ACID AND BASESDefinition and properties of Acid: Acid is defined as a substance whose aqueous solution possesses the following characteristic properties:1. It conducts electricity2. It reacts with active metal like zinc, magnesium etc to give hydrogen.3. It turns blue litmus red.4. It has sour taste.5. Its acidic properties disappear on reaction with a base.

Definition and properties of Base: Base is defined as a substance whose aqueous solution possess the following characteristic properties:1. It conducts electricity2. It turns red litmus blue. 3. It has bitter taste. 4. It has soapy(slippery) touch.5. Its basic properties disappear on reaction with an acid.

Arrhenius concept of acid and bases: It was proposed by Arrhenius in 1884.Arrhenius Concept of Acid : 1. An acid is defined as a substance which contains hydrogen ion.2. When dissolved in water it gives hydrogen ion.3. Example HC l(hydrochloric acid), HNO3 ( nitric acid).4.The acid which dissociate completely into H + and the negative ions are called strong acid. Example HCl, HNO3 .

HCl water H+(aq) + Cl- (aq)

5. Some acid which do not dissociate completely into H+ ion and negative ion are called weak acid. Example CH3COOH(acetic acid),H2CO3(carbonic acid).

6. There is an equilibrium between ions and undissociated molecules.CH3COOH Water CH3COO- + H+

Arrhenius Concept of Base : 1. A base is defined as a substance which contains hydroxyl group(OH-).2. When dissolved in water it gives hydroxyl (OH-)ion. Example NaOH (sodium hydroxide), NH4OH (ammonium hydroxide).3. The base which dissociate completely into OH -and the positive ions are called strong base. Example NaOH, KOH . NaOH water Na+(aq) + OH- (aq)4. Some base which do not dissociate completely into OH- ion and positive ion are called weak base. Example NH4OH and Al(OH)3.5. There is an equilibrium between ions and undissociated molecules.

NH4OH Water NH4+(aq) + OH-(aq)

Neutralization process:1. It is the process in which hydrogen ions and hydroxide ions join to form unionized molecules of water.2. As a result of this properties of acids and base are destroyed. HCl ( water) H+(aq) + Cl- (aq)NaOH water Na+(aq) + OH- (aq)

ThereforeNa+(aq) + OH- (aq) + H+(aq) + Cl- (aq) Na+(aq) + Cl- (aq) + H2O

Amphoteric substance :1. The substance which acts as both acid and base are called amphoteric or amphiprotic substance.

Example: Water is proton acceptor HCl + H2O H3O+ + Cl-

Example: Water is proton donor. NH3 + H2O NH4

+ + OH-

Conjugate acid base pair1. A conjugate pair of acid and a base differs by a proton (H+) only. Conjugate acid Conjugate base + H+

Acid 1 Base2 Acid 2 Base1 HCl + H2O H3O+ + Cl-

H2O + NH3 NH4+ + OH

2. Acid 1 and Base1 are conjugate pair I ( for forward reaction).3. Acid 2 and Base2 are conjugate pair II ( backward reaction).

Lewis concept of acid and base: 1. It was proposed by G.N.Lewis in 1923. 2. It is also called electronic concept.Lewis acid1.A Lewis acid is defined as a substance( an

atom, ion or molecule) which is capable of accepting a pair of electrons .

2.A Lewis acid is electron pair acceptor.Examples : BF3 , AlCl3 ( neutral molecules)

Examples : Ag+ , Cu+2,Fe+3 ( cations)

Lewis base1. A Lewis base is defined as a substance( an

atom,ion or molecule) which is capable of donating a unshared pair of electrons .

2. A Lewis base is electron pair donor. Examples : NH3 , ROH,H2O, RNH2 ( neutral

molecules). Examples : F- , Cl-,CN- ,OH-,I-, Br - ( anions).Reaction between Lewis acid and Lewis base NH3 is a Lewis base (electron donor) and BF3 is a

Lewis acid (electron acceptor). Reaction between NH3 a Lewis base (electron donor) and BF3 a Lewis acid ( electron acceptor) can be shown as :

Strength of Acid and Bases : pHThe pH and pOH scales provide a convenient way to express the acidity and basicity of dilute aqueous solutions. The pH and pOH of a solution are defined as as the negative logarithm of hydronium(H3O+) concentration in moles per litre.

Where [ H3O+] represent the number of moles of [ H3O+] per litre.

Example: If in a solution [ H3O+] = 10 –x moles per litre. Now according the equation pH = - log [ H3O+] = - log 10 -x

= - ( - x log 10) = - (- x ) (as log 10 =1) = x

Calculate the pH value of 10-2 molar HNO3solution.

HNO3 solution completely ionizes as

HNO3 + H2O H3O+ + NO3-

Now given [ H3O+] = [ HNO3] = 10-2 M

Therefore, pH= -log [ H3O+] = -log(10-2) = - (-2 x log 10) = 2

pH scale:1 pH scale is from 0 to 14.2 Neutral solution has pH =7.3 pH of acid solution is always less than 7.4 pH of basic solution is always greater than 7.

The self-ionization of water:

Water ionizes weakly to produce OH- and H+ ions, where Kw is the ionization constant of water, according to:

H2O(l) H+ (aq) + OH-(aq) Kw = 10-14

This means that the product of the concentration (molarity) of [H+] and [OH-] must always be 10-14 .

Kw = 10-14 = [H+] [OH-]

pH and pOH

The concentration of [H+] and [OH-] can be expressed as the pH and pOH: (p in pH or pOH means ‘potency’, and means that any constant ‘K’ or concentration with a ‘p’ in front of it is represented as pK = -log K , or pH = -log [H+])

pH = - log [H+]pOH = - log [OH-]

We can show that:

pKw = 14.0 =pH + pOH