Acids, Bases, and pH

Acids and Bases

• Acids produce H+ ions

• Bases produce OH- ions

Macroscopic properties of acids and bases

• Taste and feel– Acids taste sour (lemon juice, vinegar)– Bases taste bitter– Bases are slippery (soap)

• Litmus test and other color changes– Indicators change colors in the presence of an acid

or a base– Litmus: acid = red, base = blue

Submicroscopic behavior of acids

• Hydrogen ion formation

HCl H+ + Cl-

HNO3 H+ + NO3-

• Acidic hydrogen atoms– Reaction of acid with water is a transfer of a

hydrogen ion from an acid to a water molecule– Any hydrogen that can be transferred to water is

called an acidic hydrogen– It is possible for acids to have more than one

acidic hydrogen

• Acids are electrolytes– Substance that dissolve in pure water to form ions

& conduct electricity – Not all acids conduct electricity equally well• Strong acids conduct electricity better than weak acids

Submicroscopic behavior of bases

• Hydroxide ion formation can happen two ways

• Simple bases: NaOH Na+ + OH-

• Bases that accept H+ (Bronsted-Lowry):NH3+ H2O NH4

+ +OH-

• Bases are electrolytes– Strong bases conduct electricity better than weak

bases

Strengths of Acids and Bases

• Strong acids and bases ionize completely while weak acids and bases ionize only partially

Strong Bases

• A base that completely dissociates into ions when dissolved in water

• Ionic compounds that contain hydroxide ions

NaOH Na+ + OH-

Strong Acids

• An acid that completely ionizes in water

HCl H+ + Cl-

Weak Acids

• An acid that only partially ionizes in solution• Usually when acidic hydrogen is bonded to an

oxygen

H2CO3 H+ + CO32- + H2CO3

Weak Bases

• Only partially ionize in solution

NH3 + H2O NH4+ + OH- + NH3 + H2O

Types of Acid-Base Reactions

• Neutralization reaction – properties of both acid and base are neutralized when they react

• Acid + Base Salt + Water– Salt – ionic compound (metal + nonmetal)

Strong Acid + Strong Base

• Strong acids completely ionize

• Strong bases completely ionize

• Ionic equation – everything (aq) written as ions– Spectator ions– Net ionic equation

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

• H2SO4(aq) + 2KOH(aq) K2SO4(aq) + H2O(l)

• Strong acid + strong base & pH– Hydrogen ion from acid and hydroxide ion from

base form water which has a pH of 7

Strong Acid + Weak Base

• Weak bases do not completely ionize

• 3HBr(aq) + Al(OH)3(s) AlBr3(aq) + H2O(l)

Weak Acid & Strong Base

• HC2H3O2(aq) + NaOH NaC2H3O2 + H2O

Weak is not insignificant

• Most acids and bases are classified as weak

• Most of the acid-base chemistry in living systems occurs between weak acids and weak bases

Strength is not concentration

• Weak & strong compare strength• Dilute and concentrated compare

concentration• Combination of strength and concentration

determine the behavior of solution

The pH Scale

• A mathematical scale in which the concentration of H+ ions in a solution is expressed as a number from 0 – 14

Interpreting the pH scale

• pH < 7 = acidic• pH = 7 = neutral• pH > 7 = basic

• Each unit of pH represents a power of 10– Something with pH of 2 is 10 times more acidic

than something with a pH of 3

pH = -log [H+]

• What is the pH of solutions having the following ion concentrations?

1. [H+] = 1.0 x 10-2 M

2. [H+] = 3.0 x 10-6 M

• Relating H+ and OH- ion concentration• [H+ ][OH-] = 1 x 10-14

• As [H+ ] increases what happens to [OH-]?

• Calculate the [H+] or [OH] and state whether the solution is neutral, acidic, or basic– 1.0 x 10-5 M OH-

– 1.0 x 10-7 M OH-

– 10.0 M H+

pOH = -log [OH-]

• What is the pOH of a solution having the following ion concentration?

1. [OH-] = 1.0 x 10-6 M

2. [OH-] = 6.5 x 10-4 M

pH + pOH = 14

• What is the pOH of a solution whose pH is 5?

• What is the pH of a solution whose [OH-] = 4.0 x 10-3 M

• Calculate the pH and pOH of the following solutions: 1. [H+] = 0.000033 M

2. [OH-] = 0.0095 M

Bronsted-Lowry Acids & Bases

• Acid – H+ donor• Base – H+ acceptor

• HC2H3O2 + NH3 NH4 + + C2H3O2-

• Conjugate base – formed when acid donates proton

• Conjugate acid – formed when base accepts a proton

• HC2H3O2 + NH3 NH4 + + C2H3O2-

• Identify the acid, base, conjugate acid, and conjugate base in the following:

• HCO3 -1 + H2O CO3 -2 + H3O +1

Applications of Acid-Base Reactions

• Buffers regulate pH– Solution that resist changes in pH when moderate

amounts of acids or bases are added– Prepared by using a weak acid and its conjugate

base or weak base and its salt

Acid-Base Chemistry & Antacids

• pH of stomach acid = 2.5• Stomach is coated with basic mucous to

protect it • If stomach becomes too acidic mucous layer

breaks down by acid-base neutralization• Antacids are bases

Acid-Base Titrations

• Titration – process of determining molarity of an acid or a base through the use of an acid-base reaction– Molarity of one of the reactants is known but the

other is unknown– Standard solution – solution whose molarity is

known

• Standard solution is loaded into a buret– Allows an accurate measurement of the delivery

of a given volume of liquid

• Using indicators – when solution is neutral you know you have added exactly enough stock solution to react with the unknown solution– endpoint

• A 15.0 mL sample of a solution of H2SO4 with unknown molarity is titrated with 32.4 mL of 0.145 M NaOH to the endpoint. What is the molarity of the sulfuric acid solution?