•Acids were first recognized as adistinct class of compounds becauseof the common properties of theiraqueous solutions.

•Aqueous solutions have a sour taste (many are corrosiveand poisons).

•Acids change the color of acid-base indicators.

•Some acids react with active metals to release hydrogengas, H2.

Acids react with bases toproduce salts and water.

Some acids conduct an electric current.

•Used in petroleum refining, automobilebatteries, and used as a water-removingagent (can cause serious burns).

•Used in making explosives (stains skin and causes serious burns).

•Used for manufacturing fertilizers andanimal feed and for flavoring beverages.

•Used in food processing.

•Used in the manufacture ofplastics and as a fungicide.

•Aqueous solutions of bases have a bitter taste.

•Bases change the color of acid-base indicators.

•Dilute aqueous solutions of bases feel slippery.

•Bases react with acids to produce salts and water.

•Bases conduct electric current.

An Arrhenius acid is a chemical compound that increases the concentrationof hydrogen ions, H+, in aqueous solution.

Examples: HCl, H2SO4, HI, HF

An Arrhenius base is a substance that increases theconcentration of hydroxide, OH-, in aqueous solution.

Examples: NaOH, LiOH, Ca(OH)2

A Brønsted-Lowry acid is a molecule or ion that is a proton donor (H+).A Brønsted-Lowry base is a molecule or ion, that is a proton acceptor.In a Brønsted-Lowry acid-base reaction, protons are transferred from one reactant (the acid) to another (the base).

HCO3 + HOH → H2CO3 + OH-

Base Acid

A Lewis acid is an atom, ion, or molecule that accepts anelectron pair to form a covalent bond.

A Lewis base is an atom, ion, or molecule that donatesan electron pair to form a covalent bond.

A Lewis acid-base reaction is the formation of one ormore covalent bonds between an electron-pair donor andan electron-pair acceptor.

•Pure water is a weak electrolyte and undergoes self-ionization as seen below.•Remember ionization is the process of adding or removing electrons from an atom or molecule, which gives the atom or molecule a net charge.•In the self-ionization of water, two water moleculesproduce a hydronium ion and a hydroxide ion bytransfer of a proton.

-322 OH OH OH OH

hydronium ion hydroxide ion(Acid) (Base)

Concentrations of hydronium ion (H3O+) and hydroxide ion (OH-) are 1.0 x 10-7 mol/L inwater at 25 oC.

The product of [H3O+][OH-] = 1.0 x 10-14 and the product is called the ionization constant of water, Kw.

Note: [ ] means molar concentration (mol/L).

•Any solution in which [H3O+] = [OH-] is neutral.•Any solution in which [H3O+] is greater than [OH-] isacidic. Remember acids increase the concentration of H3O+ in aqueous solutions.

•Any solution in which [OH-] is greater than [H3O+] isbasic. Remember bases increase the concentration of OH- in aqueous solutions.

Instead of expressing acidity or basicity interms of the concentration of H3O+ or OH-, a quantity called pH is used.

The pH scale commonly has values from 0 to 14 with a pH value of 7 considered neutral.

pH values less than 7 are acidic and pH value greaterthan 7 are basic.

pH indicates the hydronium ion (H3O+) concentrationof a solution.

The pH of a solution is defined as the negative of thecommon logarithm of the hydronium ion concentration.

pH = -log[H3O+] or pH = [H+]

pOH (hydroxide ion concentration)pOH = -log[OH-]

pH + pOH = 14.0