Class 11th

Boron fluoride exists as BF₃ but boron hydride doesn't exist as BH₃. Give reason. In which form does it exist? Explain its structure.

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In BF₃, due to n−pπ back bonding between the vacant p-orbital of boron and filled p-orbital of fluorine. This Π− pπ back bonding is absent in case of hydrogen as it is a single electron element.

Two BH₃ molecules dimerise to form diborane.

In B₂H₆ There are two types of hydrogens present.

(I) Four hydrogens that are terminally bonded to each of two boron atoms.

(II) Two hydrogens that are bonded to both boron atoms forming a bridge in between.

The four terminal hydrogen atoms and two boron atoms lie in the same plane while bridging hydrogen lies in a plane perpend

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BCl3 exists as monomer whereas AlCl3 is gimerised through halogen bridging. Give reason. Explain the structure of the dimer of AlCl3 also.

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Both the compounds, BCl₃ and AlCl₃ are electron deficient compounds. In BCl₃, boron is smaller in size and cannot assemble four big chlorine atoms near it causing steric hindrance and making it unstable.

Hence, BCl₃ exists as a monomer only.

In AlCl₃, aluminum has 3p-orbitals through which chlorine atoms can be accommodated easily to complete its octet and dimer is formed.

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Three pairs of compounds are given below. Identify that compound in each of the pairs which has group 13 elements in a more stable oxidation state. Give reason for your choice. State the nature of bonding also.

(i) TlCl₃,TlCl (ii) AlCl₃, AlCl (iii) InCl₃, InCl (iv) What are boranes? Give chemical equation for the preparation of diborane.

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(i) TlCl is more stable than TlCl₃, due to inert pair effect, +1 oxile. oxidation state.

(ii) AlCl3, Al³⁺ is more stable than aluminum ions in +1 state.

(iii) Due to the inert pair effect, +1 oxidation state is more stable than the +3 oxidation state. So, InCl is more stable than InCl_3.

(iv) Boranes correspond to alkane-like compounds of boron. They consist of boron and hydrogen. Most common borane existing is dibecane.

_{4BF3 + 3LiAlH4?2B2H6 + 3LiF + 3AIF3}

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When aqueous solution of borax is acidified with hydrochloric acid, a white crystalline solid is formed which is soapy to touch. Is this solid acidic or basic in nature? Explain.

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When aqueous solution of borax is acidified with hydrochloric acid, boric acid is produced. As the name says, boric acid is acidic in nature but weak acid. Unlike protonic acid, boric acid is monobasic acid. It accepts electrons from the hydroxyl group of water and forms [B (OH)₄]^-.

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3. When aqueous solution of borax is acidified with hydrochloric acid, a white crystalline solid is formed which is soapy to touch. Is this solid acidic or basic in nature? Explain.

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Assertion (A): Beryllium carbonate is kept in the atmosphere of carbon dioxide. Reason (R): Beryllium carbonate is unstable and decomposes to give beryllium oxide and carbon dioxide.

(A) Both A and R are correct and A is the correct explanation of R.

(B) Both A and R are correct but A is not the correct explanation of R.

(C) Both A and R are not correct.

(D) A is not correct but R is correct.

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This is a assertion and reason type question as classified in NCERT Exemplar

(A) Explanation:

Alkaline earth metals are stable at room temperature except beryllium carbonate. It decomposes to give beryllium oxide and carbon dioxide. It is kept in an atmosphere of carbon dioxide so that equilibrium shifts to the right. The reaction is shown below. BeCO₃→ BeO+CO₂

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In the following questions a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.

Assertion (A): The carbonate of lithium decomposes easily on heating to form lithium oxide and CO₂⋅

Reason (R): Lithium being very small in size polarises large carbonate ion leading to the formation of more stable Li₂O and CO₂.

(A) Both A and R are correct and R is the correct explanation of A.

(B) Both A and R are correct but R is not the correct explanation of A.

(C) Both A and R are not correct.

(D) A is not correct but R is correct.

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This is a assertion and reason type question as classified in NCERT Exemplar

(A) Lithium beimg small in size, polarises large carbonate ion. Polarisation is the distortion of electron cloud of the anion by the cation. Thus, the carbonate of lithium decomposes easily on heating to form lithium oxide and CO₂. The reaction is shown below.

Li₂CO₃→ Li₂O+CO₂

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Account for the following observations:

(i) AlCl₃ is a Lewis acid (ii) Though fluorine is more electronegative than chlorine yet BF₃ is a weaker Lewis acid than BCl₃

_(iii)_{PbO2 is a stronger oxidising agent than SnO2}_(iv)_{The +1 oxidation state of thallium is more stable than its +3 state}

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(i) ln? AlCl₃, Aluminium is bonded to 3 chlorine atoms through three covalent bonds and six electrons are shared in the structure. To have complete Octet, AlCl₃lacks two electrons & hence act as an electron acceptor substance or Lewis acid.

(ii) Undoubtedly, fluorine has more electronegativity than chlorine, BF₃ is stronger Lewis acid than BCl₃ because of n−p π back bonding in BF₃, both the constituting atoms boron and fluorine are involve p-orbital in back bonding. On moving down the group, the size of halogen atoms increases, back bonding decreases and Lewis ac

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10 months ago

Match the elements given in Column I with the colour they impart to the flame given in Column II.

Column I	Column II
(i) Cs	(a) Apple green
(ii) Na	(b) Violet
(iii) K	(c) Brick red
(iv) Ca	(d) Yellow
(v) Sr	(e) Crimson red
(vi) Ba	(f) Blue

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This is a matching type question as classified in NCERT Exemplar

(i) → (f); (ii) → (d); (iii) → (b) : (iv) → (c) ; (v) → (e); (vi) → (a)

All alkali metal and alkaline earth metals except beryllium and magnesium gives characteristic color when introduced into flame. Due to released energy being absorbed in the visible region, alkali metals and alkaline earth metals gives characteristic color.

(i) Caesium imparts blue color when introduced into flame.

(ii) Sodium imparts yellow color when introduced into flame.

(iii) Potassium imparts violet color when introduced into flame.

(iv) Calcium imparts brick red color when introduced

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Match the compounds given in Column I with their uses mentioned in Column II.

Column I	Column II
(i) CaCO₃	(a) Density, ornamental work
(ii) Ca(OH)₂	(b) Manufacture of sodium carbonate from caustic soda
(iii) CaO	(c) Manufacture of high quality paper
(iv) CaSO₄	(d) Used in white washing

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