Chapter 17: Acids & Bases Acids Arrhenius acid A substance that produces H+ (also H3O+) ions in aqueous solution Brnsted-Lowry acid - a substance that donates H+ ions Structure of an acid Hydrogens connected to more
electronegative atoms are more likely to act as an acidic hydrogen High EN atoms: F, Cl, Br, I, O Practice Decide if the following molecules are acids or not, based on their structures. Strong acids
Strong acid - an acid that completely ionizes or dissociates in water Weak acids Weak acid - an acid that partially ionizes or dissociates in water
Bases Arrhenius base A substance that produces OH- ions in aqueous solution Brnsted-Lowry base - a substance that accepts H+ ions Arrow pushing mechanism Strong bases Strong base - a base
that completely ionizes or dissociates in water Weak bases Weak base - a base that partially ionizes or dissociates in water
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) Conjugate acid-base pairs Each acid has a conjugate base and every base has a conjugate acid Problems Identify the conjugate bases for the
following acids 1) HBr 2) H2S 3) H2CO3 Identify the conjugate acids for the following bases 4) NO25) NH3 6) OH- Polyprotic acids
Polyprotic acids - acids that can donate more than one H+ Acid ionization constant (Ka) Acid ionization constant (Ka) - the equilibrium constant for the ionization reaction of an acid with water
[ H 3O ][CH 3COO ] Ka [CH 3COOH ] Questions 1)
Write the Ka expression for the dissociation of HCN (hydrocyanic acid) in water. 2) Write the Ka expression for the dissociation of HClO2 (chlorous acid) in water. 3) If the Ka for HCN is 4.9 x 10-10 and the Ka
for HF is 3.5 x 10-4, which acid is stronger? Base ionization constant (Kb) Base ionization constant (Kb) - the equilibrium constant for the ionization reaction of a base with water
Autoionization of water Also called the self ionization of water 1 out of 10 million water molecules form ions through self ionization All aqueous solutions
contain both H3O+ and OH Ion Product Constant for Water (Kw) The numerical value obtained by multiplying the molar concentrations for hydronium
and hydroxide ions present in pure water Kw = [H3O+][OH-] = 1.00 x 10-14 @ 25 oC the concentration of H3O+ and OH are equal in pure water [H3O+] = [OH] = 10-7M @ 25C [H+] vs. [OH-] [H+] 100 10-1
H + OH- Acid 10-3 10-5
H OH + - [OH-]10-14 10-13 10-11 10-9 10-7
H OH + 10-7 10-9 Base 10-11
H 10-13 10-14 H+ + OH OH - 10-5
10-3 10-1 100 - Questions 1)
Calculate the [OH] at 25C when the [H3O+] = 1.5 x 10-9 M, and determine if the solution is acidic, basic, or neutral 2) Calculate the [H3O+] at 25C in a solution that has a [OH] 1.3 x 10-10 M and determine if
the solution is acidic, basic, or neutral 3) Calculate the [H3O+] at 25C in a solution that has a [OH] 1.0 x 10-7 M and determine if the solution is acidic, basic, or neutral
pH and pOH pH is a measure of the concentration of H+ or H3O+ in solution pOH is a measure of the concentration of OH- in solution Acidic solutions
[H3O+] and pH of an acidic solution : Strong acids The two sources of [H3O+] in a solution are from the autoionization of water and the weak/strong acid
Strong acids completely ionize in water, so the initial acid concentration is the [H3O+] Determining [H3O+] and pH of an acidic solution :
Weak acids For weak acids we must use the Ka value to determine [H3O+] from initial [HA] [ H 3O ][ A ] Ka [ HA]
x is small approximation Initial Change Equilibrium [HA] [H3O+]
[A-] [HA] -x [HA] - x 0 +x x 0
+x x [ H 3O ][ A ] Ka [ HA] x2
Ka [ HA] x If Ka is very small, the x value is very small since small amount of products made This allows simplification of denominator expression 2 [ HA] small # [ HA] x
Ka [ HA] K a [ HA] x Exceptions to x is small approximation Can only apply this rule if the [HA] is much larger than Ka
If they are close in within a factor of 1 to 100, then the % error is large, so you cant use this approximation Ka = 1x10-10 [HA] = 1 x 10-3 Can use approximation Ka = 1x10-4 [HA] = 1 x 10-3 Cant use approximation If you cant use approx. then you have to use quadratic formula like from before Practice
Determine the [H3O+] of a 0.100 M HCN which has a Ka value of 4.9 x 10-10? HCN(aq) + H2O(l) [ H 3O ][CN ] Ka [ HCN ] Initial Change Equilibrium
x2 Ka Ka [HCN] x [HCN] K a [HCN] x 4.9 10 10 0.100 x 3.2 10 6 M [ H 3O ] A way to quantify the ionization of
a weak acid Percent ionization of weak acids Example [HA] initial = 0.120 M [H3O+] equilibrium = 0.00600M
Finding pH for a mixture of acids If you have a mixture of a strong and a weak acid with relative concentrations, the weak acid becomes essentially negligible Calculate as if only the strong acid is present
If you have a mixture of two weak acids with relative concentrations, consider the weaker of the two acids to be negligible, if differing by a magnitude of 103 Calculate as if only the stronger acid is present Finding pH for mixture
of acids If both weak acids have relative concentrations and Ka values that are close Calculate the [H+] of the stronger weak acid using ice table Then take the calculated [H+] from the ice table and use it as the initial concentration in a ice table with the weaker weak acid
Practice What is the pH of a mixture of 0.100 M Benzoic acid (HC7H5O2 , Ka1 = 6.5 x 10-5) and 0.050 M Acetic acid (HC2H3O2, Ka2 = 1.8 x 10-5)? HC7H5O2 (aq) + H2O (l) H3O+ (aq) + C7H5O2-(aq) HC2H3O2 (aq) + H2O (l) H3O+ (aq) + C2H3O2-(aq) Conjugate acids and bases
Anions can be conjugate bases of acids The Kb of a conjugate base is related to the Ka of the acid it came from Ka Kb = Kw = 1014 In general Stronger acid = weaker conjugate base Weaker acid = stronger
conjugate base Acid strength trends If a Hydrogen has more of a + charge, or is more easily donated, it is a stronger acid Weaker bond to H atom, stronger acid Higher electronegativity
of atom connected to H, stronger acid The more oxygen atoms in close proximity, stronger acid Acid strength: Bond energy Acid strength: Electronegativity Acid strength: Oxygens in close proximity
Acid rain Combustion of fossil fuels release SO2 and NO2 into the atmosphere 2SO2+O2+2H2O 2H2SO4 sulfuric acid 4NO2+O2+2H2O 4HNO3 nitric acid These acids + rain = acid rain Can cause building degradation, hazardous to aquatic life, hindrance to tree survivability, overall bad for the environment
Chapter 17 Review Acids and bases Strong vs weak acids/bases Conjugate
acid/base pairs Poly protic acids Acid/base ionization constant Ka/Kb Kw ion product of water
pH and pOH ICE tables for acids, x is small approx % ionization Acid strength trends and
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