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TOPIC 12-ACIDS AND BASES
THE ARRHENIUS THEORYTHE ARRHENIUS THEORYIn an aqueous solution a strong electrolyte exists only in the form of ions, whereas a weak electrolyte exists partly as ions and partly as molecules.When the acid HCl dissolves in water, the HCl molecules ionize completeley , yielding H+ ions. When the base NaOH dissolves in water, the Na+ and OH- ions become dissociated
BRONSTED-LOWRY THEORY OF BRONSTED-LOWRY THEORY OF ACIDS AND BASESACIDS AND BASES
In their theory an acid is a proton donor and a base is a proton acceptor
BRONSTED-LOWRY THEORY OF BRONSTED-LOWRY THEORY OF ACIDS AND BASESACIDS AND BASES
BRONSTED-LOWRY THEORY OF BRONSTED-LOWRY THEORY OF ACIDS AND BASESACIDS AND BASES
We identify acids and bases in some typical acid-base reactions. In working through this example, notice the following additional features:1.Any species that is an acid by the Arrhenius theory remains an acid in the Bronsted-Lowry theory, the same is true of bases.2.Certain species, even though they do not contain the OH- group, produce OH- in aqueous solution, e.g OCl-. As such they are Bronsted-Lowry bases.The Bronsted-Lowry theory accounts for substances that can either react as an acid or a base; they are said to be amphiprotic. The Arrhenius theory does not amphiprotic. The Arrhenius theory does not account for amphiprotic behaviour.account for amphiprotic behaviour.
Bronsted-Lowry acid-base reaction is favored in the direction from the stronger to the weaker acid-base combination.
RELATIVE STRENGTHS OF SOME RELATIVE STRENGTHS OF SOME COMMON ACIDS AND BASESCOMMON ACIDS AND BASES
THE COMPARISON OF THE STRENGTH OF THE COMPARISON OF THE STRENGTH OF ACIDSACIDS
Both HCl and HClO4 are strong acids. To determine whether HCl or HClO4 is stronger acid, we need to use a solvent that is a weaker base than water, a solvent that will accept protons from the stronger of the two acids more readily than water from the weaker one. In the solvent diethyl ether, (C2H5)2O, HClO4 is completely ionized but HCl is only partially ionized. HClO4 is stronger acid than HCl.
THE SELF-IONIZATION OF WATER AND THE SELF-IONIZATION OF WATER AND THE PH SCALETHE PH SCALE
For each H2O molecule that acts as an acid another acts as a base, and hydronium, H3O+, and hydroxide, OH- ions are formed. In the reverse reaction H3O+ donates a proton to OH-. Equilibrium is displaced far to the left since H3O+ and OH- are much stronger than the other acid-base conjugate
The equilibrium constant expression for pure waterThe significance of the equation is that it applies to all aqueous solutions
pH and pOHIn 1909 the Danish biochemist Soren Sorensen proposed the term pH to refer to the « potential of hydrogen ion». He defined pH as the negative logarithm of [H+]. Restated in termS of [H3O+].
pH = - log [HpH = - log [H33OO++]]Thus, in a solution that is 0,0025 M HCl[H3O+] = 2,5 x 10-3 M and pH= -(log 2,5x 10-3)= 2,60To determine [H3O+] corresponding to pH value, we do an inverse calculation Log [H3O+] = - 4,50 [H3O+]= 10-4,50 = 3,2 x 10-5
pOH = -log [OH-] pKw= pH + pOH =14,00We say that pure water and all aqueous solutions with pH=7 are pH neutral. If the pH is less than 7,00, the solution is acidic, if the pH is greater than 7, the solution is basic or alkaline
STRONG ACIDS AND STRONG STRONG ACIDS AND STRONG BASESBASES
STRONG ACIDS AND STRONG BASESSTRONG ACIDS AND STRONG BASES
WEAK ACIDS AND WEAK BASES
SOME COMMON WEAK ACIDS
SOME COMMON WEAK BASES
DETERMINATION OF KDETERMINATION OF Kaa
Example: Butyric acid, HC4H7O2 is used to make compounds employed in artificial flavorings and syrups. A 0,25 M aqueous solution of HC4H7O2 is found to have a pH of 2,72. Determine Ka for butyric acid
initial conc
changes
equil. conc.
