acids and bases the concept of acidic and basic solutions is perhaps one of the most important...
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Acids and BasesThe concept of acidic and basic
solutions is perhaps one of the most important topics in chemistry. Acids
and bases affect the properties of foods, biochemical reactions,
pharmaceuticals, and industrial materials.
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Acids and Bases• Properties of Acids• Sour or tart taste.• Corrosive (deteriorate).• Electrolytes• Electrolytes are able to conduct an
electrical current because of the presence of ions in aqueous solutions.
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Acids and Bases• Properties of Acids (cont.)• Will react with most metals to form
hydrogen gas.• Some acids are ‘stronger’ than others• All acids contain a hydrogen that they
can give away.
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Acids and Bases• Properties of Bases• Bitter tasting• Slippery• Caustic – They will degrade biological
tissue. Chemical burns from strong bases are nasty.
• Bases form the hydroxide ion (OH-1) in water.
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Acids and Bases• Arrhenius Acids • Svante Arrhenius (1900) defined an
acid.• Acids are hydrogen containing
compounds that yield a hydrogen ion (H+) in water.
• An Arrhenius acid donate an H+ ion.
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Acids and Bases• Arrhenius Acids • HCl(s) H+
(aq) + Cl-(aq)
• H2SO4(s)
• H2O(l)
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Acids and Bases• Arrhenius Acids • Acids that have one hydrogen ion to
donate are called monoprotic.• Acids that have 2 hydrogen ions to
donate are called diprotic.• Acids that have 3 hydrogen ions to
donate are called ____protic.
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Acids and Bases• Arrhenius Bases • Compounds that produce the
hydroxide ion(s) (OH-1) in water are called Arrhenius Bases.
• NaOH(s)
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Acids and Bases• Arrhenius Acids and Bases • Problem with the Arrhenius
definition; Some bases can form OH- ions in solution but not have an OH- ion in the chemical formula.
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
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Acids and Bases• Bronsted-Lowry Acids and Bases• A better definition of an acid and a base• Bronsted-Lowry Acid – A molecule that
donates an H+ to another molecule.• Bronsted-Lowry Base – A molecule that
accepts an H+ from the B-L acid.• Identify the Bronsted-Lowry acid and base;
NH3(g) + H2O(l) NH4+
(aq) + OH-(aq)
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Acids and Bases• Bronsted-Lowry Acids and Bases• Conjugate Acid – becomes the H+ donor in
the reverse reaction.• Conjugate Base – becomes the H+
acceptor in the reverse reaction.• Identify the BL acid, base, conjugate acid
and conjugate base;HClO2(aq) + H2O(l) H3O+1
(aq) + ClO2-(aq)
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Acids and Bases• The pH Scale – Measures the
concentration of H+ ion in an aqueous solution.
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Acids and Bases• The pH Scale • Remember that a water molecules
ionizes;H2O(l) H+
(aq) + OH-(aq)
• In pure water, the concentration of H+ and OH- each is 1.0 x 10-7M
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Acids and Bases• The pH Scale
H2O(l) H+(aq) + OH-
(aq)
• Therefore, the product of [H+] and [OH-] must be equal to 1.0 x 10-14M2.
• An aqueous solution will always have the concentration of H+ and OH- equal to 1.0x10-14M2.
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Acids and Bases• The pH Scale
H2O(l) H+(aq) + OH-
(aq)
Keq = [H+] x [OH-]
Kw = [1.0x10-7] x [1.0x10-7] = 1.0x10-14
Kw is called the autoionization constant for water.
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Acids and Bases• The pH Scale
Keq = [H+] x [OH-]
[1.0x10-7] x [1.0x10-7] = 1.0x10-14
• If the addition of an acid makes the [H+] increase, then the [OH-] will decrease.
• If the addition of a base makes the [OH-] increase, the [H+] will decrease.
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Acids and Bases• The pH Scale • Calculate the [H+] if 0.05 moles of HCl
is added to 1.0 L of water.
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Acids and Bases• The pH Scale • pH is a measure of the concentration
of H+.
pH = -log[H+]
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Acids and Bases• The pH Scale • What is the pH of an aqueous solution
where [H+] = 1.0 x 10-7?
• What is the pH of an aqueous solution where [H+] = 1.0 x 10-2?
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Acids and Bases• The pH Scale • What is the pH of an aqueous solution
where [H+] = 2.7 x 10-1?
• What is the pH of an aqueous solution where [H+] = 8.0 x 10-12?
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Acids and Bases• The pH Scale • Since Kw = 1 x 10-14 = [H+] x [OH-]
14 = pH + pOH
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Acids and Bases• The pH Scale • Calculate the pOH of an aqueous
solution that has an [H+] = 1.0 x 109.
