Transcript Chapter 16 Acid-Base Titrations & pH

Chapter 15
Acid-Base Titrations & pH
15.1 Aqueous Solutions & The
Concept of pH
Self-Ionization of Water
• Autoprotolysis:
H2O (l) + H2O (l) → H3O+ (aq) + OH- (aq)
• Molarity at 25°C
1.0 x 10-7 moles H30+ per liter of solution
1.0 x 10-7 moles OH- per liter of solution
Ionization Constant for Water (KW)
• KW = [H3O+][OH-] = (1.0 x 10-7M)(1.0 x 10-7M)
= 1.0 x 10-14M2
• KW is a constant at ordinary ranges of room
temperatures
Neutral: [H3O+] = [OH-]
Acidic:
[H3O+] > [OH-]
Basic:
[H3O+] < [OH-]
Ion Concentration in Water
Calculating [H3O+] and [OH-]
• Assume that strong acids and bases are completely
ionized in solution:
1.0 M H2SO4 = 2.0 M H3O+
1.0 M Ba(OH)2 = 2.0 M OH-
pH Calculations and the Strength of
Acids and Bases
• Weak acids and weak bases cannot be
assumed to be 100% ionized
• [H30+] and [OH-] cannot be determined
from acid and base concentrations, and
must be determined experimentally
H+, OH-, and pH
pH
Scale
The pH Scale
• Due to the variations in soln’s, there are many
possible concentrations of hydronium &
hydroxide ions for solutions.
• Usually this spans 10-14 M to 1 M
• In order to compare substances, the pH scale
was developed.
• Ex:
6 M sol’n of HCl has a H3O+ molarity of 6 M
6 M sol’n of HC2H3O2 has a H3O+ molarity of 0.01M
pH
• pH is a scale in which the concentration of
hydronium ions in solution is expressed as a
number ranging from 0 to 14.
• Instead of referring to a scale of 1 to 10-14, the
pH scale is much easier to use.
• pH is the negative of the exponent of the
hydronium concentration.
Calculating pH & pOH
• pH
The negative of the common logarithm of the
hydronium ion concentration
pH = - log [H3O+]
• pOH
The negative of the common logarithm of the
hydroxide ion concentration
pOH = - log [OH-]
• pH + pOH = 14.0
pH Example
• A solution with a hydronium concentration
of 10-11 M has a pH of 11.
• What would be the pH of a solution with a
hydronium concentration of 10-6 M?
pH = 6
Finding [H3O+], [OH-] from pH, pOH
[H3O+] = 10-pH
[OH-] = 10-pOH
Interpreting the pH Scale
•
•
•
•
The pH scale is divided into 3 main areas:
If it is exactly 7, it is neutral.
If it is less than 7, it is acidic.
If it is more than 7, it is basic.
pH Scale
Interpreting the pH Scale
• As pH decreases below 7, hydronium ion
concentrations increase & hydroxide ion
concentrations decrease.
** pH values differ in factors of 10.
Ex:
An acidic sol’n w/ a pH of 3 has 10 times
the hydronium concentration as a sol’n w/
a pH of 4.
Interpreting the pH Scale
• As pH increases above 7, hydroxide ion
concentrations increase & hydronium ion
concentrations decrease.
Ex: A basic sol’n w/ a pH of 9 has 10 times
the hydroxide concentration as a sol’n w/ a
pH of 8.
• A neutral sol’n has equal concentrations of
hydronium and hydroxide ions.
pH
+
pOH
=
14
Significant Figures & pH
• Significant digits when calculations involve
logarithms are dependant only on the
number of digits to the right of the decimal.
• Example:
This concentration has 2
significant digits
[H3O+] = 2.5 x 10-3
the pH will have 2 digits to
pH ==?????
pH
2.60 So
the right of the decimal point
Practice
1. Determine the [H3O+] & [OH-] in a 0.01 M solution
of HClO4.
2. An aqueous solution of Ba(OH)2 has a [H3O+] of 1
x 10-11 M. What is the [OH-]? What is the molarity
of the solution?
3. Determine the pH of a 1 x 10-4 M solution of HBr.
4. Determine the pH of a 5 x 10-4 M solution of
Ca(OH)2.
5. What is the pH of a solution whose [H3O+] = 6.2 x
10-9 M?
6. Determine the pH of a 0.00074 M solution of
NaOH.
More Practice
7. What are the [H3O+] & [OH-] of a solution if its
pH = 9.0?
8. The pH of a solution if 10.0. What is the
concentration of hydroxide ions in the
solution? If the solution is Sr(OH)2 (aq), what is
its molarity?
9. The pH of a hydrochloric acid solution for
cleaning tile is 0.45. What is the [H3O+] in the
solution?
10.A shampoo has a pH of 8.7. What are [H3O+]
& [OH-] in the shampoo?
Practice Answers!
