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Hydrolysis of Salts and pH of Buffer Solutions. Experiment 24 Page 257 Dr. Scott Buzby Ph.D. Objectives. Learn about the concept of hydrolysis Acids Bases Hydrolysis Gain a familiarity with acid-base indicators Learn about the behavior of buffer solutions - PowerPoint PPT Presentation
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Experiment 24Page 257
Dr. Scott Buzby Ph.D.
Learn about the concept of hydrolysis Acids Bases Hydrolysis
Gain a familiarity with acid-base indicators
Learn about the behavior of buffer solutions Henderson-Hasselbalch equation
Safety Note!!! Strong acids and bases attack living tissue
and cause serious burns wear proper PPE
An acid (from the Latin acidus meaning sour) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a pH less than 7.0
Arrhenius acids a substance that increases the concentration of
hydronium ions, H3O+, when dissolved in water Brønsted-Lowry acids
A Brønsted-Lowry acid (or simply Brønsted acid) is a species that donates a proton from a Brønsted-Lowry base
Lewis acids A Lewis acid is a species that accepts a pair of
electrons from another species; in other words, it is an electron pair acceptor
Strong Acids A strong acid is an acid that dissociates completely
in an aqueous solution by losing one proton Weak Acids
A weak acid is an acid that dissociates incompletely and does not release all of its hydrogens in a solution (i.e. it does not completely donate all of its protons)
While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve glass and all metals except iridium
Strong Acids HCl - hydrochloric
acid HNO3 - nitric acid
H2SO4 - sulfuric acid HBr - hydrobromic
acid HI - hydroiodic acid HClO4 - perchloric
acid
Weak Acids HCH3O2 – acetic acid HF – hydrofluoric acid Most organic acids NH4
+ - ammonium
A base is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a pH greater than 7.0
Arrhenius bases a substance that increases the concentration of
hydroxide ions, OH-, when dissolved in water Brønsted-Lowry bases
A Brønsted-Lowry base (or simply Brønsted base) is a species that accepts a proton from a Brønsted-Lowry acid
Lewis bases A Lewis base is a species that donates a pair of
electrons to another species; in other words, it is an electron pair donor
Strong BaseA strong base is a base which hydrolyzes
completely and is able to deprotonate very weak acids in an acid-base reaction, common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals like NaOH and Ca(OH)2
Weak BaseA weak base is a chemical base that does
not ionize fully in an aqueous solution or as Brønsted-Lowry bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete
Strong Bases Potassium hydroxide
(KOH) Barium hydroxide
(Ba(OH)2) Caesium hydroxide
(CsOH) Sodium hydroxide
(NaOH) Strontium hydroxide
(Sr(OH)2) Calcium hydroxide
(Ca(OH)2) Lithium hydroxide (LiOH) Rubidium hydroxide
(RbOH) Magnesium hydroxide
(Mg(OH)2)
Weak Bases Alanine, C3H5O2NH2
Ammonia, NH3
Methylamine, CH3NH2
Pyridine, C5H5N Other weak bases are
essentially any bases not on the list of strong bases
A hydrolysis reaction is the reaction of a ion with water
Anions of weak acids (C2H3O2-) react with
water to form OH-, raising the pH of the solution
Cations of weak bases (NH4+) react with
water to form H+, lowering the pH of the solution
Salt of a strong acid and a strong baseNeither ion hydrolyzes, and the solution has
a pH of 7 (Neutral) Salt of a strong acid and a weak base
The cation hydrolyzes, forming H+ and the solution has a pH < 7 (Acidic)
Salt of a weak acid and a strong baseThe anion hydrolyzes, forming OH- and the
solution has a pH > 7 (Basic) Salt of a weak acid and a weak base
Both ions hydrolyze, and the pH of the solution is determined by the relative extent to which each ion hydrolyzes
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid
It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it For example the addition of 0.036g of HCl to 1L of
water causes the pH to drop from 7.0 to 3.0 Buffer solutions are used as a means of
keeping pH at a nearly constant value in a wide variety of chemical/biological applications Human blood has a pH ≈ 7.4, changes of as little
as ± 0.05 can be dangerous or even fatal
In a solution there is an equilibrium between a weak acid, HA, and its conjugate base, A-
When hydrogen ions (H+) are added to the solution, equilibrium moves to the left, as there are hydrogen ions (H+ or H3O+) on the right-hand side of the equilibrium expression
When hydroxide ions (OH-) are added to the solution, equilibrium moves to the right, as hydrogen ions are removed in the reaction
Thus, in both cases, some of the added reagent is consumed in shifting the equilibrium in accordance with Le Chatelier's principle and the pH changes by less than it would if the solution were not buffered
AOHOHHA 32
OHOHH 2
Henderson-Hasselbalch equation is used to determine the pH of a buffer solution
This equation is convenient for preparing buffer solutions because you can neglect the amounts of the acid and base that ionize and use the initial concentrations of the acid and the conjugate base
][
][log
acidweak
baseconjugatepKpH a
Hydrolysis of Salts – Page 265
pH of Buffer Solutions – Page 266Preparation of a buffer solutionOperation of the pH meterEffect of acid and base on the buffer pH
Report Sheet – Pages 269-273
Questions – Page 272
Pre-Lab Experiment 25 – Page 281
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