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acid equilibrium intro 2 days.notebook
1
April 19, 2016
Part one: Acid/Base Theories
Learning Goals:
• to identify acids and bases and their conjugates according to Arrhenius and BronsteadLowry Theories.
• to be able to identify amphoteric and polyprotic acids and bases.
• to write an equilibrium equation for a reaction of an acid or base with water.
ACID BASE EQUILIBRIUM
ARRHENIUS THEORY
• Acids are solutes which produce hydrogen ions, H+, in aqueous solutions
eg. HCl à H+(aq) + Cl (aq)
• Bases are solutes which produce hydroxide ions, OH, in aqueous solutions
eg. NaOH à Na+ (aq) + OH (aq)
• This works for most acids and bases, but does not explain all acids and bases (eg. Ammonia, NH3)
acid equilibrium intro 2 days.notebook
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April 19, 2016
BRONSTEADLOWRY THEORY
• An acid is a proton (H+) donor• A base is a proton (H+) acceptor
• Both are present in a system
• An acid will donate a proton to become a conjugate base,• A base will accept a proton to become a conjugate acid
The conjugate acidbase pair
• Many times proton transfer can occur in both directions, and both forward and reverse reactions involve BronsteadLowry acidbase pairs
• The pair of substances which differ by a single proton (H+) are called conjugate acidbase pairs
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April 19, 2016
Try this…..
Use the BrønstedLowry definitions to identify the two conjugate acid –base pairs in each of the following acid –base reactions.H2S ﴾aq﴿ + H2O ﴾l﴿ ß> HS
﴾aq﴿ + H3O+ ﴾aq﴿
A B CB CA
Competition for protons
• A strong acid has a very weak attraction for protons• A strong base has a very strong attraction for protons
• The stronger the acid, the weaker it’s conjugate base• The stronger the base, the weaker it’s conjugate acid
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April 19, 2016
eg. HCl + H2O <à Cl + H3O
+
HCl is a very strong acid and therefore has a very weak attraction for protons.It’s conjugate base is a very weak base and will therefore have a very weak attraction for protons
Cl + H2O ß> HCl + OH
(aq)(aq) (aq)(l)
(aq)(aq)(aq) (l)
The six strong acids…..
• Hydrochloric HCl• Hydrobromic HBr• Hydroiodic HI•Nitric HNO 3•Sulfuric H 2SO4•Perchloric HClO4
all the rest are weak acids
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April 19, 2016
Let’s try the movie showing an acid being a proton donor
Let’s try the movie showing a base being a proton acceptor
Let’s try the movie showing a conjugate acid/base pair
Amphoteric (amphoteric) substances
A substance that may act as a BronstedLowry acid in some reactions and as a BronstedLowry base in others.
NH3 (aq) + H2O(l) NH4+(aq) + OH 1
(aq)
Base_
H2CO3(aq) + H2O(l) HCO3(aq) + H3O+1
(aq)
Acid
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April 19, 2016
Polyprotic Acids
Polyprotic acids have more than one ionizable proton
H3PO4(aq) + H2O(l) ⇔ H3O+(aq) + H2PO4
-1(aq)
H2PO4-1
(aq) + H2O(l) ⇔ H3O+(aq) + HPO4
-2(aq)
HPO4-2
(aq) + H2O(l) ⇔ H3O+(aq) + PO4
-3(aq)
Pg. 492 # 1, 2Pg. 494 #4, 5
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Part two: 1) The autoionization of water, 2) Calculating acidity for strong acids and Bases
Learning Goals
• To understand how water can ionize itself• To calculate [H3O+] and [OH ] based on Kw
• To understand what makes a substance acidic, and to calculate acidity• To calculate pH for a strong acid and base, and a weak acid and base
The autoionization of water
Water can ionize itself:H2O <> H+ + OH
or H2O + H2O <> H3O+ + OH
(H3O+ hydronium ion)(OH hydroxide ion)
This occurs in very little amount (2 molecules in a billion) Kw = 1 X 1014
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Kw = [H3O+][OH]
Therefore, in neutral water, [H3O+] = [OH] = 1 X 107
H2O(l) + H2O (l) <==> H 3O+(aq) + OH
(aq)
(H2O is omitted from the equation because it is a pure liquid)
1 X 1014 = [H3O+][OH] 1 X 1014 = x2
ICE
Measuring Acidity
• Acidity is the measure of the hydrogen ion (H+) or hydronium ion (H3O+) concentration in a solution.
• In neutral water [H3O+] = 1 X 107 mol/L
• pH is created to express acidity (power of hydrogen)
• pH = log [H3O+]• pH of neutral water is 7 (try it!!)
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• Acids increase the [H3O+] concentration in a solution (decrease [OH] )
• Bases increase the [OH] concentration in a solution (decrease [H3O+])
• Kw = [H3O+][OH]
[H3O+] Kw
[OH]=
[OH]Kw
[H3O+]=
The six strongest acids….
• Completely dissociate to produce hydronium ions• Calculating the pH can be done easily, because the [acid] = [H3O+]•HCl + H2O à Cl + H3O+
(aq) (aq) (aq)(l)
Strong Bases completely dissociate to produce OH1
Hydroxides of Group 1 and 2 metals are considered strong bases
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Acid/Base EquilibriaCalculating pH and pOH
Find the pH of a STRONG acid or Base
a) Calculate the [H3O+], [OH-], pH, and pOH of a 3.87x10 -4M HNO3
solution.**Remember, Strong Acids or Bases dissociate completely
HNO3(aq) H+(aq) + NO3
-(aq)
3.00 g of NaOH is added to 7.50 L of water. Calculate the [H3O
+], [OH-], pH, and pOH
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Pg. 509 #1,2, 5 cf, 6, 7 ,8Pg. 527 #1,2
Ka for weak acids
• Weak acids do not completely dissociate into hydronium ions in water, and this affects how acidic the solution is.•“HX” weak acid, conjugate base is X
HX + H2O ß> H3O+ + X
The amount of H3O+ ions depends on the extent
of dissociation, and therefore Ka
(aq) (aq) (aq)(l)
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April 19, 2016
Measuring strength of acid: Ka• Ka – acid equilibrium constant• Larger the Ka, the greater the acid will dissociate into hydronium ions (increasing acidity)
•Ka for strong acids is so large that the reaction favours products so much that it is considered an irreversible reaction
HX + H2O ß> H3O+ + X
• Ka = [H3O+][X][HX] (at equilibrium)
Once you know [H3O+ ], you can calculate pH
and you can calculate the :
(aq) (aq) (aq)(l)
% dissociation= [H3O+]HX
X 100
acid equilibrium intro 2 days.notebook
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April 19, 2016
Calculate the pH and the % dissociation of a 0.500 M acetic acid solution.
Calculate the pH and % dissociation of a 0.500 M NH3 solution.
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