Properties of Acids• Acids have a sour taste

• They react with “active” metals

– i.e. Al, Zn, Fe, but not Ag or Au

2 Al + 6 HCl AlCl3 + 3 H2

– Corrosive

• They react with carbonates, producing CO2

– marble, baking soda, chalk, limestone

CaCO3 + 2 HCl CaCl2 + CO2 + H2O

• They change the color of vegetable dyes

– blue litmus turns red

• They react with bases to form ionic salts

Properties of Bases

• They are also known as “alkalis”

• They taste bitter

– alkaloids = plant products that are alkaline

• often poisonous

• Base solutions feel slippery

• They change the color of vegetable dyes

– different color than acids

– red litmus turns blue

• They react with acids to form ionic salts

– neutralization

Arrhenius Theory

• Bases dissociate in water to produce OH- ions and cations– ionic substances dissociate in water

NaOH(aq) → Na+(aq) + OH–(aq)

• Acids ionize in water to produce H+ ions and anions– because molecular acids are not made of ions, they

cannot dissociate – they must be pulled apart, or ionized, by the water

HCl(aq) → H+(aq) + Cl–(aq)– in formula, ionizable H written in front

HC2H3O2(aq) → H+(aq) + C2H3O2–(aq)

Arrhenius Acid-Base Reactions

• The H+ from the acid combines with the OH- from the base to make a molecule of H2O

– it is often helpful to think of H2O as H-OH

• The cation from the base combines with the anion from the acid to make a salt

acid + base → salt + water

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Problems with Arrhenius Theory

• The Arrhenius Theory does not explain why molecular substances, like NH3, dissolve in water to form basic solutions – even though they do not contain OH– ions.

• It does not explain acid-base reactions that do not take place in aqueous solution.

• The H+ ions do not exist in water. Acid solutions contain H3O+ ions– H+ = a proton!– H3O+ = hydronium ions

Brønsted-Lowery Theory

• In a Brønsted-Lowery Acid-Base reaction, an H+ is transferred

– It does not have to take place in aqueous solution

– It is a broader definition than the Arrhenius Theory provides

• Acid is a H donor, base is a H acceptor– base structure must contain an atom with an

unshared pair of electrons

• In the reaction, the acid molecule gives an H+ to the base molecule

H–A + :B :A– + H–B+

Brønsted-Lowery Acid-Base Reactions

• One of the advantages of the Brønsted-Lowery theory is that it allows reactions to be reversible

H–A + :B → :A– + H–B+

• The original base has an extra H+ after the reaction – so it could act as an acid in the reverse process

• And, the original acid has a lone pair of electrons after the reaction – so it could act as a base in the reverse process

:A– + H–B+ → H–A + :B

• A double arrow “” is usually used to indicate a process that is reversible

Conjugate Pairs• In a Brønsted-Lowery Acid-Base reaction, the original base becomes

an acid in the reverse reaction, and the original acid becomes a base in the reverse process

• Each reactant and the product it becomes is called a conjugate pair• The original base becomes the conjugate acid; and the original acid

becomes the conjugate base

H–A + :B :A– + H–B+

acid base conjugate base conjugate acid

HCHO2 + H2O CHO2– + H3O+

acid base conjugate base conjugate acid

H2O + NH3 HO– + NH4+

acid base conjugate base conjugate acid

Conjugate Acid-Base Pairs

AH + B A- + HB+

ACID

protondonor

BASE

protonacceptor

CONJUGATE BASEproton

acceptor

CONJUGATE ACIDprotondonor

ACID CONJUGATE BASE

BASE CONJUAGE ACID+ +

Types of Electrolytes

• salts = water soluble ionic compounds– Most are strong electrolytes

• acids = form H3O+1 ions in water solution

• bases = combine with H3O+1 ions in water solution

– increases the OH-1 concentration

• may either directly release OH-1 or pull H off H2O to form OH-

Strong or Weak

• A strong acid is a strong electrolyte– practically all the acid molecules ionize, →

• a strong base is a strong electrolyte– practically all the base molecules form OH– ions, either through

dissociation or reaction with water, →

• a weak acid is a weak electrolyte– only a small percentage of the molecules ionize,

• a weak base is a weak electrolyte– only a small percentage of the base molecules form OH– ions,

either through dissociation or reaction with water,

Relationship between Strengths of Acids and their Conjugate Bases

• The stronger an acid is, the weaker is the attraction of the ionizable H for the rest of the molecule.

• The better the acid is at donating H, the worse its conjugate base will be at accepting a H:

strong acid HCl + H2O → Cl– + H3O+ weak conj. base

weak acid HF + H2O F– + H3O+ strong conj. Base

A strong acid is one for which a forward reaction predominates. The relatively weak conj. Base has a low attraction for proton. In contrast, a weak acid is one for which the reverse reaction predominates. The relatively strong conjugate base has a strong attraction for protons.

