a pH <7 ?

Acids contain a lot more H+1 than OH-1. (You might be wondering why there is any OH-1 in acids. Well there is always a bit because it comes from water. Remember its equilibrium.).
As a result, [H+1] > 1 X 10-7 .
Take the log of both sides:
log [H+1] >-7. Now multiply both sides by –1. Remember math: inequality flips when multiplying by a (-).
-log [H+1] <-(-7).
pH < 7 for acidic substances.
 

d. Why Is Alkaline (basic) Stuff at a pH >7 ?

Bases contain a lot more OH-1 than H+1. Even in bases there is always a bit of H+1 that comes from water but of course there is even less than you would find in neutral water.
As a result, [H+1] < 1 X 10-7 .
Take the log of both sides:
log [H+1] <-7. Now multiply both sides by –1. Remember math: inequality flips when multiplying by a (-).
-log [H+1] >-(-7).
pH > 7 for alkaline substances.

e. The concept of pOH

[H+1] [OH-1] = 1 X 10-14.
Log both sides:
log ([H+1] [OH-1]) = -14. Remember from math : log AB = log A + log B:
log[H+1] + log[OH-1] = -14. Multiply through by (-1):
-log[H+1] - log[OH-1] = 14.
pH + pOH = 14, and note that pOH = -log[OH-1] or [OH-1] =10-pOH
Example 2: If an aqueous sodium hydroxide solution has [OH-1] = 0.01 M, what its pOH? Its pH?
pOH = -log[OH-1]
= -log[0.01] = 2
pH = 14 – pOH = 14 – 2 = 12.
Example 3:
Complete the following table:

 

[H+1] mol/L

[OH-1] mol./L

pH

pOH

Neutral, acidic, or basic

A

1 x 10-3

 

 

 

 

B

  

 2.0 X 10-4

 

 

 

C

  
 

 

 3

 

D

  

 

 6

 

 
Solution
 

 

[H+1] mol/L

[OH-1] mol./L

pH

pOH

Neutral, acidic, or basic

A

1 x 10-3
 Step 4
[OH-1] =10-pOH
= 10-11
or since [H+1][OH-1]=10-14
[OH-1] = 10-14/[H+1]
10-14/10-3=10-11

 Step1

pH =-log(1 x 10-3) = 3

 Step 3

pOH =14 – pH

=14 –3 = 11

 Step 2

Acidic, because pH<7.

B

5.0 X10-11

 2.0 X 10-4

 14-3.70 = 10.3

 3.70

  alkaline

C

10-11
 10-3

 11

 3

 alkaline

D

10-6

 10-8

 6

 8

 acidic
Notice that a difference of "only" 3 pH units between two acids is a lot bigger than the numbers seem to indicate. A pH =3 solution is 1000 times more acidic ( has a 1000 times more H+1) than a pH = 6 solution. See above table.