Chapter 14 Chapter 14 Aqueous Equilibria: Aqueous Equilibria: Acids and Acids and Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H + or increases the concentration of hydrogen ion Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH -1 increases the concentration of hydroxide ions. M + (aq) + OH (aq) MOH(aq) H + (aq) + A (aq) HA(aq) The Brønsted-Lowry Theory
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Chapter 14 Aqueous Equilibria: Acids and Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H + or increases the concentration.
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Arrhenius Acid: A substance that dissociates in water to produce hydrogen ions, H+ or increases the concentration of hydrogen ion
Arrhenius Base: A substance that dissociates in water to produce hydroxide ions, OH-1 increases the concentration of hydroxide ions.
M+(aq) + OH(aq)MOH(aq)
H+(aq) + A(aq)HA(aq)
The Brønsted-Lowry Theory
Acid DissociationAcid Dissociation What happens when an acid dissolves in water?
HCl (aq) + H2O (l) → Cl–(aq) + H3O+ (aq)
Water acts as a Bronsted-Lowry base and takes a proton (H+) from the acid.
As a result, the conjugate base of the acid and a hydronium ion are formed.
Acid-Base Concepts: The Acid-Base Concepts: The Brønsted-Lowry TheoryBrønsted-Lowry Theory
Conjugate Acid-Base Pairs: Chemical species whose formulas differ only by one hydrogen ion, H+
Brønsted-Lowry Acid: A substance that can transfer hydrogen ions, H+. In other words, a proton donor
Brønsted-Lowry Base: A substance that can accept hydrogen ions, H+. In other words, a proton acceptor
14.3 Hydrated Protons and 14.3 Hydrated Protons and Hydronium IonsHydronium Ions
H+(aq) + A(aq)HA(aq)
[H(H2O)n]+
For our purposes, H+ is equivalent to H3O+.
n = 4 H9O4+
n = 1 H3O+
n = 2 H5O2+
n = 3 H7O3+
Due to high reactivity of the hydrogen ion, it is actually hydrated by one or more water molecules.
ExamplesExamples Write a balanced equation for the dissociation of each
of the following Bronsted-Lowry acids in water
HCl (aq)
H2CO3(aq)
HSO4-(aq)
ExamplesExamples In the following equation, label each species as an acid
or a base. Identify the conjugated acid-base pair
a. NO2-(aq) + HF(aq) HNO2(aq) + F-(aq)
b. HClO4(aq) + H2O(l) H3O+(aq) + ClO4-
(aq)
14.2 Acid Strength and Base 14.2 Acid Strength and Base StrengthStrength
H3O+(aq) + A(aq)HA(aq) + H2O(l)
BaseAcidBaseAcid
Weaker acid + Weaker baseStronger acid + Stronger base
With equal concentrations of reactants and products, what will be the direction of reaction?
The reaction almost goes completely to the right, to the formation of a weaker acid. Why?
Acid Strength and Base Acid Strength and Base StrengthStrengthWeak Acid: An acid that is only partially dissociated in water and is thus a weak electrolyte.
ExamplesExamples If you mix equal amount concentrations of reactants
and products, decide which species (reactants or products) are favored at the completion of the reaction
◦ H2SO4(aq) + NH3(aq) NH4+(aq) + HSO4
-
(aq)
◦ HF(aq) + NO3-(aq) HNO3(aq) + F-(aq)
◦ H2S(aq) + F-(aq) HF(aq) + HS-(aq)
14.4 Dissociation of 14.4 Dissociation of WaterWater
Water autoionizationWater autoionization As we have seen, water is amphoteric. In pure water, a few molecules act as bases and a few act
as acids. This is referred to as autoionization. H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH- (aq
Kw = [H3O+] [OH−] at 25oC
This special equilibrium constant is referred to as the ion-product constant for water, Kw
Do you expect to have the same value of Kw at a temperature that is differ than 25oC?
Dissociation of WaterDissociation of Water Acidic:
[H3O+] > [OH]
Neutral:
[H3O+] = [OH]
Basic:
[H3O+] < [OH]
ExamplesExamples At 50oC the value of Kw is 5.5 x 10-14. What are the
concentration of H3O+ and –OH in a neutral solution at 50oC?
Calculate the [-OH] in a solution with [H3O+] = 7.5 x 10-5M. Is this solution basic, acidic or neutral?
The pH ScaleThe pH ScaleThe hydronium ion concentration for lemon juice is approximately 0.0025 M. What is the pH when [H3O+] = 0.0025 M?
pH = log(0.0025) = 2.60
2 significant figures
2 decimal places
ExamplesExamples
Calculate the [H3O+] and pH of a solution with [-OH] = 6.4 x 10-8M
Calculate the concentration of H3O+ and –OH for a solution with a pH of 5.50
Ways to measure pHWays to measure pH Three ways to measure pH Litmus paper
◦ red-to-blue:
◦ basic, pH > 8
blue-to-red:◦ acidic, pH < 5
An indicator A pH meter
14.6 Measuring pH14.6 Measuring pHAcid-Base Indicator: A substance that changes color in a specific pH range. Indicators exhibit pH-dependent color changes because they are weak acids and have different colors in their acid (HIn) and conjugate base (In) forms.
H3O+(aq) + In(aq)HIn(aq) + H2O(l)
Color BColor A
14.7 pH of strong acids and 14.7 pH of strong acids and strong basesstrong bases A strong monoprotic acds – 100% dissociated in
aqueous solution (HClO4, HCl, HBr, HI, H2SO4 HNO3 etc…)◦ Contains a single dissociable proton
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
◦ pH = - log H3O+
◦ [H3O+] = [A-] = initial concentration of the acid
◦ Undissociated [HA] = 0
Strong BasesStrong Bases Alkali metal hydroxide, MOH