Acids and Bases
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The Chemistry of Acids and Bases
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3 definitions of Acids/BasesnThe theory of acids/bases has changed over time due to research and experimentationn1) Arrehnius (oldest)n2) Bronstead-Lowrey (newer)n3) Lewis (newest)
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1.Arrhenius Definition - 1887n Acids produce hydrogen cations (H+) or hydronium ions (H3O+) when dissolved in water (HCl → H+ + Cl-)
n Bases produce hydroxide anions (OH-) when dissolved in water.
(NaOH → Na+ + OH-)n Only in aqueous solutions.n Aqueous solutions are liquids that have moleculess dissolved in WATER
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Svante Arrhenius (1859-1927)
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Svante Arrheniusn He was a Swedish chemist (1859-1927), and a Nobel prize winner in chemistry (1903)n One of the first chemists to explain the chemical theory of the behavior of acids and bases
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2. Brønsted-Lowry - 1923n A broader definition than Arrheniusn Acid is hydrogen-ion donor (H+ or proton);
base is hydrogen-ion acceptor.n Acids and bases always come in pairs.n HCl is an acid.
–When it dissolves in water, it gives it’s proton to water.
HCl(g) + H2O(l) ↔ H3O+(aq) + Cl-(aq)
n Water is a base; makes hydronium ion.
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Johannes Brønsted Thomas Lowry (1879-1947) (1874-1936)
Denmark England
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Why Ammonia is a BasenAmmonia can be explained as a base by using Brønsted-Lowry:
NH3(aq) + H2O(l) ↔ NH41+
(aq) + OH1-(aq)
Ammonia is the hydrogen ion acceptor (base), and water is the hydrogen ion donor (acid).
This causes the OH1- concentration to be greater than in pure water, and the ammonia solution is basic
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Acids and bases come in pairsn A “conjugate base” is the charged
particle of the original acid, after it donates it’s hydrogen ion
n A “conjugate acid” is the charged particle formed when the original base gains a hydrogen ion
n Thus, a conjugate acid-base pair is related by the loss or gain of a single hydrogen ion.
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Acids and bases come in pairsn General equation is: HA(aq) + H2O(l) ↔ H3O+
(aq) + A-(aq)
n Acid + Base ↔ Conjugate acid + Conjugate basen NH3 + H2O ↔ NH4
1+ + OH1-
base acid c.a. c.b.n HCl + H2O ↔ H3O1+ + Cl1-
acid base c.a. c.b.n Amphoteric – a substance that can act as
both an acid and base- as water shows
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3. Lewis Acids and Basesn Gilbert Lewis focused on the donation or acceptance of a pair of electrons during a reaction
n Lewis Acid - electron pair acceptorn Lewis Base - electron pair donorn Most general of all 3 definitions; acids don’t even need hydrogen!
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Gilbert Lewis (1875-1946)
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AcidsandBases
What Happens When an Acid Dissolves in Water?
• Water acts as a base and abstracts a proton (H+) from the acid.
• As a result, the conjugate base (Cl-) of the acid and a hydronium ion (H3O+) are formed.
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Arrhenius Acid
1+
+ +
hydronium ion
H3O+
1-
chloride ion
Cl-
water
H2O
hydrogen chloride
HCl
(an Arrhenius acid)
Any substance that releases H+ ions as the only positive ion in the aqueous solution.
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Definitions
Arrhenius - In aqueous solution…
HCl + H2O → H3O+ + Cl– •Acids form hydronium ions (H3O+)
H
HH H H
H
Cl ClO O–+
acidCourtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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Definitions
Arrhenius - In aqueous solution…
•Bases form hydroxide ions (OH-)
NH3 + H2O → NH4+ + OH-
H
H
HH H
H
N NO O–+
HH
H H
baseCourtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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Neutralization
Neutralization is a chemical reaction between an acid and a base to produce a salt (an ionic compound) and water.
