3 Theories of Acids & Bases Arrhenius Theory of Acids & BasesProperties of acids are due to the presence of H+ ionsExample:HCl H+ + Cl-Properties of bases are due to the presence of OH- ionsNaOH Na OH-
4 HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) H+ ions in waterH+ ions are bare protonsThese H+ ions react strongly with the nonbonding pair of electrons in a water moleculeThis forms the hydronium ion, H3O+Oftentimes H+ and H3O+ are used interchangeablyHCl H+ + Cl-HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)
5 Problems with Arrhenius Arrhenius theory has limitations:Only deals with aqueous solutions (solutions in water)Not all acids and bases produce H+ and OH- ionsNH3 for example is a baseBrønsted and Lowry proposed a definition based on acid base reactions transferring H+ ion from one substance to another
7 Theories of Acids & Bases Brønsted-Lowry TheoryAcids are substances that donate H+ ionsAcids are proton donorsBases are substances that accept H+ ionsBases are proton acceptorsExample:HBr + H2O H3O Br-A B
8 Brønsted-Lowry Theory The behavior of NH3 can now be understood:NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq)Since NH3 becomes NH4+, it is a proton acceptor (or a Brønsted-Lowry base)H2O becomes OH-, which means it is a proton donor (or a Brønsted-Lowry acid)
9 Brønsted-Lowry Theory Conjugate Acid-Base PairsAn acid and a base that differ only in the presence or absence of H+ are called a conjugate acid-base pair.Every acid has a conjugate base.Every base has a conjugate acid.HX is the conjugate acid of X-H2O is the conjugate base of H3O+
10 Brønsted-Lowry Theory These pairs differ by only one hydrogen ionExampleIdentify the Brønsted-Lowry acid, base, conjugate acid and conjugate baseNH3 + H2O NH OH-B A CA CBNH3 acts as a Brønsted base by accepting a proton.Water acts as a Brønsted acid by donating a proton.
11 Brønsted-Lowry Theory ExampleHCl (g) + H2O (l) ↔ H3O+(aq) + Cl- (aq)HSO HCO3- ↔ SO H2CO3ABCACBABCBCA
12 Theories of Acids & Bases Lewis Acids & BasesAcids are electron acceptorsBases are electron donorsExample:H2O + NH3 OH- + NH4+Is really:H2O + :NH3 OH- + H:NH3+Electron pairdonor(NH3)Electron pairacceptor(H+)
13 Summary Of Theories Acids release H+ Bases release OH- ArrheniusAcids release H+Bases release OH-Defines acids & bases in H2OBrønsted-LowryAcids – proton donorBases – proton acceptorCan define acids & bases in solvents other than H2OLewisAcids – electron acceptorBases – electron donorDefines acids & bases without a solvent
14 The Self-Ionization of Water Even pure water contains a small number of ions:H2O (l) ↔ H3O+ (aq) + OH- (aq)In pure water, the concentrations of the ions (H3O+ and OH-) are equal.[H3O+]=[OH-]= 1x10-7 M
15 The Self-ionization of Water Writing the equilibrium expression for the self- ionization of water gives:Plugging in the concentrations in pure water, this gives an equilibrium constant of 1x10-14this is referred to as the ion product constant of waterThis ion product constant of water is given the symbol Kw
16 The Self-ionization of water Example #1What is the H3O+ concentration in a solution with [OH-] = 3.0 x 10-4 M?Kw = [H3O+][OH-]1x10-14 = [H3O+][3.0x10-4]
17 Example #2If the hydroxide-ion concentration of an aqueous solution is 1.0 x 10-3 M, what is the [H3O+] in the solution? Kw = [H3O+][OH-]1x10-14 = [H3O+][1.0x10-3]
18 The pH scaleDeveloped by Søren Sørensen in order to determine the acidity of alesUsed in order to simplify the concept of acids and basesThe pH scale goes from 1 to 14A change in one pH unit corresponds to a power of ten change in the concentration of hydronium (H3O+) ionsA pH = 2.0 has 10 times the concentration of H3O+ than a pH = 3.0, and 100 times greater than pH = 4
20 pH = -log [H3O+] or [H3O+] = 10-pH Calculations of pHpH can be expressed using the following equation:pH = -log [H3O+] or [H3O+] = 10-pHExample #1What is the pH of a solution with M H3O+? Is this solution an acid or a base?Acid
21 pH = -log [H3O+] Kw = [H3O+][OH-] Calculations of pHExample #2What is the pH of a solution with the concentration of hydroxide ions M? Is this an acid or a base?pH = -log [H3O+] Kw = [H3O+][OH-]Base
26 Strength of Acids & Bases When a solution is considered strong, it will completely ionize in a solutionNitric acid is an example of strong acid:HNO3 (l) + H2O (l) NO3- (aq) + H3O+ (aq)In a solution of nitric acid, no HNO3 molecules are presentStrength is NOT equivalent to concentration!
27 Strength of Acids & Bases Knowing the strength of an acid is important for calculating pHIf given concentration of strong acid (such as HNO3) assume it is the same as the concentration of hydronium, H3O+, ionsGiven concentration of a strong base, assume it has the same concentration as the hydroxide, OH-, ions
33 Strong Acids 6 of 7 strong acids are monoprotic (HX) Exists only as H ions and X ionsHI(aq) H+(aq) + I-(aq)2M HI = [H+]= [I-] = 2MDetermining pH of Strong AcidsFor Strong Acids: pH = -log [H+]For monoprotic strong acids: [H+] = [X]
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