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Acids, Bases and Salts
• Acids give up hydrogen ions (H+) in a water solution.
• Bases give up hydroxide ions (OH-) in a water solution.
Properties of Bases
• Bitter taste• Feel slippery if you touch them• pH > 7.0• Strong bases:
– Sodium hydroxide (NaOH)– Potassium hydroxide (KOH)
• Weak bases:– Sodium bicarbonate (NaHCO3)
Properties of Acids• Sour taste • pH < 7.0• React with metals to form H2 gas• Strong acids:
– Sulfuric acid (H2SO4)– Hydrochloric acid (HCl)
• Weak acids: – Acetic acid (vinegar – CH3COOH))– Citric acid (in fruits)– Ascorbic acid (Vitamin C)
Naming Acids
• Binary Acid = 2 elements (HCl)– Begin with hydro-.– Use name of 2nd element and end with –ic.– HCl is hydrochloric acid.
• Oxyacid = H + O + 3rd element (H2SO4)
– Usually incorporates the polyatomic ion name into the acid name.
– Written with H first, then the polyatomic ion.
– H2SO4 is sulfuric acid.
Acid Strength• Strong acids ionize completely in water.
– Many H3O+ ions– Strong electrolyte
• Weak acids do not ionize completely in water.– Some H3O+ ions– Weak electrolyte– The ionized H3O+ ions are being bonded back to the
negative species (reverse reaction) at the same time some acid molecules are being ionized to form H3O+ ions .
• Strong and weak bases are similar, except ion involved is OH- instead of H3O+ .
Classifying strength of electrolytes from solubility
(general rules for compounds)• Strong electrolyte
– Water soluble AND ionic– Strong acid, water soluble and not ionic– Strong base, water soluble and not ionic
• Weak electrolyte– Weak acid or base, water soluble and not
ionic
• Nonelectrolyte– None of the above
Cl
The Hydronium Ion• Ionization is the creation of ions from a
molecular compound.– If the force of attraction between solvent molecules
and parts of the solute are stronger than the covalent bonds of the solute, the solute breaks into ions.
– In water, HYDRATION produces heat, which provides energy to break more covalent bonds.
• H + attracts other molecules or ions • In water, H+ becomes a proton bonded to the
oxygen of a water molecule.• The hydronium ion is H3O+.
ClOO+ +-+
H H H
H
H
H
Acid-Base Systems
Type Acid Base Examples
Arrhenius
H+ or H3O+ producer
OH- producer
HNO3 + H2O H3O+ + NO3
-
NH3 + H2O NH4+ + OH-
Brønsted-Lowry
proton (H+) donor
proton (H+) acceptor
HCl + NH3 NH4+ + Cl-
Lewis electron pair acceptor
electron pair donor
BF3 + F- BF4-
BF3 is Lewis acid,
F- is Lewis base
Brønsted-Lowry Acids
• Brønsted-Lowry acids donate a proton.
• A monoprotic acid donates one proton per molecule, such as HCl and HClO4.
• A polyprotic acid donates more than one proton per molecule, such as H2SO4 and H3PO4.
– H2SO4 is a diprotic acid.
– H3PO4 is a triprotic acid.
Conjugate acids and bases• The species remaining after a Brønsted-Lowry
acid gives up its proton is the conjugate base of that acid: Take off one H from the acid.
• The species remaining after a Brønsted-Lowry base accepts its proton is the conjugate acid of that base: Take off one H. Add an H to the base.
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO- (aq)
acid base conjugate conjugate
(proton donor) acid base
Examples of conjugate acids and basesHCO3
- (aq) + H2O(l) H2CO3 (aq) + OH- (aq)
base acid conjugate conjugate
(proton acid base
acceptor)
HF(aq) + H2O(l) H3O+(aq) + F- (aq)
acid base conjugate conjugate
(proton acid base
donor)
Salts
• An acid and base combined together react to neutralize each other.