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)base acid salt water
Some neutralization reactions:H2SO4(aq) + NaOH(aq) Na2SO4 + H2O(l)
sulfuric acid sodium hydroxide sodium sulfate water
HC2H3O2(aq) + Ca(OH)2(aq) Ca(C2H3O2)2 + H2O(l)acetic acid calcium hydroxide calcium acetate water
2 2
2 2
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Salts
Salts - Ionic compounds containing a positive ion other than the hydrogen ion and a negative ion other than the hydroxide ion. i.e., a metal and a non-metal
NaCl(s) + H2O(l) Na1+(aq) + Cl1-(aq)
Formulas and names of common saltsFormulas and names of common saltsFormulas and names of common salts
SALT FORMULA Common Name
sodium chloride NaCl (table) salt
sodium nitrate NaNO3 Chile saltpeter
sodium bicarbonate NaHCO3 baking soda
potassium carbonate K2CO3 potash
ammonium chloride NH4Cl sal ammoniac
NaCl
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Neutralization
ACID + BASE → SALT + WATER
HCl + NaOH → NaCl + H2O
HC2H3O2 + NaOH → NaC2H3O2 + H2O
•Salts can be neutral, acidic, or basic.•Neutralization does not mean pH = 7.
weak
strong strong
strong
neutral
basic
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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ACID + BASE → SALT + WATER
HCl + NaOH → NaCl + H2O
HC2H3O2 + NaOH → NaC2H3O2 + H2O
•Salts can be neutral, acidic, or basic.•Neutralization does not mean pH = 7.
weak
strong strong
strong
neutral
basic
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
ACID BASE
NEUTRAL
pH scale: measures acidity/basicity
Acids have a pH between 0 and 7
Bases have a pH between 7 and 14
“Neutral” pH is 7
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Properties of Acidsn They taste sour (don’t try this at home).n They can conduct electricity.
–Can be strong or weak electrolytes in aqueous solution
n React with metals to form H2 gas.n Change the color of indicators (for
example: blue litmus turns to red).n React with bases (metallic hydroxides)
to form water and a salt.
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Properties of Acidsn They have a pH of less than 7 (more on
this concept of pH in a later lesson)n They react with carbonates and
bicarbonates to produce a salt, water, and carbon dioxide gas
n How do you know if a chemical is an acid?–It usually starts with Hydrogen.–HCl, H2SO4, HNO3, etc. (but not water!)
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Acids Affect Indicators, by changing their color
Blue litmus paper turns red in contact with an acid (and red paper stays red).
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Acids have a
pH less
than 7
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Acids React with Active Metals
Acids react with active metals to form salts and hydrogen gas:
HCl(aq) + Mg(s) → MgCl2(aq) + H2(g)
This is a single-replacement reaction
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Acids React with Carbonates and Bicarbonates
HCl + NaHCO3
NaCl + H2O + CO2
Hydrochloric acid + sodium bicarbonate
salt + water + carbon dioxide
An old-time home remedy for relieving an upset
stomach
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Effects of Acid Rain on Marble(marble is calcium carbonate)
George Washington:BEFORE acid rain
George Washington:AFTER acid rain
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Sulfuric Acid = H2SO44 Highest volume
production of any chemical in the U.S. (approximately 60 billion pounds/year)
4 Used in the production of paper
4 Used in production of fertilizers
4 Used in petroleum refining; auto batteries
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Nitric Acid = HNO34 Used in the production
of fertilizers4 Used in the production
of explosives4 Nitric acid is a volatile
acid – its reactive components evaporate easily
4 Stains proteins yellow (including skin!)
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Hydrochloric Acid = HCl4 Used in the “pickling” of
steel4 Used to purify
magnesium from sea water
4 Part of gastric juice, it aids in the digestion of proteins
4 Sold commercially as Muriatic acid
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Phosphoric Acid = H3PO44 A flavoring agent in
sodas (adds “tart”)4 Used in the
manufacture of detergents
4 Used in the manufacture of fertilizers
4 Not a common laboratory reagent
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Acetic Acid = HC2H3O2 (also called Ethanoic Acid, CH3COOH)
4 Used in the manufacture of plastics
4 Used in making pharmaceuticals
4 Acetic acid is the acid that is present in household vinegar
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Properties of Bases (metallic hydroxides)
nReact with acids to form water and a salt.
nTaste bitter.nFeel slippery (don’t try this either).nCan be strong or weak electrolytes in aqueous solution
nChange the color of indicators (red litmus turns blue).
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Examples of Bases(metallic hydroxides)
Sodium hydroxide, NaOH (lye for drain cleaner; soap)
Potassium hydroxide, KOH (alkaline batteries)
Magnesium hydroxide, Mg(OH)2 (Milk of Magnesia)
Calcium hydroxide, Ca(OH)2 (lime; masonry)
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Bases Affect Indicators
Red litmus paper turns blue in contact with a base (and blue paper stays blue).