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Acids and Bases• The pH Scale • Calculate the [H+] of an acid with a pH
of 4.
• Calculate the [H+] of a base with a pH of 10.8.
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Acids and Bases• The pH Scale • Calculate the [OH-] of an acid with a
pH of 2.
• Calculate the [OH-] of a base with a pH of 12.9.
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Acids and Bases• Strong Versus Weak Acids• What makes some acids ‘strong’ and
some ‘weak’?
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Acids and Bases• Strong Versus Weak Acids• We can quantify the relative strength
of an acid by using it’s equilibrium expression (Ka).
• Ka = [products]x = [H+] x [conj. base] [reactants]y [acid]
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Acids and Bases• Strong Versus Weak Acids• Write the Ka expression for HCl.
• Ka =
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Acids and Bases• Strong Versus Weak Acids• Write the Ka expression for H3PO4.
• Ka =
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Acids and Bases• Strong Versus Weak Acids• As the [H+] increases, Ka increases.
Therefore the greater the value of Ka, the more [H+] present, the stronger the acid.
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Acids and Bases• Weak Acids
The pH of a 1.0 M solution of formic acid, HCOOH, is 2.38. Calculate the Ka for formic acid.
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Acids and Bases• Weak AcidsCalculate the Ka of a 0.220 M solution of H3AsO4
that has a pH of 1.50.
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Acids and Bases• Weak AcidsCalculate the Ka of a 0.0400 M solution of HClO2
that has a pH of 1.80.
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Acids and Bases• Weak Acids
Calculate the pH of a 8.6 x 10-3 M solution of H3PO4. Ka of H3PO4 is 1.4 x 10-3.
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Acids and Bases• Salt Hydrolysis• Sometimes an ion from a salt can
make an aqueous solution acidic or basic.
• What happens when sodium bicarbonate dissolves in water?
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Acids and Bases• Salt Hydrolysis• NaHCO3(aq) Na+
(aq) + HCO3-(aq)
• HCO3-(aq) + H2O(l) H2CO3(aq) + OH-(aq)
Now there will more OH- than H+ in the solution making it basic.
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Acids and Bases• Salt Hydrolysis• Will a solution of ammonium chloride,
NH4Cl, be acidic or basic?
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Acids and Bases• Buffers• A buffer is a solution that resists
changes in pH when either an acid or a base is added.
• Buffers consist of either a weak acid with one of its salts, or a weak base with one of its salts.
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Acids and Bases• Buffers• For example, if a solution is make by
dissolving carbonic acid (weak acid) and sodium bicarbonate (salt of the acid) we get the following;
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3
-1(aq)
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Acids and Bases• Buffers
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3
-1(aq)
• If we add a base to this buffered solution, the H3O+ will scoop it up.
H3O+(aq) + OH-
(aq) 2H2O(l)
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Acids and Bases• Buffers
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3
-1(aq)
• If we add an acid to this buffered solution, the HCO3
-1 will scoop it up.
H+(aq) + HCO3
-1 (aq)
H2CO3(aq)
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Acids and Bases• Buffers• Write the chemical reaction for the
phosphoric acid – dihydrogen phosphate buffer reaction.
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Acids and Bases• Neutralization Reactionso Occurs when an equal number of H+ ions
and OH- ions react.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
o The products of a neutralization reaction are always a salt and water.
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Acids and Bases• Neutralization Reactions
KOH(aq) + HNO3(aq)
Ca(OH)2(aq) + HClO3(aq)
H2SO4(aq) + LiOH(aq)
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Acids and Bases• Titrations – Reacting an acid with a
base to a specific pH.
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Acids and Bases• Titrations o The purpose of a titration is to determine
the concentration of either the acid or the base being titrated.
o Usually, the buret contains a known concentration of base while the acid is of an unknown concentration.
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Acids and Bases• Performing a Titration
1. Add a desired amount of the acid into the flask.
2. Load the buret with the base of a known concentration.
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Acids and Bases• Performing a Titration
3. Record the initial volume of the base in the buret. A buret can give a volume reading to 0.01 mL.
4. Add a few drops of the indicator.
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Acids and Bases• Performing a Titration
5. Slowly dispense the base into the acid. Your goal is to change the color of the acid from clear to pink with the addtion of one drop of the base.
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Acids and Bases• Titrations o After the titration is completed, we use the
equation MAVA = MBVB to determine the concentration of the acid.
MA = Concentration of Acid
MB = Concentration of Base
VA = Volume of Acid
VB = Volume of Base
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Acids and Bases• Titrations o Calculate the concentration of 25.0 mL of an
HCl solution that was titrated with 0.25 M NaOH. The initial volume in the buret read 3.50 mL and the final volume read 18.20 mL after the titration was completed.