• Neutralization is a process which produces produces that are not acids or bases.
• H+ and OH- make water.• The remaining reactants combine to make a salt.• A salt is an ionic compound made of a metal
and a nonmetal.• Soap and detergent are organic salts.
Neutralization
HCl + NaOH NaCl + H2O
HNO3 + KOH KNO3 + H2O
H++ OH- H2O
ACID + BASE YIELDS SALT + WATER
saltacid base water
Neutralization• The reaction between an acid and a base which
produces a salt and water. • Recall: A salt is an ionic compound: metal +
nonmetal which is likely to dissociate in water.• Ex: You take a dose of the antacid magnesium
hydroxide, Mg(OH)2, to relieve excess stomach acid, HCl.
• Equations for this neutralization:
2HCl(aq) + Mg(OH)2 (s)MgCl2(aq) + 2H2O(l)
2H+(aq) + Mg(OH)2 (s) Mg2+(aq) + 2H2O(l)
Ionic Equations for Acid-Base Reactions
• H+(aq) + OH-(aq) H2O (l)
• Strong acids ionize completely.
• HCl H+ + Cl-
• Weak acids do not ionize completely.
• HF H+ + F-
Water and ammonia are a weak acid and a weak base, respectively.
• NH3(aq) + H2O (l) NH4+(aq) + OH- (aq)
Measuring Strength of Acids and Bases
• pH is a measurement which indicates acidic or basic strength, measuring the concentration of H+ ions.
• More H+ ions = lower pH = acid.
• An indicator changes color depending on pH of a solution.
• pH paper is treated with indicators to change color when dipped in a solution.
1: acid 7: neutral 14: base
pH and acidity• pH is related to the concept of concentration of
hydronium ions found in water.• Water and all its solutions contain hydronium ions and
hydroxide ions.
• Acidic solutions: More H3O+ ions.
• Basic solutions: Fewer H3O+ ions / more OH- ions.
• If pH = 2:
H3O+ ion concentration = 0.01 M
OH- ion concentration = 0.000000000001 M• If pH = 12:
H3O+ ion concentration = 0.000000000001 M
OH- ion concentration = 0.01 M
The pH scale
• pH stands for “power of hydronium ion.”
pH = -log [H3O+]
• pH value is the exponent on the power of 10 with its sign changed.
pH examples
• Example 1: – 0.001 mol H3O+ = 1 x 10-3 mol H3O+
– Concentration is 0.001M
– pH = -log [H3O+] = -log (1x 10-3 ) = -(-3.0) = 3.0
– pH is 3.0 - This is an acid.
• Example 2: – 0.000 000 01 mol H3O+ = 1 x 10-8 mol H3O+
– Concentration is 0.000 000 01M
– pH = -log [H3O+] = -log (1x 10-8 ) = -(-8.0) = 8.0
– pH is 8.0 - This is a base.
pOH
• pOH is the negative of the logarithm of the hydroxide ion (OH-) concentration.
pOH = -log [OH-]
pH + pOH = 14.0 at 25 deg. C
Buffers• Why is the pH of some lakes unaffected by
acid rain even when they are downwind of big polluters?
• The lakes are surrounded by soils which neutralize the acidic precipitation.
1. One way to neutralize acid is to add a base. Limestone (CaCO3) is a weak base.
2. Another way to neutralize either an acid or a base is to add a buffer.
• A buffer is a substance or combination of substances capable of neutralizing limited quantities of either acids or bases.
Buffers• Calcium carbonate around a lake contaminated
with acid rain would react this way:
CaCO3 + H3O+ Ca 2+ + HCO3- + H2O
• The polyatomic ion HCO3- acts as a buffer:
HCO3- + H3O+ 2H2O + CO2
HCO3- + OH- H2O + CO3
2-
• Another buffer is the hydrogen phosphate ion. Possible sources of this ion are Na2HPO4 and NaH2PO4.
H3O+ + H2PO4 2-
H2PO4 - + H2O