IEO Level 2- English Olympiad (SOF) Class 9 Coaching Programs
β³ π― Online Tests (2 Tests [50 Questions Each]): NTA Pattern, Analytics & Explanations
Click Here to View & Get Complete Material
Rs. 200.00
3 Year Validity (Multiple Devices)
π Study Material (303 Notes): 2024-2025 Syllabus
Click Here to View & Get Complete Material
Rs. 450.00
3 Year Validity (Multiple Devices)
π― 250 MCQs (& PYQs) with Full Explanations (2024-2025 Exam)
Click Here to View & Get Complete Material
Rs. 200.00
3 Year Validity (Multiple Devices)
Chemistry: Ionic Compounds: General Concepts of Acids and Bases, Limitations of Arrhenius Concept
General Concepts of Acids and Bases
The concepts of acids and bases are proposed by 3 concepts namely Arrhenius, Bronsted and Lowry & Lewis concepts.
Arrhenius Concept
- It is developed by Svante Arrhenius in 1884 and it most commonly used concept of acids ad bases.
- It defines acids as a substance that is capable of producing H+ ions. It is represented as follows: HA (aq) H+ (aq) + A- (aq)
Where HA = acid, A- = Acid molecule without hydrogen ion
The Arrhenius concept of HCl is represented below:
HCl (aq) H+ (aq) + Cl- (aq)
The hydrogen ion binds with water molecule and form Hydronium ion (H3O) . H+ + H2O H3O+
- Hydronium ion is also called as ox onium ion or hydroxonium ion.
- Arrhenius concept is useful to understand acid-base behavior.
Limitations of Arrhenius Concept
- It is limited to only aqueous solutions and require dissociation of solutions
- It doesnΥt explain acidic behavior of substance without hydrogen or hydroxide group. E. g. AlCl3
Bronsted and Lowry Concept
- The main idea of this concept is that acid β base reactions can be interpreted as proton-transfer reactions.
- According to this concept an acid is defined as a proton (H+ ) donor, and a base is defined as a proton acceptor.
- It is clear that hydrogen containing molecules or ions are capable of donating protons in case of acids, whereas, bases are one that accepts protons.
- E. g. 1: In a reaction between ammonia and hydrogen fluoride, ammonia acts as base while hydrogen fluorides acts as acid.
- NH3 + HF NH4+ + F-
- E. g. 2: In the reaction of HCl with NH3 in a solvent like water, the equation is given below:
- NH3 + H3O+ + Cl- H2O + NH4+ + Cl-
- In this, Hydronium ions give proton to ammonia molecule which results in water and NH4+ . So, Hydronium is proton donor or acid and ammonia is proton acceptor or base. Here, water plays important role in transferring proton from HCl to NH3.
- Bronsted β Lowry concept, acid and bases can neither be ions or molecular substances.
- In acid-base equilibrium both forward and reverse reactions involves in proton transfers.
- E. g. 1: Let us consider the reaction of NH3 with H2O. In the forward reaction, NH3 accepts a proton (as its base) from H2O (is acid) . In the reverse reaction, NH4+ donates a proton (acid) to OHβ . (base) . NH3 becomes NH4+ ion by gaining a proton, whereas NH4+ ion becomes NH3 molecule by losing a proton. The species NH4+ and NH3 are called conjugate acid-base pair.
- The acid in conjugate acid-base pair is called as conjugate acid of the base and base is called as conjugate base of acid. i.e.. NH4+ is the conjugate acid of NH3, and NH3 is the conjugate base of NH4
- The members of each conjugate pair are designated by the same numerical subscript e. g. , pair is marked as 2 while the pair is marked as 1
- E. g. 2: . When HCI is dissolved in water, the HCI molecules give up H + ions that can be accepted by any of the two bases such as Cl β and H2O. HCl + H2O H3O+ + Cl-
- Here. HCI is completely ionized in dilute aqueous solution. A stronger acid (HCl) has a weaker conjugate base, Cl β and the weaker acid, (HF) has a stronger conjugate base, F β .
- In a conjugate acid-base pair, the weaker acid stronger is its conjugate base and like versa.
- Amphiprotic species are species that can act as both acid and base.
- Bronsted β Lowry concept are more advantageous that Arrhenius concept.
Lewis Concept
- G. N. Lewis proposed the above concept in the year 1923.
- According to him, an acid is any atom, molecule or ion that can accept an electron pair from any other atom , molecule or ion.
- A Lewis base on the other hand is any atom , molecule or ion that can donate a pair of electrons.
- E. g. 1: Let us take an example AlCl3 + NH3 Cl3Al NH3
- In the above example AlCl3 is an electron deficient species. It accepts an electron pair from a molecule of NH3 which has a lone pair of electrons on N atom. Thus, AlCl3 is a Lewis acid and NH3 is a Lewis base.