PROPERTIES OF ACID AND BASES

Acids:

1.    Acids have a sour taste, vinegar gives the taste of acetic acid and lemons and other citrous fruits contain citric acid.

2.    They change the colour of blue litmus to red.

3.    Acids react with certain metals, such as (Zn, Mg and Fe) to produce (H2) gas, for example, when dilute (HCL) reacts with (Zn) metal, .produces (H2) gas.
                     Zn(s) + 2HCl(aq)
------à ZnCl2(s) + H2(g)

4.    Acids react with carbonates and bicarbonates, such as Na2 CO3, CaCO3 and NaHCO3 to produce CO2 gas.
                CaCO3(s) + 2HCl(aq)
------à CaCl2(aq) +H2O(l) + CO2(g)
             NaHCO3(s) +   HCl(aq)
------à  NaCl(aq) +H2O(l) + CO2(g)

5.    Acids react with oxides and hydroxide of metals, forming salt and water.
                  FeO(s) + 2HCl(aq)
------à FeCl2(aq) + H2O(l)
              Fe(OH)3(s) + 3HCl(aq)
------à FeCl3(aq) + 3H2O(l)

6.    Aqueous acid solutions, conduct electricity.

7.    They react with bases to form salts and water.

Bases:

1.    Bases have bitter taste.

2.    Bases have slippery touch.

3.    They change the colour of red litmus to blue

4.    Aqueous solutions of bases, conduct electricity.

5.    They react with acids to form salts. When acids and bases are mixed in right proportions, the acidic and basic properties disappear and new substances salt and water are formed and the reaction is neutralization reaction.

9.2.1 Acids and Bases (Arrhenius theory):

Svante-Arrhenius, a Swedish chemist in (1887) first gave the clue to chemical nature of acids in his theory of ionization. It states, that an acid can be defined as a substance that yields hydrogen (H+) ions when dissolved in water. A base can be defined as a substance that yields hydroxide (OH-) ions, when dissolved in water. Thus (HCL) and (H2SO4) are acids and (NaOH) and (KOH) are bases.

Arrhenius acid
HCl(aq) < ----- >  H+(aq) + CI (aq)

Arrhenius base
NaOH(aq)   < ------ > Na+(aq) + OH (aq)

Bronsted Lowry Theory (Proton-Donor and Acceptor Theory):

Arrhenius's definitions of acids and bases are limited in that, they apply only to water (aqueous) solutions and it also does not account for the basicity of ammonia (NH3), that doesn't contain (OH) group. Broader definitions, which were proposed by the Danish Chemist Johannes and English Chemist Thomas Lowry, in 1923, describe, that An acid is a substance having a tendency to donate one or more protons and is substance, having a tendency to accept (add) protons.

Bronsted-Lowry acid = A substance that can donate (H+)

Bronsted-Lowry base = A substance that can accept (H+)

In the above examples, the (HCL) and water (H2O), are proton donors and act as Bronsted-Lowry acids, whereas (H2O) and ammonia (NH3) are proton acceptors and are known as Bronsted Lowry bases.

LEWIS CONCEPT OF ACIDS AND BASES

Electronic explanation of Acids and Bases): In 1923 G. N. Lewis proposed a more general concept of acids and bases. It explains the behaviour of acids in terms of electrons rather than protons, since electrons are responsible for chemical bonding. Lewis acids include not only (H+) protons, but also other cations and neutral molecules, having vacant valence orbitals that can accept a pair of electrons donated by Lewis base.

According to the Lewis theory, an acid is any species (molecule or ion) which can accept a pair of electrons, and base is any species (molecule or ion) which can donate a pair of electrons. An acid-base reaction, in which electron pair donor is base and electron-pair acceptor is acid, they form a co-ordinate covalent bond between the two.

Lewis Acid = An electron pair acceptor.

Lewis Base = An electron pair donor.

For example: when ammonia (NH3) reacts with proton (H+) to form ammonium ion (NH+4), in which the nitrogen of (NH3) donates a pair of electrons whereas the (H+) accepts that pair of electrons for bond formation, this is shown by curved arrow.

Another example is provided by the reaction of ammonia (NH3) with boron tri-fluoride (BF3), in which nitrogen of (NH3.) donate an electron pair and (B) of BF3, which lacks a pair of electrons to complete its outer most shell (octet), accepts that pair of electrons and form a co-ordinate covalent bond.

