Acids
• Arrhenius Model
• Produce hydrogen ions aqueous solution.
• HCl  H+ (aq) + Cl- (aq)
• Acids you SHOULD know:
• Strong Acids
• Hydrochloric acid, HCl
• Nitric acid, HNO3
• Sulfuric acid, H2SO4
• Weak Acids
• Phosphoric acid, H3PO4
• Acetic acid, HC2H3O2
Properties of Acids
• Acids taste sour
• Acids affect indicators
• Blue litmus turns red
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Methyl orange turns red
Acids have a pH lower than 7
Acids are proton (hydrogen ion, H+) donors
Acids react with active metals, produce H2
Acids react with carbonates
Acids neutralize bases
Bases
• Arrhenius Model
• Produce Hydroxide ions.
• NaOH  Na+ (aq) + OH- (aq)
• Examples of Bases
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Sodium hydroxide (lye), NaOH
Potassium hydroxide, KOH
Magnesium hydroxide, Mg(OH)2
Calcium hydroxide (lime), Ca(OH)2
Properties of Bases
• Bases taste bitter
• Bases affect indicators
• Red litmus turns blue
• Phenolphthalein turns purple
• Bases have a pH greater than 7
• Bases are proton (hydrogen ion, H+)
acceptors
• Solutions of bases feel slippery
• Bases neutralize acids
Bronsted – Lowery Model
• ACID: Substance that can donate
proton (H+1).
• BASE: Substance that can accept
proton (must contain lone pair of
electrons).
The pH Scale
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The pH scale measures how acidic or basic a substance is.
The pH scale ranges from 0 to 14. A pH of 7 is neutral.
A pH less than 7 is acidic. A pH greater than 7 is basic.
The pH scale is logarithmic and as a result, each whole
pH value below 7 is ten times more acidic than the next
higher value.
• For example, pH 4 is ten times more acidic than pH 5 and 100
times (10 times 10) more acidic than pH 6.
• The same holds true for pH values above 7, each of which is
ten times more alkaline (another way to say basic) than the
next lower whole value.
Calculating pH or pOH
• pH = -log[H+]
• Example [H+] = 1.0 x 10-9
• Answer pH=9.0
• pOH = -log[OH+]
• Example [OH+] = 1.0 x 10-6
• Answer pOH = 6
• pOH + pH = 14.0
Acids Neutralize Bases
• HCl + NaOH  NaCl + H2O
• Neutralization reactions ALWAYS
produce a salt and water.
• H2SO4 + 2NaOH  Na2SO4 + 2H2O
• 2HNO3 + Mg(OH)2  Mg(NO3)2 + 2H2O
Bases Neutralize Acids
Milk of Magnesia contains
magnesium hydroxide, Mg(OH)2,
which neutralizes stomach acid,
HCl.
2 HCl + Mg(OH)2
MgCl2 + 2 H2O
Titration:
• A technique for
determining the
concentration of
a solution.
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Acid Base Titration
• Titration is a method for determining the
concentration of a solution by reacting a
known volume of that solution with a
solution of known concentration.
• In a titration procedure, a measured
volume of an acid or base of unknown
concentration is placed in a beaker or flask,
and initial pH recorded.
• A neutralization reaction is a reaction in
which an acid and a base in an aqueous
solution react to produce a salt and water.
• A salt is an ionic compound made up of a
cation from a base and an anion from an
acid.
• Neutralization is a double-replacement
reaction.
Titration Procedure
1) A measured volume of an acidic or basic solution of
unknown concentration is placed in a beaker. The electrodes
of a pH meter are immersed in this solution, and the initial
pH of the solution is read and recorded.
2) A buret is filled with the titrating solution of known
concentration. This is called the standard solution, or titrant.
3) Measured volumes of the standard solution are added slowly
and mixed into the solution in the beaker. The pH is read and
recorded after each addition. This process continues until the
reaction reaches the equivalence point, which is the point at
which moles of H+ ion from the acid equal moles of OH- ion
from the base.
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• Standard Solution or titrant– An acid/
base solution whose concentration is
known.
• End point: The point at which an
indicator changes color.
• Equivalence point: The point at which
moles of H+ ion from the acid equals
moles of OH– ion from the base. An
abrupt change in pH occurs at the
equivalence point.
Acid-Base Indicators
• Chemists often use a chemical dye rather than a pH meter to detect
the equivalence point of an acid-base titration.
• Chemical dyes whose colors are affected by acidic and basic
solutions are called acid-base indicators.
• Many natural substances act as indicators.
– If you use lemon juice in your tea, you might have noticed that the
brown color of tea gets lighter when lemon juice is added.
– Tea contains compounds called polyphenols that have slightly ionizable
hydrogen atoms and therefore are weak acids.
– Adding acid in the form of lemon juice to a cup of tea lessens the degree
of ionization, and the color of the un-ionized polyphenols becomes more
apparent.
• Chemists have several choices in selecting indicators.
– Bromthymol blue is a good choice for the titration of a strong acid with a
strong base, and phenolphthalein changes color at the equivalence point
of a titration of a weak acid with a strong base.
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• Chemical dyes
whose color are
affected by acidic
and basic solutions are
called acid-base indicators.
Indicators and Titration End Point
• Many indicators used for titrations are weak acids.
– Each has its own particular pH or pH ranges over which it
changes color.
• The point at which the indicator used in a titration changes
color is called the end point of the titration.
– It is important to choose an indicator for a titration that will
change color at the equivalence point of the titration.
– Remember that the role of the indicator is to indicate to you,
by means of a color change, that just enough of the titrating
solution has been added to neutralize the unknown solution.
• Equivalence point ≠ End point!
– BUT for strong-strong titrations, the pH change is so steep
and so large, that the are approximately equal.
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Titration with an Indicator
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