Acids and bases

S E C T I O N 1

Properties of Acids and Bases

� Many foods have acid in them !

� Sour milk – lactic acid � Vinegar – acetic acid � Citrus fruits – citric acid � Apples – malic acid � Grape juice – tartaric acid

� Many household items are bases !

� Ammonia � Sodium hydroxide (lye) in drain and oven cleaners � Milk of magnesia (antacid) to relieve stomach pain

(Mg(OH)2)

Acids

� First recognized as separate class of compounds b/c of common properties of aqueous solutions !

1. Aqueous solutions of acids have a sour taste. � Should never be used as a test to evaluate a

chemical substance � Many are corrosive (destroy tissue/clothing) � Many are poisonous

2. Acids change the color of acid-base indicators.

� When pH paper used, paper turns certain color

3. Some acids react with active metals to release hydrogen gas.

� Metals above hydrogen in activity series go through single-replacement reactions with certain acids !

4. Acids react with bases to produce salts and water. � When equal amounts of acid and base reacted,

previous properties disappear � Acid is neutralized � Produces water and ionic compound (salt)

5. Some acids conduct electric current. !� Acids that form many ions in aqueous solution are

elctrolytes

Naming Acids (Acid Nomenclature)

� Binary acid à acid that contains only two different elements: hydrogen, and one of the more electronegative elements !

� Ex. Hydrogen halides – HF, HCl, HBr, HI

� Oxyacid à acid that is a compound of hydrogen, oxygen, and a third element, usually a nonmetal !

� Ex. Nitric acid – HNO3 !

� One class of ternary acids (contain 3 elements) � Usually written as one or more H followed by

polyatomic ion

Prefix Suffix Prefix Suffix

Hypo- -ous Hypo- -ite

None -ous None -ite

None -ic None -ate

Per- -ic Per- -ate

Name of oxyacid Name of oxyanion

Some Common Industrial Acids

� Properties of acids make them important chemicals in lab and industry !

� Sulfuric acid � Nitric acid � Phosphoric acid � Hydrochloric acid � Acetic acid

Sulfuric Acid

� Most commonly produced industrial chemical in the world !

� More than 47 million tons made each year in US alone !

� Used in large quantities in petroleum refining and metallurgy !

� Also in manufacture of fertilizer

� Also needed for large number of industrial processes !

� Making metals � Paper � Paint � Dyes � Detergents � Many chemical raw materials � Used in car batteries

� b/c it attracts water, concentrated H2SO4 is an effective dehydration agent

� Can remove water from gases that it doesn’t react with !

� Sugar and other organic compounds dehydrated by H2SO4

Nitric Acid

� Pure nitric acid is volatile, unstable liquid rarely used in industries or labs

� Stains proteins yellow

� Acid has suffocating odor � Stains skin � Can cause serious burns !

� Used in making explosives � Also used to make rubber, plastics, dyes,

pharmaceuticals

� Initially it is clear !

� If you leave it, it will turn yellow !

� Slight decomposition to brown nitrogen dioxide gas

Phosphoric Acid

� Phosphorous is essential element for plants and animals

� Most phosphoric acid used directly for making fertilizers and animal feed

� Dilute phosphoric acid has pleasant but sour taste and is not toxic

� Used as flavoring agent in beverages � Cleaning agent for dairy equipment � Important in making detergents and ceramics

Hydrochloric Acid

� Stomach makes it to help digestion !

� In industry, useful to “pickle” iron and steel (remove surface impurities) !

� Used in industry as general cleaning agent � Used in food processing � Used in activation of oil wells, recovery of magnesium

from sea water, and in making other chemicals

� Dilute solution of HCl (referred to as muriatic acid) can be found in hardware stores

� Used to maintain correct acidity in swimming pools and cleaning stone (masonry) !

� Vixol – solution of HCl

Acetic Acid

� Concentrated acetic acid is clear, colorless, strong-smelling liquid known as glacial acetic acid !

� Name comes from freezing point (17℃)   !

� Can form crystals in cold room � Fermentation of certain plants makes vinegars

containing acetic acid � White vinegar – 4-8% acetic acid

� Important in industry in making chemicals used in making plastics !

� It’s a raw material in production of food supplements – ex. Lysine (essential amino acid) !

