Acids SS1 Chemistry Lesson Note

An acid is a substance which in an aqueous solution produces a hydroxonium ion (H3O+) or hydrogen ion (H+) as the only positive ion. Also, acids can be referred to as proton donors.

CLASSES OF ACIDS

There are two classes of acids:

1. Organic acids occur as natural products in plants and animal material.

2. Inorganic acid: Inorganic acid can be prepared from mineral elements or inorganic matter.

An acid is also defined as a substance which produces hydroxonium

H+(aq)    +    H2O(l)       H3O+(aq)

Acid can be diluted or concentrated depending on the amount of water added. A dilute acid is an acid produced when a large amount of water is added to a small amount of acid. Concentrated acid is an acid produced when only a small amount of water is added to a relatively large amount of acid.

STRENGTH OF AN ACID

The strength of an acid can either be weak or strong.

1. Strong acids: are acids which ionize completely in an aqueous solution and such acid solution has a high concentration of H+. Examples are HCl, H2SO4 and HNO3.

H2SO4   =    2H+ + SO42-

HNO3    =     H+ + NO3-

HCl        =       H+ + Cl-

2. Weak acids: are acids which ionize or dissociate slightly or partially in an aqueous solution and such acid solutions have a low concentration of hydrogen ions. Examples are ethanoic acid (CH3COOH), H2CO3, H3PO4, and H2SO3.

H2CO3  =         2H+        +   CO32-

H3PO4  =         3H+        +   PO43-

CH3COOH =  H+         +   CH3COO-

H2SO3   =     2H+         +   SO32-

BASICITY OF AN ACID

The basicity of an acid is the number of replaceable hydrogen ions, H+, in one molecule of the acid. Eg:

I. Hydrochloric acid is monobasic

II. Tetraoxosulphate (vi) acid is dibasic

III. Tetraoxophosphate(v) acid is tribasic

IV. Ethanoic acid is monobasic 

PHYSICAL PROPERTIES OF ACID

1. They have a sour taste.
2. They turn blue litmus paper to red.
3. They are corrosive, especially the strong acid.
4. In aqueous solution, they conduct electricity.

CHEMICAL PROPERTIES OF ACID

1. Reaction with metals: They react with metals to liberate hydrogen gas and salt of metal i.e.

i. Acid + Metal

ii. Salt + Hydrogen gas.

E.g.  2HCl(aq)   +   Zn(s)                      ZnCl2(aq)     +    H2(g)

H2SO4(aq)  +   Mg(s)                     MgSO4(aq)    +    H2(g)

2. They react with soluble bases to form salt and water only. This reaction is known as neutralization.

Acid + Base

Salt + water

E.g  H2SO4(aq) + 2KOH(aq)           K2SO4(aq) + 2H2O(l)

2HCl(aq) + CaO(s)           

CaCl2(aq) + H2O(l)

3. They react with  trioxocarbonates (iv) salts to liberate carbon (iv) oxide, salt and water  i.e. 

Acid   +  trioxocarbonate (iv)            

Salt  +     Water   +    CO2

E.g  2HCl (aq) +    Na2CO3(aq)                   

2NaCl(aq)  +      H2O(l)  +  CO2(g)

PREPARATION OF ACIDS

Acid can be prepared by using the following methods:

1. Dissolving an acid anhydride in water: Acid anhydride is an oxides of non-metal that dissolve in water to produce the corresponding acids e.g. SO2, CO2, CO, NO2, SO3.

SO2(g)      +        H2O(l)              H2SO3(aq)

CO2(g)    +        H2O(l)              H2CO3(aq)

SO3(g)    +        H2O(l)              H2SO4(aq)

2. Combination of constituent elements.

(a). Burning hydrogen in chlorine, in the presence of activated charcoal as the catalyst, yields HCl gas which dissolves readily in water to give HCl acid.

H2(g)     +         Cl2(g)            activated charcoal       2HCl(g)

(b)Heating hydrogen gas and bromine vapour, in the presence of platinum as the catalyst, produces hydrogen bromide which dissolves readily in water to form hydrobromic acid.

H2(g)         +          Br2(g)                  Platinum         2HBr(g)

3. By displacement of a weak or more volatile acid from its salt by a stronger or less volatile acid. For example:

(a)Displacement of the more volatile hydrogen chloride from metallic chloride by the less volatile concentrated tetraoxosulphate (vi) acid.

NaCl(s)   +  H2SO4(aq)     NaHSO4(aq)        +         HCl(aq)

(b)Displacement of weaker trioxoborate (iii) acid from ‘borax’ by tetraoxosulphate (vi) acid.

Na2B4O7(s) +  H2SO4(aq)    +   5H2O(l)      Na2SO4(aq)    +    4H3BO3(aq)

Borax         Trioxoborate (iii) acid