Acids and bases questions

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Properties of acids and bases

We have already considered the thermodynamic aspects of the release of a proton by a molecule in the gas phase. The heat of the ionization of a proton is actually the proton affinity of the corresponding anion. Such proton affinity data are now available with a fair degree of accuracy to enable a good comparison of relative acid-strengths in the gas phase.

Acids and bases posses in the sense of some opposite properties, for example, acids turn blue litmus red and bases turn red litmus blue.

Neutralization of acids and bases

Question
What is the acidic order of oxoacids of phosphorus?

Answer

Oxidation state rule applied to the oxoacids of phosphorus (H₃PO₂, H₃PO₃, and H₃PO₄) it is predicted that the acidic character of these acids should be in the order

H₃PO₂ㄑH₃PO₃ㄑH₃PO₄
Experimental observation suggested the reverse order
H₃PO₂ ≥ H₃PO₃〉H₃PO₄

The experimental order can be explained when we consider the structures of this phosphorus acids
Study structure of Hypophosphorous, phosphorus, and phosphoric acid
Hypophosphorous, phosphorus, and phosphoric acid
H₃PO₂ is a monobasic acid. The proton attached to oxygen has a far greater chance of dissociation than any directly bonded hydrogen.

The structure of H₃PO₂ involves one protonated oxygen and another unprotonated oxygen. H₃PO₃ is dibasic and hence has two protonated oxygens and one unprotonated oxygen. H₃PO₄ is tribasic, has three protonated oxygens and one unprotonated oxygen.

In this series, therefore, the number of unprotonated oxygen, which are the vehicles for the enhancement of acidity, is the same for all the three acids. But dissociable protons increase from one in H₃PO₂ to three in H₃PO₄.

Therefore the overall inductive effect of the unprotonated oxygen decreases from H₃PO₂ to H₃PO₄. Hence the acidity slightly falls off in the order

H₃PO₂ ≥ H₃PO₃〉H₃PO₄

Acid-base neutralization

Question
What will be the effect of adding KNH₂ to liquid ammonia with respect to acidity?

Answer
2NH₃ NH₄⁺ + NH₂⁻

KNH₂ K⁺ + NH₂⁻
Due to common ion affects the equilibrium shift to left and decrees the acidity of NH₃.

Question
Why all alkali are based but all bases are not alkali?

Answer
All the bases are dissolved in water to produce alkali whereas all the bases are not dissolved in water but all the alkali are dissolved in water. Thus all alkali are based but all bases are not alkali. Na₂O is a base because it dissociates in water to produce NaOH.
NaOH, KOH and Ca(OH)₂ dissolved in water to produce OH⁻ ion. Thus all these hydroxides are alkali.
NaOH ⇆ Na⁺ + OH⁻

KOH ⇆ K⁺ + OH⁻

Ca(OH)₂ ⇆ Ca⁺² + 2OH⁻
Al(OH)₃, Fe(OH)₃, Zn(OH)₂, etc does not dissolve in water but reacts with acids to produce salt and water. These are bases but not alkali.

Question
Arrange these oxides in order of their acidic nature: N₂O₅, As₂O₃, Na₂O, MgO.

Answer
Acidic oxides react with water to give oxoacids. The higher the oxidation number and the higher the electronegativity the greater the central element will force to reaction with water to give the oxoacid.
Of all these oxoacids nitrogen has the highest oxidation number and electronegativity. Thus the order of acidic nature and oxidation number are
Na₂O MgO As₂O₃ N₂O₅
+1 +2 +3 +5
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What is ph in chemistry?

Question
The pH of a solution is 4.5. Calculate the concentration of H⁺ ion.
Answer
We have from the definition,
pH = - log[H⁺] = 4.5
or, log[H⁺] = - 4.5
∴ [H⁺] = 3.16 × 10⁻⁵

Question
Calculate the [H⁺], [OH⁻] and pH of a solution prepared by diluting 20 ml of 0.1M HCl to one lit.
Answer
[H⁺] = (20×0.1)/1000
= 0.002 = 2×10⁻³

[OH⁻] = (1×10⁻¹⁴)/[H⁺]
= (1×10⁻¹⁴)/(2×10⁻³) = 0.5×10⁻¹¹

pH = -log[H⁺]
= -log2×10⁻³
= 3-log2
= 2.7
Question
Calculate the [H⁺], [OH⁻] and pH  scale of a solution obtained by dissolving 28 gm KOH to make 200 ml of a solution.

Answer
[OH⁻] = (28×1000)/(56×200) = 2.5 M
(Molecular Weight of KOH = 56gm)

[H⁺] = (1×10⁻¹⁴)/[OH⁻] = (1×10⁻¹⁴)/(2.5)
= 4×10⁻¹⁵pH = -log[H⁺]
= - log4 × 10⁻¹⁵
= (15 - log4)

Conjugate acid base pairs

Question
Name the conjugate acid-base pairs of HX⁻¹ and X⁻².

Answer
Conjugate acid of a species is the one that is obtained on the addition of a proton and the conjugate base of a species is one that is obtained on the release of a proton.
H₂O + H₂X HX⁻¹ + H₃O⁺
Acid1
Base2 Base1 Acid2
In the above reaction, HX⁻ acts as a base and its conjugate acid is H₂X.
HX⁻ + H₂O X⁻² + H₃O⁺
Acid1
Base2 Base1 Acid2
In this acid-base neutralization, reaction HX⁻ is an acid thus its conjugated base is X⁻². In the same way, the conjugate acid of X⁻² is HX⁻ but X⁻² cannot have any conjugated base because there is no proton that can release.

