18AcidsandBasestitrationhydrolisisp3.pptx

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SALT HYDROLYSIS

Salt Hydrolysis: reaction of the anions or cations in a salt with water to form an acidic or basic solution.

Although salts are products of neutralization reaction,

they do not all form neutral aqueous solutions

Salt + water = ? ======

======

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Anion Hydrolysis

A-_{+ H}

20(l) <=> HA(aq) + OH-(aq)

The anion is a conjugate base, when the acid is weak this conjugate is strong enough to hydrolyze water

The release of OH-_{causes an increase of the PH}

Cation Hydrolysis

Example:

The salt of a weak acid (CH₃COOH) and a strong base (NaOH) Completely

ionizes in solution:

CH₃COONa  CH₃COO-_{+ Na}+

CH₃COO- acts as a base

CH₃COO-_{+ H}

2O => CH3COOH + OH

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Example:

Ammonium Chloride (NH₄Cl)

NH₄Cl is the salt of a strong acid (HCl) and a weak base (NH₃)

Completely ionizes in solution:

NH₄Cl + H₂O_(l)  NH₄+

(aq) + Cl-(aq)

NH₄+_{acts as an acid in water}

NH₄+

(aq) + H2O (l) => H+(aq) + NH3(aq)

METAL IONS

Metal ions are small and have two or three positive charges can hydrolyze water

Example: AL3+

<==>

3+

======

=> O-H bond is broken to release H+

-3+ 2+

+ H+

[Al(H₂O)₆]3+

(aq) <==>

[Al(H

2

O)

5

OH]

2+(aq)

+ H

+

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Metals ions that are able to hydrolyze are Be2+_{, Al}3+_{and transition}

metals as Fe3+

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Acid Base Titration is used for neutralization reaction

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Obtaining titration curves

When a base is added to an acid the PH will change

To follow the reaction we measure:

The pH and the volume of the base added (concentration base known)

We will look at four main titration curves.

1 Strong Acid and strong Base

2 Weak Acid and strong Base

3 Strong Acid and weak Base

4 Weak Acid and Weak Base

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Titration of a Strong Acid with a Strong Base

NaOH (aq) + HCl (aq) Na

+

_{(aq) + Cl}

-

_{(aq) + H}

2

O(l)

Add standard NaOH to an unknown solution of HCl

p

H

NaOH Volume (ml)

2

4

6

8

10

12

Phenolphthalien

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The correct graph shows the following for a Strong acid with a strong Base

1 Initial pH = 1 (PH of a strong acid)

2 pH = Changes gradually until equivalence point

3 There is a sharp jump in PH at equivalence point (and between 3-11)

4 After equivalence point the pH is high

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p

H

NaOH Volume (ml)

2

4

6

8

10

12

Titration of a Weak Acid with a Strong Base

NaOH (aq) + CH

₃

CO

₂

H (aq)

_Na

+

(aq) + CH

3

CO

2-

(aq) + H

2

O(l)

Add standard NaOH to an unknown solution of CH

₃

CO

₂

H

Buffer Region

CH₃CO₂-_{accumulates, making a}

CH₃CO₂H/CH₃CO₂-_buffer Equivalence Point

Mol H+_{= mol OH}-_added

Midpoint (Half of the acid neutralized) [CH3CO2H] = [CH3CO2- ]

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The correct graph shows the following for a weak acid with strong Base

1 Initial PH is fairly high (PH of a weak acid)

2 PH stays relative constant until equivalence (buffer region) 3 There is a jump in PH at equivalence (and between 7-11)

not a big jump as with titration of a strong acid with a strong base.

4 After equivalence point the ph flattens out at high value (strong base)

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A

Titration of a Strong Acid with a weak Base

A

B

Titration of a Strong Acid with a weak Base

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The correct graph shows the following for a Strong acid with a weak Base

1 Initial PH = 1 (PH of a strong acid)

2 PH stays relative constant through buffer region

3 There is a jump in PH at equivalence from PH3 to PH 7

4 After equivalence the curve flattens out at a fairly low PH

5 PH at equivalence <7

The correct graph shows the following for a Weak acid with a weak Base

1 Initial PH is fairly high (PH of a weak acid)

2 Addition of base causes the PH to rise gradually

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pH 0 1 2 3 4 5 6

pH 7 8 9 10 11 12 13

INDICATORS

Indicators signal change in PH

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Range and Color Changes of Some Common Acid-Base Indicators

Indicators

pH Scale

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Methyl orange red 3.1 – 4.4 yellow

Methyl red red 4.4 6.2 yellow

Bromthymol blue yellow 6.2 7.6 blue

Neutral red red 6.8 8.0 yellow

Phenolphthalein colorless 8.0 10.0 red colorless beyond 13.0