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