9.2.2 Neutralization:

If we add an acid to a base drop by drop, the acidic character of the acid decreases gradually. A stage will come when the resultant solution will become neutral to litmus. This stage is called neutralization. In other words when equivalent quantities of an acid and a base are mixed, salt and water are formed. A common example is the reaction between HCL and NaOH.

HCl(aq) + NaOH(aq) < ----- > NaCl(aq) + H2O(l)

The hydrogen ion (H+) which is responsible for acidic properties reacts with the hydroxide ion (OH-), which is responsible for the basic properties, producing neutral water (H-OH). Because the only change that takes place is the reaction of the hydrogen (H+) ion and hydroxide ion (OH-), the neutralization may be expressed simply as:

H+ + OH- ------à  H2O

The neutralization is an exothermic reaction.

E.g. HCl (aq) + NaOH (aq) ------à  NaCl (aq) + H2O(l) + 13700 cal/mol (57.24KJ/mol)

In case that either acid or base is not completely ionized, i.e. (weak acid or weak base) the neutralization reaction may not go to completion and the heat of neutralization may be less than 13700 cal. For example, when strong acid (HCl) reacts with weak base Ca (OH)2, the amount of heat evolved is 24700 cal/mol.

2 HCl + Ca (OH)2  ------à CaCl2 + 2H2O + 24700 cals

Fig. 9.1 shows when solutions of hydrochloric acid and sodium hydroxide are mixed, only hydrogen ions and hydroxide ions react forming water. Therefore we may write the equation: H+(aq) + OH-(aq) ------à HOH(l)

REMEMBER

A reaction in which an acid and a base form an ionic compound (salt and water) is called neutralization reaction. If water is formed, the reaction can be classified as a double-replacement reaction.

9.2.3 Mono and Poly Acids and Bases (Mono Basic and Poly Basic Acids) (Basicity of Acids)

As we have seen that an acid yields the protons. Different acids have different number of protons (acidic-hydrogen) per molecule and yield different number of (H3O+) ion in a solution.

The common acids like (HCl) (HNO3) and (CH3COOH) contain only one acidic hydrogen atom per molecule. When dissolved in water 1 mole of each of these acids is capable of producing 1 mole of hydrated FT = (H3O+) ions, and in order to neutralize this solution 1 mole of (OH-) ions is required. Consequently these acids are called mono-basic acids more commonly called mono-protic acids

E.g. H-Cl(aq) + H OH(l) ----- >H3O+ (aq) + Cl-(aq)

The number of replaceable or ionizable hydrogen atoms present in molecule of an acid is called 'Basicity" of the acid. Sulphuric acid (H2SO4) contains two acidic hydrogen atoms per molecule. It can neutralize two molecules of hydroxide (OH) ions. Consequently sulphuric acid (H2SO4) is called dibasic acid (Diprotic acid). Sulphuric acid (H2SO4) dissociates in two steps:

H2SO4 (aq) + H2O (l) < ----- > H3O+ (aq) + HSO-4(aq)

HSO-4 (aq) + H2O (l) < ----- > H3O+ (aq) + SO-24(aq)

The acid like phosphoric acid (H3PO4), which contains three acidic hydrogen atoms per molecule can neutralize three molecules of (OH-)ions, so phosphoric acid (H3PO4) is called tri basic acid or (Tri-protic acid).

H3PO4 (aq) + H2O (l) < ----- > H3O+ (aq) + H2PO-14(aq)

H2PO-14(aq) + H2O (l) < ----- > H3O+ (aq) + HPO-24 (aq)

H2PO-24(aq) + H2O (l) < ----- > H3O+ (aq) + PO-34(aq)

 

Acids that contain two, three or more acidic hydrogen per molecule are called poly-basic acids, or more commonly poly-protic acids.

9.2.4 Mono-Acid and Poly-Acid Bases (Acidity of Bases):

Similarly, bases that produce 1 mole of (OH") ions per mole of base (such as NaOH and KOH) are called mono-acid bases. Bases that produce 2 moles of (OH) ions per one mole of base (such as Ca(OH)2 and Ba(OH)2 ) are called tri-acid bases, and bases that produce 3 moles of (OH-) ions per 1 mole of base (such as Al (OH)3 and Cr (OH)3 are called tri-acid bases. The number of ionizable or replaceable (-OH-) ions, present in a molecule of base is called acidity of the base. Bases that contain two three or more hydroxide (OH-) ions per molecule are called poly-acid bases.

 
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