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?