� Acetic acid is also used as fungicide

Bases

1. Aqueous solutions of bases taste bitter. � Ever had soap in your mouth? � NEVER use this as a test � Many bases are caustic – they attack skin and

tissues !2. Bases change the color of acid-base indicators.

3. Dilute aqueous solutions of bases feel slippery. � Like soap !

4. Bases react with acids to make salts and water. !5. Bases conduct electric current. � Like acids, bases form ions in aqueous solutions so

are electrolytes

Arrhenius Acids and Bases

� Svante Arrhenius – Swedish chemist, 1859-1927 !

� Understood aqueous solutions of acids and bases conduct electric current !

� He concluded acids and bases must produce ions in solution

� Arrhenius acid à chemical compound that increases the concentration of hydrogen ions in aqueous solution

� Acid ionized in solution and increases the number of H+ present !

� Arrhenius base à substance that increases the concentration of hydroxide ions (OH-) in aqueous solution

� Some bases are ionic hydroxides � These dissociate in solution to release OH- into

solution !

� Others are substances that react with water to remove H+, leaving OH- in solution

Aqueous Solutions of Acids

� The acids described by Arrhenius are molecular compounds with ionizable hydrogen atoms !

� Their water solutions are called aqueous acids !

� All pure aqueous acids are electrolytes

� Acid molecules are polar enough so that one or more hydrogen ions are attracted by H2O !

� Negatively charged anions are left behind !

� Hydrogen ion in aqueous solution is best represented by hydronium, H3O+

Ex. HNO3

� Hydrogen atoms combine with water to form hydronium !!!

� HCl ionizes in a similar way

Strength of Acids

� Strong acid à one that ionizes completely in aqueous solution

� Ex. HClO4, HCl, HNO3 !

� Strength depends on polarity of bond between hydrogen and other elements – how easily it breaks !

� Strength increases with increasing polarity and decreasing bond energy

� Weak acid à acids that are weak electrolytes !

� Aqueous solutions contain hydronium ions, anions and dissolved acid molecules !

� Ex. HCN (hydrocyanic acid)

� In aqueous solution, ionization and reverse reaction of HCN happen at same time !

� Most of solution is made of hydrogen cyanide and water

� Strong acids assumed to ionize completely to give up one H+ !!!!!!!

� Number of H+ in formula does not affect strength

� Molecules with multiple hydrogen atoms may not give them up easily !

� Ex. Phosphoric acid has 3 hydrogens � Doesn’t give any of them up easily � So is weak acid

� Organic acids à contain acidic carboxyl group (-COOH) !

� Generally weak acids !

� Ex. Acetic acid (CH3COOH) � Ionizes slightly in water

� 1 molecule of acetic acid contains 4 hydrogen atoms � Only 1 ionizes !

� Hydrogen in carboxyl is the “acidic” hydrogen � Forms hydronium ion

Aqueous Solutions of Bases

� Most bases are ionic compounds made of metal cations and the hydroxide anion

� b/c they are ionic, they dissociate to some extent when in solution !

� Alkaline à when a base completely dissociates in water to yield aqueous OH- ions

� NaOH is water-soluble and dissociates as follows !!!

� Remember sodium is an alkali metal � All alkali metal hydroxides form alkaline solutions

� Not all bases are ionic compounds !

� Ammonia (common household cleaner) is molecular � It’s a base b/c it produces hydroxide ions when it

reacts with water molecules

Strength of Bases

� Strength of base also depends on extent of dissociation into solution !

� Ex. KOH is strong base – it dissociates completely !!!

� Strong bases are also strong electrolytes

� Bases that are not very soluble don’t make a large number of OH- when added to water !

� Some metal hydroxide are not very soluble in water � They cannot make strong alkaline solutions !

� Alkalinity depends on concentration of OH- in solution

� Unrelated to number of OH- ions in undissolved compound

� Consider ammonia – highly soluble but weak electrolyte

� Concentration of OH- ions in solution relatively low � So, ammonia is weak base !

� Many organic compounds that contain N atoms also weak bases

S E C T I O N 2

Acid-Base Theories

� As scientists investigated acid-base behavior, they found some substances acted as acids or bases when not in water solution !

