Transcript Document

Strength of Acids and Bases
• The strength of an acid (or base) is determined by
the amount of IONIZATION
• This is not the concentration
• If the acid (or base) completely dissociates to
produce H+ (or OH-) ions in solution, STRONG ACID
(or BASE)
• If the acid (or base) dissociates to a slight extent in
aqueous solution, WEAK ACID (or BASE)
STRONG ACIDS
• A strong acid contains a relatively weak
conjugate base
• One that has a low attraction for protons
• HNO3, HCl, H2SO4 and HClO4 are among the
common strong acids
Weak Acids
• Weak acids are much less than 100% ionized in water
• Contains a relatively strong conjugate base
– High attraction for protons
• One of the best known is acetic acid (CH3CO2H)
STRENGTH OF
ACIDS AND
THEIR
CONJUGATE
BASES
Strong and Weak Bases
• Strong Base
– 100% dissociated in water
NaOH (aq) ---> Na+ (aq) + OH- (aq)
• Weak base
– Less than 100% ionized in water
– One of the best known weak bases is
ammonia
NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq)
COMMON ACIDS AND BASES
WATER WILL IONIZE ITSELF
• Some small amount of water will always dissociate,
whether pure water or part of a solution
• How much will dissociate is predicatable using the
following math relationship:
H+ OH− = 1 × 10−14 = K w
• Kw is called the ION-PRODUCT constant for water
– In any aqueous solution, no matter what it contains,
the product of [H+] and [OH-] must equal 1.0 x 10-14
• This is an important relation when understanding
acids/bases
• Please notice:
– It is an inverse relation
• When [H+] increases, [OH-] decreases and vice versa
– It is the basis for the pH scale!
DETERMINING THE ACIDICITY OF A
SOLUTION
• [H+] is critical in many chemical reactions
• Because [H+] in an aqueous solution is typically quite small, a
logarithmic pH scale is used!
• By definition:
pH = – log [H+], [H+] = 10-pH
• The pH scale, similar to the Richter scale, describes a wide range
of values
– Because the pH scale is a log scale based on 10, the pH
changes by 1 for every power-of-10 change in [H+]
– An earthquake of “6” is 10 times as violent as a “5”
• Thus, the pH scale condenses possible values of [H+] to a 14
point scale
• Also, it is easier to say pH = 7 vs. [H+] = 1 x 10–7
How to Read the pH Scale
Under 7 = acid
7 = neutral
Over 7 = base
CALCULATIONS WITH PH
pH = - log [H+]; [H+] = 10-pH
(Remember that the [ ] mean Molarity)
• What is the pH if [H+]= 6.3 x 10–5?
Ans: 4.2
• What is the [H+] if pH = 7.4?
3.98x10–8 M
MORE PRACTICE
Calculate the pH of each of the following:
• A 0.15 M solution of hydrochloric acid
• A 3.00 X 10-7 M solution of nitric acid
Determining the Basicity of a Solution
pOH
• Since acids and bases are opposites, pH and
pOH are opposites!
• pOH does not really exist, but it is useful for
changing bases to pH
• pOH looks at the perspective of a base:
pOH = - log
[OH ]
• Since pH and pOH are on opposite ends:
pH + pOH = 14
pH
[H+]
[OH-]
pOH
PRACTICE!
• The pH of rainwater collected in a certain region of
the northeastern United States on a particular day
was 4.82. What is the H+ ion concentration of the
rainwater?
• The OH- ion concentration of a blood sample is 2.5
x 10-7 M. What is the pH of the blood?
Calculating [H3O+], pH, [OH-], and pOH
• A chemist dilutes concentrated hydrochloric acid to
make two solutions: (a) 3.0 M and (b) 0.0024 M.
Calculate the [H3O+], pH, [OH-], and pOH of the two
solutions at 25°C.
• What is the [H3O+], [OH-], and pOH of a solution with
pH = 3.67? Is this an acid, base, or neutral?
• Problem above with pH = 8.05?
pH TESTING
• There are several ways to test pH
– Blue litmus paper
• red = acid
– Red litmus paper
• blue = basic
– pH paper
• multi-colored
– pH meter
• 7 is neutral, <7 acid, >7 base
– Universal indicator
• multi-colored
– Indicators
• Phenolphthalein
– Natural indicators like red cabbage, radishes
pH Indicators
• Indicators are dyes that can be
added that will change color in
the presence of an acid or base
• Some indicators only work in a
specific range of pH
• Once the drops are added, the
sample is ruined
• Some dyes are natural, like
radish skin or red cabbage
Paper Testing
• Paper tests like litmus paper and pH paper
– Put a stirring rod into the solution and stir
– Take the stirring rod out
– Place a drop of the solution from the end
of the stirring rod onto a piece of the
paper
– Read and record the color change
– Note what the color indicates
– You should only use a small portion of the
paper
• You can use one piece of paper for
several tests
pH Meter
• Remember that acids and bases
are electrolytes
– Strong acids/bases are STRONG
electrolytes
– Weak acids/bases are WEAK
electrolytes
• Tests the voltage of the electrolyte
• Converts the voltage to pH
• Very cheap, accurate
• Must be calibrated with a buffer
solution
ACID-BASE REACTIONS
TITRATIONS
• To analyze the acid or base content of a solution, chemists often
perform a TITRATION
– A titration involves the delivery of a measured volume of a
solution of known concentration (TITRANT) from a buret into
the solution being analyzed (ANALYTE)
– Titrant is added slowly to the analyte until exactly enough has
been added to just react with all of the analyte
• EQUIVALENCE POINT
– [H+] = [OH-]
– Indicator or pH meter is used to determine equivalence point
– Neutralization is said to occur when the acid and base have
same numbers of moles present in the combined solution
Setup for Titrating an Acid with a Base
HOW TO SOLVE A TITRATION-TYPE
PROBLEM
• Based on stoichiometry
– Write a balanced chemical equation
– Calculate the number of moles of the standard solution present in the solution
• Standard solution is the solution in the buret and has a known concentration
• Will always be given a volume and concentration of the standard solution
– Use the molar relationship from the equation (STOICHIOMETRY) to convert from
moles of standard solution into moles of unknown solution
– Calculate the unknown quantity (molarity or volume) of the unknown solution by
using the relationship M = n/V using the number of moles found in step 3
– Sometimes, you will need to convert from moles to other quantities such as
volume of a gas (L) and grams
• Remember your stoichiometry road map!
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 1
• Boric acid (H3BO3) is a solid acid that
is good for controlling ants. It also
neutralizes bases. How many grams
of boric acid would be needed to
neutralize 35.00 grams of calcium
hydroxide?
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 2
• 35.62 mL of NaOH is neutralized with
25.2 mL of 0.0998 M HCl by titration
to an equivalence point. What is the
concentration of the NaOH?
NEUTRALIZATION (TITRATION)
EXAMPLE PROBLEM 3
• 25 drops of citric acid are titrated with .112 M
KOH until the indicator changes from yellow to
blue. 48 drops of KOH are added before a
color change occurs.
What is the
concentration of the citric acid? What is the pH
of the citric acid?
**20 drops = 1 mL**