Common Acids

Chemical Name Formula Uses Strength

Nitric Acid HNO3 explosive, fertilizer, dye, glue Strong

Sulfuric Acid

H2SO4 explosive, fertilizer, dye, glue,

batteries Strong

Hydrochloric Acid HCl metal cleaning, food prep, ore

refining, stomach acid Strong

Phosphoric Acid H3PO4 fertilizer, plastics & rubber,

food preservation Moderate

Acetic Acid HC2H3O2 plastics & rubber, food preservation, Vinegar

Weak

Hydrofluoric Acid HF metal cleaning, glass etching Weak

Carbonic Acid H2CO3 soda water Weak

Boric Acid H3BO3 eye wash Weak

Structures of Acids

• Binary acids have acid hydrogens attached to a nonmetal atom

– HCl, HF

• Oxyacids (most common) have acid hydrogens attached to an oxygen atom

- H2SO4, HNO3

Note that sulfuric acid is diprotic acid: can furnish two protons.

• Carboxylic acids (organic acids) have a -COOH (carboxyl group)

HC2H3O2, H3C6H5O3

only the first H in the formula is acidic the H is on the -COOH

Common Bases

Chemical Name

Formula Common

Name Uses Strength

sodium hydroxide

NaOH lye,

caustic soda soap, plastic,

petrol refining Strong

potassium hydroxide

KOH caustic potash soap, cotton, electroplating

Strong

calcium hydroxide

Ca(OH)2 slaked lime cement Strong

sodium bicarbonate

NaHCO3 baking soda cooking, antacid Weak

magnesium hydroxide

Mg(OH)2 milk of

magnesia antacid Weak

ammonium hydroxide

NH4OH, {NH3(aq)}

ammonia water

detergent, fertilizer,

explosives, fibers Weak

Structure of Bases

• Most ionic bases contain OH- ions– NaOH, Ca(OH)2

• Some contain CO32- ions

– CaCO3 , NaHCO3

• Molecular bases contain structures that react with H+

– mostly amine groups

Amphoteric Substances

• amphoteric substances can act as either an acid or a base– having both a transferable H and an atom with a lone pair of

electrons

• HCl(aq) is acidic because HCl transfers an H+ to H2O, forming H3O+ ions– water acts as a base, accepting an H+

HCl(aq) + H2O(l) → Cl–(aq) + H3O+(aq)

• NH3(aq) is basic because NH3 accepts an H+ from H2O, forming OH–(aq)– water acts as acid, donating H+

NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)

Or: H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

In this reaction one water molecule acts as an acid by furnishing a proton, and the other acts as a base by accepting the proton.

Autoionization of Water

• Water is actually an extremely weak electrolyte– therefore there must be a few ions present

• About 1 out of every 10 million water molecules form ions through a process called autoionization

H2O H+ + OH– (for simplicity)

H2O(l) + H2O(l) H3O+(aq) + OH–(aq)

• All aqueous solutions contain both H+ and OH–

– the concentration of H+ and OH– are equal in water– [H+] = [OH–] = 10-7M @ 25°C

Ion Product of Water• The product of the H+ and OH– concentrations is always the same

number. The number is called the ion product of water and has the symbol Kw

• [H+] x [OH–] = 1 x 10-14 = KwAs the [H+] increases the [OH–] must decrease so the product stays constant– inversely proportional

• Neutral solutions have equal [H+] and [OH–]– [H+] = [OH–] = 1 x 10-7

• Acidic solutions have a larger [H+] than [OH–]– [H+] > 1 x 10-7; [OH–] < 1 x 10-7

• Basic solutions have a larger [OH–] than [H+]– [H+] < 1 x 10-7; [OH–] > 1 x 10-7

pH• The acidity/basicity of a solution is often expressed as pH

• pH = -log[H+], [H+] = 10-pH

– exponent on 10 with a positive sign– pHwater = -log[10-7] = 7– need to know the [H+] concentration to find pH

• pH < 7 is acidic; pH > 7 is basic and pH = 7 is neutral

Converting Between [H+] and pH[H+] = 1.23 x 10-4, Calculate the pHType 1.23 x 10-4 then ‘log’, then ‘-’pH = 3.91

pH = 9.4, Calculate [H+]Type ‘-9.4’, then ‘2nd F’, then ‘10x’[H+] = 3.98 x 10-10

Are these solutions acidic, basic, or neutral?The first is acidic, the second is basic

NOTE: To find the pH of a strong acid, use the concentration of the acid itself to be equal to the concentration of H+, since strong acids undergo complete ionization into [H+] and the anion.

Converting Between [H+] and pH

Complete the following table:

[H+] pH Basic/neutral/acidic?4.28 x 10-10

1.21.0 8.9

pH and pOH

• The p scale is also used to find the pOH.

pOH= -log [OH-]

Example: find the pH and the pOH for a solution of

1.0x10-4 M H+

pH = - log 10-4 = 4

pOH = -log 10-10= 10 (Note that pH + pOH =14)

We know [H+] [OH-] = 1.0x10-14

If we take the –log of both sides of the equation:

pH + pOH = 14

Buffers

• Buffers are solutions that resist changes in pH when small amounts of acid or base are added.

• They resist changing pH by neutralizing any added acid or base.

• Buffers are made by mixing together a weak acid and its conjugate base– or weak base and its conjugate acid

How Buffers Work

• The weak acid present in the buffer mixture can neutralize added base.

• The conjugate base present in the buffer mixture can neutralize added acid.

• The net result is little to no change in solution pH.