Q.What is Lewis acid ? Give one example.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Lewis Acid-Base Theory
Gilbert N. Lewis's 1923 concept is the most general of the three, defined by the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. A Lewis acid is typically a positive ion or an electron-deficient molecule; a Lewis base is typically an anion or a neutral molecule with at least one lone pair.
In BF3+NH3→F3B←NH3, boron's vacant 2p orbital accepts the lone pair donated by nitrogen, forming a new coordinate covalent bond: BF3 is the Lewis acid and NH3 is the Lewis base. In Cr3++6H2O→[Cr(H2O)6]3+, each water molecule donates a lone pair to Cr3+, so Cr3+ is the Lewis acid and H2O is the Lewis base -- the same logic underlies every coordination compound, where the ligands are Lewis bases and the central metal ion is the Lewis acid.
Families of Lewis acids: electron-deficient molecules (BF3,AlCl3,BeF2), all metal ions (Fe2+,Cr3+,Cu2+), molecules with a polar double bond (SO2,CO2,SO3), molecules whose central atom can expand its octet via empty d orbitals (SiF4,SF4,FeCl3), and carbonium ions (CH3+).
Families of Lewis bases: molecules with one or more lone pairs (NH3,H2O,R−O−H), all anions (F−,CN−,SO42−), molecules with a carbon-carbon multiple bond (CH2=CH2,CH≡CH), all metal oxides (CaO,MgO,Na2O), and carbanions (CH3−). …
The Lewis theory defines acids and bases in terms of electron-pair transfer rather than proton transfer, widening which species can be classed as acids.
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Lewis acids are electron-pair acceptors; BF3 is a classic example.
G.N. Lewis extended the acid-base concept beyond proton transfer: a Lewis acid is any molecule or ion that can accept a pair of electrons to form a new coordinate (dative) covalent bond, while a Lewis base is a species that can donate an electron pair.
A Lewis acid typically has either an incomplete octet (e.g. BF3, AlCl3) or a central atom that can expand its valence shell, or is a simple cation (e.g. H+, Al3+).
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Showing the 12 most recent of 14 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following can act as Lewis acid?(a) OH-(b) F-(c) BCl3(d) H-
›Reveal solutionSolution
A Lewis acid is an electron-pair ACCEPTOR. Among the options, BCl3 is the only species that is electron-deficient and can accept a lone pair; OH-, F-, and H- are all electron-pair DONORS (Lewis bases).
Lewis acid-base theory: a Lewis acid accepts an electron pair (has an empty/low-energy orbital available), while a Lewis base donates an electron pair (has a lone pair to give).
Check each option:
- OH-: has lone pairs on oxygen to donate -- Lewis base.
- F-: has lone pairs to donate -- Lewis base. …
- CBSE 2026Set ANNUAL1 markQ.BF3 is a ................. acid.
›Reveal solutionSolution
BF3 is electron-deficient at boron, so it readily accepts a lone pair from a donor species — the hallmark of a Lewis acid.
Boron in BF3 has only 6 electrons around it (3 bond pairs, no lone pair), giving it an empty 2p orbital. According to the Lewis concept, a species that can accept an electron pair to complete its octet is a Lewis acid. BF3 readily accepts a lone …
- CBSE 2026Set ANNUAL1 markQ.Write true or false: G.N. Lewis defined base as electron pair donor.
›Reveal solutionSolution
True. In the Lewis theory, a base is defined as an electron pair donor, and an acid as an electron pair acceptor.
G.N. Lewis proposed a broader definition of acids and bases based on electron pairs rather than protons:
- Lewis acid: a species that can accept a lone pair of electrons (electron-deficient species), e.g., BF3, AlCl3, H+.
- Lewis base: a species that can donate a lone pair of electrons, e.g., NH3, H2O, OH-, halide ions. …
- CBSE 2025Set ANNUAL1 markMCQQ.Metal ion like Ag+, Cu+ etc. acts as(a) Brönsted acid(b) Brönsted base(c) Lewis acid(d) Lewis base
›Reveal solutionSolution
Metal ions such as Ag+ and Cu+ act as Lewis acids.
A Lewis acid is defined as a species that can accept a lone pair of electrons to form a coordinate (dative) bond. Metal cations like Ag+ and Cu+ have empty (vacant) orbitals and a positive charge, allowing them to accept electron pairs from Lewis bases (e.g. NH3, CN-, H2O) to form complex ions such as [Ag(NH3)2]+ or [Cu(NH3)4]2+. This makes them Lewis acids, n …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following is not a Lewis acid?(a) AlCl3(b) BF3(c) NH3(d) FeCl3
›Reveal solutionSolution
NH3 is a Lewis base (electron-pair donor), not a Lewis acid.