1. [H3O+] = 1 x 10-2 M, [OH-] = 1 x 10-12 M
2. [OH-] = 1 x 10-3 M, [Ba(OH)2] = 5 x 10-4 M
3. pH = 4.0
4. pH = 11.0
5. pH = 8.21
6. pH = 10.87
7. [H3O+] = 1 x 10-9 M, [OH-] = 1 x 10-5 M
8. [OH-] = 1 x 10-4 M, [Sr(OH)2] = 5 x 10-5 M
9. [H3O+] = 0.35 M
10.[H3O+] = 2 x 10-9 M, [OH-] = 5 x 10-6 M
Chapter 15
Acid-Base Titrations & pH
15.2 Determining pH & Titrations
Indicators and pH Meters
• Acid-Base Indicators
Compounds whose colors are sensitive to pH
• Transition Interval
pH range over which an indicator color
change occurs
• Indicators are useful when they change color
in a pH range which includes the endpoint of
the reaction
Using Indicators to Measure pH
• A pH meter is the most accurate
way to measure pH.
• Measures voltage difference
between two electrodes
• It will determine the exact pH of a sol’n.
• There are also colored dyes that will change in a
predictable way according to a standard chart.
These are called indicators.
pH
Indicators
and their
ranges
Acid-Base Titration
• Titration
Controlled addition of the measured amount of a
solution of a known concentration required to react
completely with a measured amount of sol’n of
unknown concentration
• Equivalence Point
The point at which the solutions used in a titration
are present in chemically equivalent amounts
• Titration Curves:
End point
The point in a titration at which the rxn is just
completed
Titration Curves
Molarity and Titration
• Standard Solution
A solution that contains the precisely known
concentration of a solute, used in titration to
find the concentration of the solution of
unknown concentration
• Primary Standard
A highly purified solid compound used to
check the concentration of the known solution
in a titration
Calculations with Molar Titrations
1. Start with the balanced equation for the
neutralization reaction and determine the
chemically equivalent amounts of the acid and
base
2. Determine the moles of acid (or base) from the
known solution used during the titration
3. Determine the moles of solute of the unknown
solution used during the titration
4. Determine the molarity of the unknown solution
Titration Calculations
More Practice!
1. How many moles of HCl are in 31.15 mL
of a 0.688 M solution?
2. How many moles of NaOH would
neutralize 20.0 mL of a 13.9 M solution of
H2SO4?
3. How many milliliters of a 2.76 M KOH
solution contain 0.0825 mol of KOH?
More Practice!
4. A 25.00 mL sample of a solution of RbOH is
neutralized by 19.22 mL of a 1.017 M solution of
HBr. What is the molarity of RbOH?
5. If 29.96 mL of a solution of Ba(OH)2 requires
16.08 mL of a 2.303 M solution of HNO3 for
complete titration, what is the molarity of the
Ba(OH)2 solution?
6. You have a vinegar solution believed to be 0.83
M. You are going to titrate 20.00 mL of it with a
NaOH solution known to be 0.519 M. At what
volume of added NaOH would you expect to see
an endpoint?
Answers!
1.
2.
3.
4.
5.
6.
2.14 x 10-2 mol HCl
5.56 x 10-2 mol NaOH
29.9 mL
0.7819 M RbOH
0.6180 M Ba(OH)2
32 mL NaOH
Buffers
• Buffers have many
important biological
functions. They keep a
solution at a constant pH,
when manageable amounts
of acid of base are added.
• Ex: Your blood is a buffer! Its pH is very
slightly basic at 7.4. Even though you may eat
many different types of foods or medicines,
your blood pH stays relatively stable, varying
only about 0.1. That means your blood
controls its own pH!
Buffers
• Buffers contain ions or molecules that
react with hydronium or hydroxide if they
are added to the solution. That means,
even if you add an acid or a base, your pH
will stay the same.
• To make a buffer, you combine a weak
acid or a weak base with its corresponding
salt.
Buffers
• Example: Ammonia is combined with its
salt, NH4Cl, in sol’n:
• If acid is added to this solution, ammonia
reacts with the H+ :
NH3 (aq) + H+ (aq) → NH4+ (aq)
• If a base is added to this solution, the
NH4+ from the dissolved salt will react with
the OH- :
NH4+ (aq) + OH- (aq) → NH3 (aq) + H2O (l)
Buffers
• Blood’s pH is regulated by many systems,
but dissolved CO2 is a very important
method. Carbonic acid, H2CO3, and the
hydrogen carbonate ion, HCO3-, are both
dissolved in your blood.
CO2 (g) + H2O (l) → H2CO3 (aq)
• If you add OH- :
H2CO3 (aq) + OH- → HCO3- (aq) + H2O (l)
• If you add H+ :
HCO3- (aq) + H+ → H2CO3 (aq)
Buffers
• Your lungs control the amount of
carbon dioxide in your body. If your
body takes in too much carbon
dioxide, your blood may become too
acidic so you may yawn to lower the
concentration of carbonic acid by
expelling CO2.
Buffers
• If you hyperventilate, too much CO2 is
expelled, which causes the concentration
of carbonic acid to become too low, and
your blood may become too basic.
Breathing into a paper bag will increase
the concentration of CO2 in your lungs
and restore the proper pH.