� b/c Arrhenius definition requires aqueous solutions, definitions of acids and bases had to be revised

Brønsted-Lowry Acids and Bases

� Brønsted-Lowry acid à a molecule or ion that is a proton donor !

� b/c H+ is proton, all Arrhenius acids donate a proton and so are Brønsted-Lowry acids as well

� Substances other than molecules can also donate protons � These are not Arrhenius acids but are included in

Brønsted-Lowry acids

� Hydrogen chloride acts as Brønsted-Lowry acid when dissolved in ammonia !!!!

� HCl transfers proton to ammonia molecule � Forms ammonium

� Electron-dot formulas show similarity of reaction to reaction with HCl in water !!!!!!

� In both reactions, HCl is Brønsted-Lowry acid

� Water can also be Brønsted-Lowry acid � Ex. Water donating proton to ammonia molecule

� Brønsted-Lowry base à molecule or ion that is a proton acceptor !

� In reaction with HCl and ammonia, ammonia accepts proton from HCl

� It is a Brønsted-Lowry base

� Arrhenius hydroxide bases (NaOH) not necessarily Brønsted-Lowry bases !

� As compounds, they are not proton acceptors !

� OH- made in solution is Brønsted-Lowry base !

� It is the species that accepts proton

� Brønsted-Lowry acid-base reaction à protons transferred from one reactant (the acid) to another (the base)

Monoprotic Acids

� Monoprotic acid à an acid that can donate only 1 proton (hydrogen ion) per molecule !

� Ex. HClO4, HCl, HNO3

Polyprotic Acids

� Polyprotic acid à an acid that can donate more than one proton per molecule !

� Ex. H2SO4, H3PO4 � Ionization occurs in stages � Acid loses hydrogens one at a time

� Diprotic acid à acid that can donate 2 protons per molecule !

� Triprotic acid à acid that can donate 3 protons

Lewis Acids and Bases

� Both Arrhenius and Brønsted-Lowry definitions assume that acid contains or make hydrogen ions !

� 3rd classification (based on bonding and structure) includes acids as substances that do not have hydrogen at all

� Lewis definition emphasizes role of electron pairs in acid-base reactions

� Lewis acid à atom, ion, or molecule that accepts an electron pair to form a covalent bond

� Broadest definition of 3 definitions � Applies to any species that can accept electron pair to

form covalent bond with another species !

� Lone proton is Lewis acid in reactions where it forms covalent bond

� The formula for Lewis acid doesn’t need to have hydrogen

� Even silver ion can be Lewis acid

� Any compound where central atom has 3 valence electrons and forms 3 covalent bonds can react as Lewis acid

� It accepts pair of electrons to form 4th covalent bond (completes electron octect)

� Ex. Boron trifluoride – excellent Lewis acid � Forms 4th covalent bond with many molecules and

ions

� Lewis definition applies to species in any phase (s, l, g, aq) !

� Ex. BF3 and NH3 in gas phase

� Lewis base à atom, ion, or molecule that donates an electron pair to form a covalent bond

� Anion is Lewis base in reaction where it forms covalent bond by donating electron pair

� Ex. BF3 reacting with F- � F- donates electron pair to BF3 (acts as Lewis base)

� Lewis acid-base reaction à formation of one or more covalent bonds between an electron-pair donor and an electron-pair acceptor !

� Even though 3 definitions are different, many compounds categorized as acids or bases according to all 3

Ex. NH3

� Arrhenius base b/c OH- ions created when ammonia is in solution !

� Brønsted-Lowry base b/c it accept a proton in acid-base reaction !

� Lewis base in all reactions where it donates lone pair of electrons to form covalent bond

S E C T I O N 3

Acid-Base Reactions

Conjugate Acids and Bases

� Brønsted-Lowry definitions provide basis for studying proton-transfer reactions !

� If Brønsted-Lowry acid gives up proton, remaining ion/molecule can accept it

� This makes it a conjugate base !

� Conjugate base à the species that remains after a Brønsted-Lowry acid has given up a proton

� Ex. Fluoride ion is conjugate base of HF !!!!

� In this reaction, water molecule is Brønsted-Lowry base

� It accepts proton to form H3O+ (which is acid)

� Conjugate acid à species that is formed when a Brønsted-Lowry base gains a proton

� In general, Brønsted-Lowry acid-base reactions are equilibrium systems

� Both forward and reverse reactions occur � They involve two acid-base pairs (conjugate acid-base

pairs)

Strength of Conjugate Acids and Bases

� Degree of reaction between Brønsted-Lowry acid and base depends on strengths of acids and bases !