Calculation of pH of a weak base without Calculation of pH of a weak base without neglecting the value of Xneglecting the value of X
What is the pH of a methylamine(CH3NH2) solution that is 0,0025 M? Kb= 4,2 x 10-4
Acid Concentration, M
Percent ionization gives the proportion of ionized molecules.The percent ionization of a weak acid or a weak base increases as the solution becomes more dilute.
PERCENT IONIZATION
Percent ioniz.= [H3O]+
[HA]
X 100pH of the solution
PERCENT IONIZATION
Example: What is the percent ionization of acetic acid in 1,0 M, 0,1 M, 0,010 M HC2H3O2? Ka=1,8x10-5
POLYPROTIC ACIDS
Some acids have more than one ionizable H atom per molecule. These are called polyprotic acids. For example : Phosphoric acid, H3PO4 , it has three ionizable H atoms; it ionizes in three steps and for each step we can write an ionization equation and an ionization constant expression with a value of Ka :
1. Ka1 is so much larger than Ka2 and Ka3 that essentially all the H3O+ is produced in the first ionization step alone.
2. So little of the H2PO4- formed in the first ionization step ionizes any further
that we can assume [H2PO4-]=[H3O+]
3. [HPO42-] BC Ka2 regardless of the molarity of the acid
POLYPROTIC ACIDSPOLYPROTIC ACIDS
Calculating Ion Concentrations in a Polyprotic Acid Solution: For a 3,0 M H3PO4 solution, calculate a) [H3O+] b) [H2PO4
-] c) [HPO42-] and d) [PO4
3-]Solution:
IONIZATION CONSTANTS OF SOME COMMON POLYPROTIC ACIDS
BUFFER SOLUTIONS
There are some water solutions, called buffer solutions, whose pH values change only very slightly upon the addition of small amounts of either an acid or a base.What buffer solutions require are two components, one of which is able to neutralize acids, and the other bases.
But, of course, the two components must not neutralize each other. This rules out mixtures of a strong acid and a strong base. Instead, common buffer solutions are described either as a mixture of1.A weak acid and its conjugate base2.A weak base and its conjugate acid
An important example of a buffered system is that found in blood, which is maintained at a pH=7,4 and called HCO3/H2CO3 buffer solution. It helps that the pH of the blood remains constant at 7,4 ( A change of the value 0,2 in pH in blood may cause lethal disinfections or diseases, therefore it has a very important role in human health.)Other applications with buffer solutions are protein studies which aim to measure enzymatic activities. These studies are performed under buffer media since the magnitude and kind of electric charges of the protein molecules depend on the pH.
BUFFER SOLUTIONS
ACID-BASE INDICATORS
An acid-base indicator is a substance whose color depends on the pH of the solution to which it is added. When a small amount of indicator is added to the solution, the indicator does not affect the pH of the solution because so little of it is present.
Acid-base indicators find their greatest use where only an approximate pH determination is needed. E.g in soil testing kits to establish the pH of soil.In swimming pools, the optimal pH=7,4 and phenol red is common indicator used in testing swimming pool water.
Indicator Low pH color Transition pH range High pH color
Gentian violet (Methyl violet 10B)
yellow 0.0–2.0 blue-violet
Leucomalachite green (first transition)
yellow 0.0–2.0 green
Leucomalachite green (second transition)
green 11.6–14 colorless
Thymol blue (first transition) red 1.2–2.8 yellow
Thymol blue (second transition)
yellow 8.0–9.6 blue
Methyl yellow red 2.9–4.0 yellow
Bromophenol blue yellow 3.0–4.6 purple
Congo red blue-violet 3.0–5.0 red
Methyl orange red 3.1–4.4 orange
Bromocresol green yellow 3.8–5.4 blue
Methyl red red 4.4–6.2 yellow
Methyl red red 4.5–5.2 green
Azolitmin red 4.5–8.3 blue
Bromocresol purple yellow 5.2–6.8 purple
Bromothymol blue yellow 6.0–7.6 blue
Phenol red yellow 6.4–8.0 red
Neutral red red 6.8–8.0 yellow
Naphtholphthalein colorless to reddish 7.3–8.7 greenish to blue
Cresol Red yellow 7.2–8.8 reddish-purple
Phenolphthalein colorless 8.3–10.0 fuchsia
Thymolphthalein colorless 9.3–10.5 blue
Alizarine Yellow R yellow 10.2–12.0 red