Acids and Bases

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V.WEEK

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5th WEEK

 Acid, base, and buffer solutions

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Acids and Bases



Chemists have long been trying to identify substances as acids or bases.



When we look at history, we can say that Antoine Lavoisier first states that the basic element in acids is oxygen. Because

“oxygen ” is also meant to produce acid in greek.



David Humphrey's finding in 1810 is that acids generally contain hydrogen elements.



In 1884, there was a radical change in the acid-base definition with the work of Svante Arrhenius.



Today, we explain the acids and bases with Arrhenius,

Bronsted-Lowry and Lewis theories.

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Arrhenius Theory



According to the definition of Arrhenius; acids: compounds that produce hydrogen ions with water, bases are defined as compounds that produce hydroxyl ions with water.



Arrhenius, in its aqueous solutions, defined as completely

(100%) ionisable, ie acid (s) and acid (s), which can give all (H

+) protons or (OH-) hydroxyl ions, and acid and base solutions

that are partially ionisable in aqueous solutions.

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 However, the acid-base definition of Arrhenius has a limiting aspect in itself.

Considering the weak sample of ammonia (NH3) in the NH3, where is OH-?

 Apart from the definition of Arrhenius, NH3 is present in aqueous solutions as NH4OH compound.

 It is limited to aqueous solutions only.

 It is not able to explain the acid and its basicity of some substances in aqueous solution.

 Hydrogen ion in aqueous solution was not considered.

• As regards acid-base reactions of Arrhenius;

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Lowry-Brönsted Theory

 In 1923, the British Lowry and the Danish Brönsted described the acid as a proton donor and the base as proton acceptor.

• This concept of acid-base is very comprehensive from the concept of Arrhenius and is applied in every environment where proton transfer is present. As in Arrhenius acids, a Lowry-Brönsted acid can be formulated as HnA. Therefore, all Arrhenius acids are also Lowry-Brönsted acids.

• Lowry-Brönsted should be considered as a chemical species that contains an electron pair that can be easily given at least to bind the proton to itself.

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 Lowry-Brönsted acid-base reaction



• According to the forward reaction, HA is acid due to giving proton and B is base due to gaining proton. According to the reaction in the reverse direction, HB + is acid and A is base. As can be seen, a Lowry-Brönsted acid-base reaction has two acids and two bases. The acid-base pair which is different from each other in a proton is called the conjugated acid-base pair.

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Lewis Theory



In 1923, Lewis identified the acid as an electron pair acceptor

and some as an electron pair donor. A coordinate-covalent bond

is formed as a result of the transfer of the electron pair from B

to A by A Lewis acid and B Lewis base.

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Ionization of water



Since the water conveys the electric current, it should be remembered that there are ions in the water.

• Since the event is an equilibrium event, the equilibrium constant is called the equilibrium constant of water (Ksu).

• As the acid will give H + ion to the water [H +] = [OH-], the balance will be impaired [H +]> [OH-]

• Base gives OH- ion to water or gain H+ ion from water. The equivalence will be in [H +] <[OH-]

direction.

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pH and pOH

 pH is ion concentration. The pH is found by the negative logarithm of hydrogen ion activity, while the pOH is found by the negative logarithm of hydroxyl ion activity.pH= -log[H

⁺

], pOH= -log[OH

^-

],

 pH + pOH=14

 Hydrogen or hydroxyl concentration of a known solution of pH or pH can be determined

 [H

⁺

] = [OH

^-

]=10

^-7

M (pure water is considered an absolute standard for neutrality).

 pH= -log[10

^-7

], pH=7

pOH=-log[10

^-7

], pOH=7

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Acid Base

1) [H

⁺

] >[OH

^-

] 1) [H

⁺

] <[OH

^-

] 2) [H

⁺

] >10

^-7

M 2) [H

⁺

] <10

^-7

M 3) [OH

^-

] <10

^-7

M 3) [OH

^-

] >10

^-7

M 4) pH < 7 4) pH > 7

5) pOH > 7 5) pOH < 7

6) pH < pOH 6) pH > pOH

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Ionization of Weak Acids



Zayıf asitlerde ya da bazlarda yüzde yüz iyonlaşma olmaz ve bir denge söz konusudur (K

a

ve K

b

).

• Similar equations are obtained for weak bases by acting on weak acids.

• K

_a

x K

_b

= K

_water

, pKa + pKb= 14

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Example: The pH of the 0.1 M HA acid is 3’. According to this, what is the Ka of this acid?

If pH=3

HA ↔ H

⁺

+ A

^-

0.1-x

x x

[H

⁺

]=1x 10

^-3

[A

^-

]= 1x 10

^-3

Ka= [1x 10

^-3

]

²

/[0.1- 1x 10

^-3

] Ka= 1x 10

^-6

Neutralization:

It is the process of reacting the base with any acid to form a salt.

Acid + Base ---> Salt + H2O

The hydrogen ions from the acid and the hydroxyl ions from the

base are neutralized. If the mole number of hydrogen ions from

the acid is equal to the number of moles of hydroxyl ions from

the base, the environment becomes neutral.

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Buffers

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