The Chemistry of Acids and Bases

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)

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

Svante Arrhenius (1859-1927)

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

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.

Johannes Brønsted Thomas Lowry (1879-1947) (1874-1936)

Denmark England

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

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.

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

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!

Gilbert Lewis (1875-1946)

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.

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.

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

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

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

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

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

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

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

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.

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!)

Acids Affect Indicators, by changing their color

Blue litmus paper turns red in contact with an acid (and red paper stays red).

Acids have a

pH less

than 7

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

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

Effects of Acid Rain on Marble(marble is calcium carbonate)

George Washington:BEFORE acid rain

George Washington:AFTER acid rain

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

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!)

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

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

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

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).

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)

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)

Bases have a

pH greater than 7

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.

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

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

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

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

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

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

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

carbonic acid, H2CO3

-- carbonated beverages

-- CO2 + H2O H2CO3

dissolveslimestone(CaCO3)

rainwaterin air

H2CO3: cave formation H2CO3: natural acidity of lakes

H2CO3: beverage carbonation

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.

Some of the many pH Indicators and theirpH range

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

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

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

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

Measuring pH (cont.)

pH meter -- measures small voltages in solutions -- calibrated to convert voltages into pH -- precise measurement of pH