Dissociation of Water. Hydrolysis of salt.

Dissociation of Water. Hydrolysis of salt.

Self-ionization of water. Concept of pH. Indicators. Hydrolysis of salt. Hydrolysis constant. Degree of Hydrolysis.

Explain what is meant by the autoionization of water, and write the ion-product-constant expression for this process.

H₂O can act as both a proton donor and acceptor for itself. A proton can be transferred from one water molecule to another, resulting in the formation of one hyroxide ion (OH-) and one hydronium ion (H₃O⁺).

2H₂O ®H₃O+(aq) + OH^-(aq)

This is called the autoionization or dissociation of water. This equilibrium can also be expressed as:

H₂O ®H⁺(aq) + OH^-(aq)

In the above equilibrium, water acts as both an acid and a base. The ability of a species to act as either an acid or a base is known as amphoterism.

The concentrations of H₃O⁺ and OH^- produced by the dissociation of water are equal. The corresponding equilibrium expression for this would be: KC = {[H⁺][OH^-] / [H₂O]}

Rearranging gives: K_C (55.5 M) = [H⁺][OH^-] = K_w

So now we can eliminate [H₂O] from the equation and we are left with: K_W=[H⁺][OH^-]

Where K_W designates the product (55.5 M)K_C and is called the ion-product constant for water. At 25^oC, K_W is equal to 10^-14.

The ion-product constant always remains constant at equilibrium (as the name implies). Consequently, if the concentration of either H⁺ or OH^- rises, then the other must fall to compensate. In acidic solutions, [H⁺] > [OH^-], and in basic solutions [H⁺] < [OH^-].

Rather than expressing [H⁺] as some very small number, it is often more convenient to describe in terms of pH, defined as: pH = - log [H⁺]

For example, a neutral solution at 25^oC contains equal concentrations of H⁺ ions and OH^- ions, where [H⁺] = 10^-7M. Thus, the pH of the solution is obtained by:

pH = - log 10^-7 = 7

From this it is obvious that at 25^oC:

pH < 7 solution is acidic

pH = 7 solution is neutral

pH > 7 solution is basic

Notice that the pH of a solution measures the concentration of dissociated protons, and not the total concentration of acid in a solution.

The negative log scale is useful for measuring other minute quantities, for example to measure [OH^-]: pOH = - log [OH^-]

Knowing this, we obtain the following useful expression: pH + pOH = -log K_W = 14

The Relationship Between pH and pOH:

pH	pOH	[H^+] mol/l	[OH^-] mol/l
		1.0	10^-14
		0.01	10^-12
		0.0001	10^-10
		10^-6	10^-8
		10^-8	10^-6
		10^-10	0.0001
		10^-12	0.01
		10^-14	1.0

Indicators are substances whose solutions change colour due to changes in pH. These are called acid-base Indicators.

Indicator name	Colour in Acid	Colour in Base
Litmus	Red	Blue
Methil Orange	Red	Yellow
Phenolphthalein	colourless	Pink
Universal indicator	Red	purple

Hydrolysis OF SALTS. Any reaction in which water is decomposed with all or part of its decomposition portions combining with the products. Water is added to decompose something.

The ions of salts can have an influence on the pH of a solution. Ions that come from a strong acid or base do not influence the pH. Since strong acids and bases are 100% ionized in water, the ions are unable to reform the molecular acid or the base in water.

Add water: HCl + H₂O® H₃O⁺ + Cl^-

Hydrolysis: NaOH + H₂O® Na⁺ + OH^- + H₂O

NaCl is the salt that comes from a strong acid and a strong base. Salts that contain ions that come from a weak acid or base. A salt containing the anion of the weak acid and the cation from a strong base.

Add water: KNO₂(s) + H₂O® K⁺ + NO₂^- + H₂O

Hydrolysis: NO₂^- + H₂O® HNO₂ + OH^-

A basic solution.

Salts that contain ions that come from a weak acid or base. A salt containing the cation of the weak base and the anion from a strong acid.

Add water: (CH₃)₃NHCl (s) + H₂O à (CH₃)₃NH⁺ + Cl^- + H₂O

Hydrolysis: (CH₃)₃NH⁺ + H₂O ↔ (CH₃)₃NHOH + H⁺

An acidic solution.

Salts that contain ions that come from a weak acid and weak base.

A salt containing the cation of the weak base and the anion from a weak acid.

Add water: CH₃COONH₄ + H₂O à CH₃COOH + NH₄OH

Hydrolysis: CH₃COO^- + NH₄⁺ + H₂O à CH₃COOH + NH₄OH

solution will be neutral.

NH₄CN(aq) K_a(NH₄⁺) = 5.6 x 10^-10 K_b(CN^-) = 2.04 x 10^-5

Then the K_a and K_b of the acid or base from which the ions come from must be compared. Since K_b(CN^-) is greater than K_a(NH₄⁺), the solution is basic.

Hydrolysis constant

The general equation for the hydrolysis of a salt (BA), Applying the law of chemical equilibrium, we get

[HA][BOH]/[BA][H₂O] = K,

where K is the equilibrium constant.

Since water is present in very large excess in the aqueous solution, its concentration [H₂O] may be regarded as constant so,

[HA][BOH]/[BA] = K[H₂O = Kh]

where Kh is called the hydrolysis constant.

Degree of hydrolysis

It is defined as the fraction (or percentage) of the total salt which is hydrolysed at equilibrium. For example, if 90% of a salt solution is hydrolysed, its degree of hydrolysis is 0.90 or as 90%. It is generally represented by ‘h’.

Types of salt	Exp. for Kh	Exp. for h	Exp. for pH
Salt of weak acid and strong base	Kh=Kw/Ka	h=√(Kh/C)	pH=–1/2[log Kw+log Ka– log C]
Salt of strong acid and weak base	Kh=Kw/Kb	h=√(Kh/C)	pH=–1/2[log Kw–log Kb+log C]
Salt of weak acid and weak base	Kh=Kw/KaKb	h = √(Kh)	pH=–1/2[log Ka+ log Kw–log Kb]

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Дата добавления: 2015-11-04