A Lewis acid is a species that can accept a pair of electrons (electron-deficient, often with an incomplete octet). AlCl3, BF3, and FeCl3 all have a central atom with fewer than 8 valence electrons, so they readily accept an electron pair, making them Lewis acids. NH3 …
- CBSE 2024Set ANNUAL1 markMCQQ.Species NH3 is(a) Lewis acid(b) Lewis base(c) Both (A) and (B)(d) None of these
›Reveal solutionSolution
A Lewis base is an electron-pair donor; ammonia's nitrogen carries a lone pair that it readily donates, so NH3 is a Lewis base.
By the Lewis definition:
- Lewis acid = electron-pair acceptor (typically has an empty orbital, e.g. BF3, H+).
- Lewis base = electron-pair donor (has a lone pair available to share, e.g. NH3, H2O, OH-). …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is Lewis acid?(a) OH^-(b) NH3(c) BF3(d) NH4^+
›Reveal solutionSolution
A Lewis acid is an electron-pair ACCEPTOR. BF3 has an incomplete octet on boron (only 6 electrons), so it readily accepts a lone pair from a Lewis base, e.g. forming BF3·NH3.
Lewis's definition: a Lewis acid accepts an electron pair; a Lewis base donates an electron pair.
- OH− has a lone pair to donate → Lewis base
- NH3 has a lone pair on N to donate → Lewis base
- NH4+ already has a complete octet on N with no lone pair and no readily available vacant orbital → not a typical Lewis acid …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is not a Lewis base?(a) HO-(b) BCl3(c) F-(d) H2O
›Reveal solutionSolution
A Lewis base donates an electron pair; BCl3 instead accepts one, so it is a Lewis acid, not a Lewis base.
A Lewis base is a species that can donate a lone pair of electrons.
- OH−: has lone pairs on oxygen to donate — Lewis base. …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is not a Lewis acid?(a) AlCl3.6H2O(b) AlCl3(c) SnCl4(d) FeCl3
›Reveal solutionSolution
AlCl3.6H2O is not a Lewis acid because its aluminium centre is already coordinatively saturated by water.
A Lewis acid is an electron-pair acceptor. AlCl3, SnCl4 and FeCl3 are all classic Lewis acids: the central metal atom has an incomplete octet / empty low-lying orbitals available to accept an electron pair from a Lewis base.
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- CBSE 2019Set ANNUAL1 markMCQQ.Match the columns — Column A: (A) Arrhenius. Choose the correct matching term from Column B.(i) Estimation of nitrogen(ii) Estimation of halogen(iii) V ∝ T(iv) Concept of Ionisation(v) Gain of electrons
›Reveal solutionSolution
Arrhenius is associated with the concept of ionisation — his theory defined acids as substances that release H+ ions and bases as substances that release OH− ions in aqueous solution.
Svante Arrhenius proposed that electrolytes, when dissolved in water, dissociate (ionise) into positively and negatively charged ions, and that this ionisation is responsible for their electrical conductivity and their acidic/basic …
- CBSE 2018Set ANNUAL1 markQ.(b) Why is AlCl3 considered as Lewis acid?
›Reveal solutionSolution
AlCl3's central Al has an incomplete octet, making it an electron-pair acceptor (Lewis acid).
Step 1 -- electron count on Al in AlCl3: Aluminium has 3 valence electrons, all used to form 3 single covalent bonds with the 3 chlorine atoms. This leaves aluminium with only 6 electrons in its valence shell (3 bond pairs), 2 electrons short of a complete octet.
Step 2 -- Lewis acid definition: A Lewis acid is any species that can accept a pair of electrons (an electron-pair acceptor) to form a new covalent (coordinate/dative) bond, typically because it has an empty orbital or an incomplete octet. …
- CBSE 2018Set ANNUAL1 markMCQQ.An example of Lewis acid is:(a) MgCl2(b) BCl3(c) NaCl(d) SnCl4
›Reveal solutionSolution
BCl3 is an electron-deficient molecule (boron has only 6 electrons around it), so it readily accepts a lone pair of electrons — the defining property of a Lewis acid.
A Lewis acid is any species that can accept a pair of electrons to form a new covalent (coordinate) bond. Checking the options:
- MgCl2 and NaCl are simple ionic salts; in basic classification they are not treated as Lewis acids. …
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