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Acid Base Reaction

Chemical compounds are both, stable and reactive. Generally, the reactions in chemical compounds occur when there is more stability obtained in the process. All the compounds are formed due to the oxidation or reduction processes.

Many reactions that occur are as a result of either oxidation or reduction or both. According to the modern electronic concept, oxidation is the process of losing electron's and reduction is the process of gaining electrons. This can also be interpreted as the gain of a positive charge resulting in oxidation and the loss of a positive charge in reduction.

This means all electropositive elements get reduced when they transform from their compound metals. Similarly, all the electronegative elements will get oxidized in their compounds and come to the elemental state. With this background one can conclude that the ionic compounds are a combination of an electropositive component and an electronegative component. The simplest electropositive component in water is a proton and the negative component is a hydroxide ion. Both combine to form H-O-H or H2O. H+ is considered an acid component and OH- is considered a basic component. Together they form a neutral compound called water.

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Acid Base Reaction Definition

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Acids are substances that are sour in taste and turn blue litmus red, methyl orange indicator into red color and react with bases. They generally liberate hydrogen when treated with reactive metals. There are strong acids, weak acids, mono basic acids, di basic acids and tri basic acids. There are organic acids and inorganic or mineral acids. Strong acids have low pH, low pKa values and a high Ka value. Weak and less reactive acids will have lower values of Ka and high value of pKa. Their pH is around 5.0 - 6.5.

Bases are substances which turn red litmus blue and methyl orange to yellow. They are soap to touch and they react with acids to form salts. Water soluble bases are called alkalies. In modern definitions, acids are the proton donors or a pair of electron acceptors, while bases are those which are proton acceptors or a pair of electron acceptors.

Acid base reaction can be defined as the reaction in which when equivalent quantities of are made to react a salt and water are formed. This reaction is called neutralization reaction. Thus a salt is a product other than the solvent formed when an acid and a base react. 

Definitions


The Lewis definitions of acid and base describe an acid as an "electron pair acceptor" and a base as an "electron pair donor"

A Bronstead-Lowry acid is a molecule or ion that donates a proton and a Bronstead base is a molecule or ion that accepts a proton.

According to Arrhenius theory acids are molecules that dissociate in water to make the hydronium ion (H3O+). Bases, on the other hand, are molecules that dissociate to form hydroxide ions (OH-).

Acid Base Neutralization Reaction

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Acid base neutralization reaction is a reaction in which an acid reacts with a base to form salt and water. Polyfunctional acids and bases when react together there can be normal, acidic and basic salts that are formed. 

Normal salts are formed by the complete neutralization of acids and bases. When a multi protic acid is not completely neutralized by a base an acidic salt is formed. When a poly-functional base is incompletely neutralized by acid a basic salt is formed.

Acid Base Reaction Examples
  • Hydrochloric acid combines in equivalent proportions with sodium hydroxide which is a base and forms a neutral salt sodium chloride and water. This reaction is represented by equation:
HCl + NaOH NaCl + H2O
  • Equi molecular amounts of sulfuric acid and potassium hydroxide react with each other and forms an acidic salt potassium hydrogen sulphate. When one molecular mass of sulfuric acid and two molecular masses of potassium hydroxide are made to react a neutral salt potassium sulphate is formed. These reactions can be represented by the equations respectively.
H2SO4 + KOH KHSO4 + H2O
H2SO4 + 2KOH
K2SO4 + 2H2O

These two examples are the acid base reactions.

Bronsted Acid Base Reaction

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Removal of an electron from hydrogen atom produces H+ ion. This ion does not exist in free state and it is a proton. But it can be transferred from one substance to another. 

According to Bronsted Lowry theory a proton donor is an acid and a proton acceptor is a base. A typical Bronsted Lowry acid base reaction can be represented by the equations.

Proton Donor - HCl and NH3
Proton Acceptor - H2O
HCl + H2O H3O+ + OH-
NH3 + H2O NH4+ + OH-

Lewis Acid Base Reaction

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Lewis theory is based on the electronic configurations of elements and compounds and this theory gives an explanation to those compounds that act as acids and bases though they are not having any proton. 

According to this theory the compound which accepts a pair of electrons is an acid and the compound which donates a pair of electrons is a base. Neutralization reaction involves the formation of a coordination bond. Compound Boron tri fluoride is having a pair of electrons short in its structure which shows a willingness to accept a pair of electrons. Hence it is a Lewis acid. Ammonia molecule has a pair of electrons which it can donate in forming a co ordinate bond and hence it is a Lewis base. These two compounds react to form a salt which is shown by the equation.

H3N: + BF3 H3N->BF3

Lewis Acid Base Theory

The Lewis concept of acids and bases does not depend on the presence of a particular element as do Bronstead - Lowry and Arrhenius systems. Acidic or basic properties are instead described to the behavior of valence electrons, which all substances possess. Because the Lewis acid-base system incorporates a broader range of reactions as acid-base reactions, there is a greater range of reactivity patterns between Lewis acids and bases. 

An example is borontrifluoride ammonia ad duct, BF3.NH3. The BF3 molecule has a planar triangular structure with some double bond character in each B-F bond as fluorine is considered to be the most electronegative element.

The boron atom in BF3 is quite positive and the boron is frequently described as electron-deficient. The lone pair in the HOMO of the ammonia molecule combines with the empty LUMO of the BF3 which has very large, empty orbital lobes on boron to form the ad duct.
Arrhenius acids form hydrogen ions in aqueous solution, whereas, arrhenius bases form hydroxide ions in aqueous solution and the reaction of hydrogen ions and hydroxide ions to form water is considered to be the universal aqueous acid-base reaction. 

The ions accompanying the hydrogen and hydroxide ions form a salt, so the overall Arrhenius acid-base reaction can be written as
Acid + Base $\to $ Salt + Water

For example,

Hydrochloric acid + Sodium hydroxide $\to $ Sodium chloride + Water

H+ + Cl- + Na+ + OH- $\to $ Na+ + Cl- + H2O

This explanation works well in aqueous solution but it is inadequate for non-aqueous solutions and for gas and solid phase reactions in which H+ and OH- may not exist.
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Bronsted Lowry Theory

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The definition of acid and base is expanded to the Arrhenius list of acids and bases to include the gases HCl and NH3, along with many other compounds. This definition also introduced the concept of conjugate acids and bases, differing only in the presence or absence of a proton and described all reactions as occurring between a stronger acid and base to form a weaker acid and base.

H3O+ + NO2- $\to $ H2O + HNO2
(acid-1) (base-2) (base-1) (acid-2
)

Conjugate acid-base pair

Acid Base
H3O+ H2O
HNO2
NO2-

Species differing from each other by a proton are termed as conjugate base and acid. If the acid is strong the conjugate base is weak and vice versa.

A wide variety of Bronsted - Lowry acids and base are known. For example, cations such as NH4+

NH4+ + H2O NH3 + H3O+
(acid)


Similarly, a different type of species can act as bases in their reactions with water.

NH3 + H2O NH4+ + OH-
(base)
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