� Ex. HCL – strong acid (gives up proton readily) � Cl- has little tendency to attract and keep proton � Cl- is very weak base

� Important conclusion: the stronger an acid is, the weaker its conjugate base; the stronger a base is, the weaker its conjugate acid !

� This allows strengths of different acids and bases to be compared to predict outcome of reaction

� Ex. Perchloric acid (HClO4) and water !!!!

� Hydronium ion too weak an acid to compete with perchloric acid in donating proton (HClO4 stronger acid)

!!!

� Perchlorate ion and H2O are both bases !

� Because HClO4 is very strong acid, ClO4- is very weak base !

� So H2O gets proton

� H3O+ ion concentration lower than previous example b/c acetic acid is weak acid

� CH3COOH doesn’t compete successfully with H3O+ ion in donating protons to base

� Acetate ion is stronger base than H2O � So H2O molecule doesn’t compete successfully with

CH3COO- in accepting proton � H3O+ is stronger acid � CH3COO- is stronger base � Reaction to left more favorable

� Favored reactions always toward the weaker acid and base !

� Second important conclusion: proton-transfer reactions favor the production of the weaker acid and the weaker base !

� For reaction to reach completion, reactants must be stronger acids/bases than products

Amphoteric Compounds

� Water can be acid or base � Amphoteric compound à any species that can

react as either an acid or a base

� Whether an acid or base depends on strength of acid/base it is reacting with !

� If water reacts with compound that is stronger acid than itself, then it will be a base !

� If water reacts with a compound that is a weaker acid than itself, it will be an acid

-OH in a Molecule

� Molecular compounds with –OH groups can be acidic or amphoteric

� This is the hydroxyl group � For compound to be acidic a water molecule must be

able to attract a hydrogen atom from a hydroxl group � This happens more easily when O-H bond very polar � Any feature of molecule that increases polarity of O-H

bond increases acidity of molecular compound

� More-electronegative atoms in upper right corner of periodic table form compounds with acidic hydroxyl groups !

� All oxyacids are molecular electrolytes that contain 1 or more O-H bonds !

� Ex. Chloric and perchloric acids

!!!!

� Notice all oxygen atoms bonded to chlorine atom � Each hydrogen bonded to oxygen atom � Aqueous solutions of these are acids b/c O-H bonds

broken as hydrogen attracted by water molecules

� Behavior of compound affected by number of O atoms bonded to atom connected to –OH group

� Larger number of O atom, more acidic compound will be !

� Electronegative O atoms draw electron density away from O-H bond and make it more polar

!!!!

� Notice as more oxygen added, it becomes more acidic

� In acetic acid – not ethanol – second O bonded to C connected to –OH group !

� This is why acetic acid is acidic but ethanol isn’t

Neutralization Reactions

� Ex. Sodium bicarbonate and tartaric acid in baking powder

� When in solution, they react to make CO2 � Escaping CO2 used to make breads rise !

� Ex. Antacid (Eno) soothes overly acidic stomach

Strong Acid-Strong Base Neutralization

� Reaction between aqueous HCl and NaOH

� Solution of 1 mol NaOH dissociates as follows: !!!

� Solution of 1 mol NaCl dissociates as follows:

� If two solutions are mixed, a reaction happens between aqueous ions

� NaCl and water are produced � Overall ionic equation:

� b/c they are on both sides of equation, Na+ and Cl- are spectator ions

� Only participants in reaction are H3O+ and OH- � Net ionic equation:

� There are equal numbers of H3O+ and OH- ions in this reaction so they are completely converted to water

� Neutralization à the reaction of hydronium ions and hydroxide ions to form water molecules

� Salt is also produced � Salt à ionic compound composed of a cation

from a base and an anion from an acid

Acid Rain

� Many industrial reactions make gases like NO, NO2, CO2, SO2 and SO3

� These dissolve in atmospheric water to make acidic solutions that fall as rain or snow

� Marble in many buildings and statues are made of calcium carbonate CaCO3

� Acid rain: !!!

� Products: salt, water, and CO2