Acids and bases questions and answers
To solve the acids and bases questions and answer first we study acids and bases properties and concepts.
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 acids 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.
The acidity of oxoacids of phosphorus
The oxidation number of phosphorus in the oxoacids of phosphorus
But with the increasing oxidation number acidic character also increases. Thus the acidic character of these acids should be in the order
H3PO4 < H3PO3 < H3PO2
Experimental observation suggested the reverse order
H3PO2 ≥ H3PO3〉H3PO4
The experimental order explained when we consider the structures of this phosphorus acids
The structure of H3PO2 suggested that it is a monobasic acid. Because the proton attached to oxygen has a far greater chance of dissociation than any directly bonded hydrogen.
- Thus in H3PO2 involves one protonated oxygen and another unprotonated oxygen.
- But in H3PO3 has two protonated oxygens and one unprotonated oxygen thus it a dibasic acid.
- H3PO4 is tribasic, has three protonated oxygens and one unprotonated oxygen.
In this series, the number of unprotonated oxygen, which is responsible 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
Difference between bases and alkalis
Alkalis are the substances dissolved in water to produce bases but all the bases are not dissolved in water. Thus all the alkalies are bases but all the bases are not alkalies.
Na2O is an alkali and a base because it dissolved in water to produce NaOH.
Na2O + H2O ⇆ 2NaOH
NaOH, KOH and Ca(OH)2 dissolved in water to produce OH– ion. Thus these hydroxides have properties of alkalies and bases.
NaOH ⇆ Na+ + OH–
KOH ⇆ K+ + OH–
Ca(OH)2 ⇆ Ca+2 + 2OH–
But Al(OH)3, Fe(OH)3, Zn(OH)2, etc does not dissolve in water but reacts with acids to produce salt and water. Thus these are bases but not alkalis.
Acidic and basic oxides periodic table
Acidic oxides react with water to give respective oxoacids. Thus 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.
Electronegativity increases left to right in the period of the periodic table, thus generally acidic character also increases left to right in the periodic table.
But electronegativity decreases top to bottom in the group of the periodic table, thus generally acidic character also decreases top to bottom in the periodic table.
Among Na2O, MgO, As2O3, and N2O5, nitrogen has the highest oxidation number and electronegativity. Thus the order of acidic nature of these oxides
How does the hydrogen ion concentration relate to pH?
Let the pH of a solution = 4.5 and the concentration of H+ ion = CH⁺.
From the definition of pH
∴ CH⁺ = 10-4.5 = 3.16 × 10-5
When hydrogen and hydroxyl ion concentration and pH of a solution prepared by diluting 20 ml of 0.1M HCl.
CH⁺ = (20 × 0.1)/1000
= 0.002 = 2 × 10-3
COH⁻ = (1 × 10-14)/CH⁺
= (1 × 10⁻¹⁴)/(2 × 10-3)
pH = – log(CH⁺) = – log (2 × 10-3)
∴ pH = 2.7
Define conjugate acid base pair
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.
In the above reaction, HX– acts as a base and its conjugate acid is H2X.
But 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.