Question
Bisulphate ion can be viewed both acid-base, explain.

Answer
Bisulphate ion is HSO₃⁻. It may lose a proton to give the conjugate base SO3⁻², is an acid. Again it may add on a proton to give the conjugate acid, thus showing its base character.

Lewis definition of acids and bases

Question
Arrange in order of Lewis acidity : (i) BF₃ (ii) BCl₃ (iii) BI₃ (iv) BBr₃

Answer
These boron halides have pi-interaction between filled p- orbitals of the halogen and empty p-orbital of boron. The effectiveness of the pi-interaction falls off with the increasing size of halogens so that it is the strongest in BI₃.

When boron halides receive an electron pair the pi-bond between boron and halogen has to be ruptured in order to make room for a coordinate bond.

H₃N→BF₃
Thus with BF₃, it will be hardest to rupture the pi-bond. Therefore the order of the Lewis acidity is
BF₃ㄑBCl₃ㄑBBr₃ㄑBI₃

Question
Explain - NH₃ behaves as a base but BF₃ is acid.


Answer
In NH₃, the central N atom has lone pair of electrons This lone pair coordinate to empty orbital, is termed as a base according to Lewis.

The compounds with less than an octet for the central atom are Lewis acids. B in BF₃ contains six electrons in the central atom, and it is an acid.

F₃B + :NH₃ ⇆ F₃B ← :NH₃
Question
Explain why tri-covalent phosphorus compounds can serve both as Lewis acids and also as bases?

Answer
Tri-covalent phosphorus compounds like PCl₃ have a lone pair of electrons in phosphorus. This lone pair may coordinate with a metal ion thus allowing the compound to serve as a Lewis base.

Ni + 4PCl₃ ⇆ [Ni(PCl₃)₄]⁰
Again the quantum energy level of phosphorus has provision for d-orbitals which can receive back donated electrons from electron reach low oxidation state of a metal ion. In this latter case, the tri-covalent phosphorus compound serves as a Lewis acid.

Question
Bi-positive tin can function both as a Lewis acid and a Lewis base. Explain?

Answer
The Lewis representation of SnCl₂ shows a lone pair on tin through it is as yet short of an octet. Ligands, particularly donor solvents with lone pairs, may coordinate to tin giving complexes. Here SnCl₂ behaves as a Lewis acid.

Again interaction of platinum group metal compounds with SnCl₂ (SnCl₃⁻) as donor leads to the formation of coordinate complexes.

[(Ph₃P)₂PtCl(SnCl₃)], [RuCl₂(SnCl₃)₂]⁻²

Where SnCl₂ acts as a Lewis base.

Question
Can SiCl₄ and SnCl₄ function as Lewis acids?

Answer
Both silicon and tin are members of Group IV of the periodic table and their quantum energy levels admit of d-orbitals. As a result, they can expand their valence shell through SP³d² hybridization and can give rise to six-coordinate complexes. In fact complex are,
[SiCl₄{N(CH₃)₃}₂], [SiCl₂(Py)₄]Cl₂, [SnCl₄(Bpy)]
Thus SiCl₄ and SnCl₄ can function as Lewis acids.

Question
SO3 Behaves as acid and H2O as a base. Explain.

Answer
SO₃ like BF₃ has less than an octet and will be an acid according to Lewis. But in H₂O oxygen atom contain lone pairs to donate the Lewis acid. Oxygen and sulfur contain six electrons in their valence shell and therefore regarded as Lewis acids.

The oxidation of SO₃⁻² to SO₄⁻² ion by oxygen and S₂O₃⁻² ion by sulfur is the acid-base neutralization reaction.
SO₃⁻² + [O] ⇆ [O←S₂O₃]⁻²

SO₃⁻² + [S] ⇆ [S←S₂O₃]⁻²

Soft and hard acids and bases

Question
Explains why Hg(OH)2 dissolved readily in acidic aqueous solution but HgS does not?

Answer
In the case of Hg(OH)₂ and HgS, Hg is a soft acid and OH⁻ and S⁻² is hard to base and soft base respectively.

Evidently, HgS (soft acid + soft base) will be more stable than Hg(OH)₂ (soft acid + hard base).
More stability of HgS than that of Hg(OH)₂ explains why Hg(OH)₂ dissolved readily in acidic aqueous solution but HgS does not.

Question
Classify the following as soft and hard acids and bases. (i) H- (ii) Ni+4 (iii) I+ (iv) H+

Answer
  1. The hydride ion has a negative charge and is far too large in size compared to the hydrogen atom. Its electronegativity is quite low and it will be highly polarisable by virtue of its large size. Hence it is the soft base.
  2. The quadrivalent nickel has quite a high positive charge. Compared to the bivalent nickel its size will be much smaller. Its electronegativity will be very high and polarisability will below. Hence it is hard acid.
  3. Mono-positive iodine has a low positive charge and has a large size. It has a low electronegativity and high polarizability. Hence it is soft acids.
  4. H⁺ has the smallest size with a high positive charge density. It has no unshared pair of electrons in its valence shell. All these will give a high electronegativity and very low polarizability. Hence hydrogen ion is a hard acid.

Acids and bases questions and answers, Properties and neutralization, pH scale, and Lewis, soft, hard acids and bases for study chemistry

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