Phenolphthalein turns purple in base (pH above 9.0)
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Bases have a
pH greater than 7
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Bases Neutralize Acids
Milk of Magnesia contains magnesium hydroxide, Mg(OH)2, which neutralizes stomach acid, HCl.
2 HCl + Mg(OH)2
MgCl2 + 2 H2OMagnesium salts can cause diarrhea (thus they are used as a laxative) and may also cause kidney stones.
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less than 7
Acids and Bases
pH taste ______
react with ______
proton (H1+) donor
Both are electrolytes.
turn litmus lots of H1+/H3O1+
react w/metals
pH taste ______
react with ______
proton (H1+) acceptor
turn litmus
lots of OH1– don’t react w/metals
sour
bases
red
greater than 7bitter
acids
blue
(they conduct electricity in soln)
litmus paper
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Strengthn Acids and Bases are classified according
to the degree to which they ionize or dissociate (break apart into ions) in water:–Strong are completely ionized in aqueous solution; this means they ionize 100 %
–Weak ionize only slightly in aqueous solution
nStrength is very different from Concentration
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StrengthnStrong - completely ionizes when dissolved (100 % ionization)
nKOH (Potassium Hydroxide) is a strong base- it completely ionizes (100%) in water.
nHCl (Hydrochloric Acid) is a strong acid - it completely ionizes (100%) in water
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ConcentrationnConcentration: How many acid/base
molecules are present in a solution
nConcentrated = A LOT of acid/base molecules. High concentration is above 1.0 M (M = Molar = moles/L)
nDilute = A little of acid/base molecules. Low concentration is below 1.0 M
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Strength vs. ConcentrationnThe words concentrated and dilute tell
how much of an acid or base is dissolved in solution - refers to the number of moles of acid or base in a given volume
nThe words strong and weak refer to the extent of ionization of an acid or base. Strong = 80% - 100% ionization
n Weak = 10% - 30 % ionization
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Common AcidsStrong Acids
stomach acid;
(dissociate ~100%)
hydrochloric acid: HCl H1+ + Cl1– --
pickling: cleaning metals w/conc. HCl
sulfuric acid: H2SO4 2 H1+ + SO42–
-- #1 chemical; (auto) battery acid
explosives;nitric acid: HNO3 H1+ + NO3
1– -- fertilizer
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Common Acids (cont.)Weak Acids (dissociate very little)
acetic acid: CH3COOH H1+ + CH3COO1– --
hydrofluoric acid: HF H1+ + F1– --
citric acid, H3C6H5O7 --
ascorbic acid, H2C6H6O6 -- lactic acid, CH3CHOHCOOH --
vinegar; naturally made by apples
used to etch glass
lemons or limes; sour candy
vitamin C
waste product of muscular exertion
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carbonic acid, H2CO3
-- carbonated beverages
-- CO2 + H2O H2CO3
dissolveslimestone(CaCO3)
rainwaterin air
H2CO3: cave formation H2CO3: natural acidity of lakes
H2CO3: beverage carbonation
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How to measure pH with wide-range paper
1. Moisten the pH indicator paper strip with a few drops of solution, by using a stirring rod.
2.Compare the color to the chart on the vial – then read the pH value.
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Some of the many pH Indicators and theirpH range
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Acid-Base IndicatorsnAlthough useful, there are limitations to indicators:–usually given for a certain temperature (25 oC), thus may change at different temperatures
–what if the solution already has a color, like paint?
– the ability of the human eye to distinguish colors is limited
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Acid-Base IndicatorsnA pH meter may give more accurate pH values–some are large, others portable–works by measuring the voltage between two electrodes; typically accurate to within 0.01 pH unit of the true pH
–Instruments need to be calibrated
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Indicators chemicals that change color,depending on the pH
Two examples, out of many:
litmus…………………
phenolphthalein……..
red in acid,blue in base
clear in acid,pink in base acid base
acid base
pinklear
b
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Measuring pH
litmus paper phenolphthalein Basically, pH < 7 or pH > 7.
pH paper -- contains a mixture of various indicators
--
--
each type of papermeasures a range of pH
pH anywhere from 0 to 14
universal indicator -- is a mixture of several indicators --
4 5 6 7 8 9 10
R O Y G B I V
pH 4 to 10
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Measuring pH (cont.)
pH meter -- measures small voltages in solutions -- calibrated to convert voltages into pH -- precise measurement of pH