Important Questions and Answers: Ionic equilibrium

1. What do you mean by equilibrium state of reversible reaction?

Ans:  The reaction in which the rate of forward reaction is proceeds in equal speed to the rate of backward reaction is called equilibrium state of reversible reaction.

i.e., At equilibrium,

Rate of forward reaction (rf) = Rate of backward reaction (rb)

In equilibrium state, the concentration of reaction does not change with time.

2. All Arrhenius acids are also Bronsted acids but all Arrhenius bases are not Bronsted bases. Explain.

Ans: According to Arrhenius concept, an acid is a substance which gives hydrogen ion (H+) when dissolved in water. Similarly, according to Bronsted concept, the substance which gives proton is called acid .ie., H+ ion. Therefore, Arrhenius acids are also Bronsted acids.

According to Arrhenius concept,  a base is a substance that gives OH ion when dissolved in water. While according to Bronsted concept, a base is a substance that accept proton but may or may not contain OH ion. Example: KOH is base according to Arrhenius concept but it cannot be considered base as it cannot accept proton.

So, all Arrhenius acids are also Bronsted acids but all Arrhenius bases are not Bronsted bases.

3. Why is aqueous Na2CO3 basic?

Ans:  Na2CO3 when dissolved in water gives strong base i.e.,  NaOH and weak acid i.e., Na2CO3. As we know that strong base ionizes completely, the concentration of OH ion becomes much greater than H+ ion. So, the aqueous solution of Na2CO3 is basic.  The reactions involved in this process are given below:

$$\ce{Na2CO3 + 2H2O-> 2NaOH + H2CO3}$$

$$\ce{2NaOH-> 2Na^+ + 2OH^-}$$

$$\ce{H2CO3->2H^+ + CO3^–}$$

4. Why is aqueous solution of FeCl3 acidic?

Ans: FeCl3 when dissolved in water gives strong acid i.e.,  HCl and weak base i.e., Fe(OH)3. As we know that strong acid ionizes completely, the concentration of H+ ion becomes much greater than OH ion. So, the aqueous solution of FeCl3 is acidic.  The reactions involved in this process are given below:

$$\ce{FeCl3 + 3H2O-> Fe(OH)3 + 3HCl}$$

$$\ce{Fe(OH)3-> Fe^+++ + 3OH^-}$$

$$\ce{3HCl->3H^+ + 3Cl^-}$$

5. Water is Lewis base as well as Bronsted acid. Why?

Ans: As water contains two lone pairs of electrons. So, it can donate a pair of electron and act as Lewis base. Also, water gives H+ ions to Bronsted base. So, it acts as Bronsted acid. Hence, water can be called as Lewis base as well as Bronsted acid.

6. Write the conjugate acid and conjugate base of NH3.

Ans: We know that Bronsted acid + H+ → Conjugate acid of the base

NH3 + H+ NH4

So, conjugate acid of NH3 is NH4+.

Also, Bronsted acid → Conjugate base of the acid + H+

NH3 → NH2+ + H2

So, conjugate base of NH3 is NH2.

 7. Explain why sodium chloride precipitates from a saturated salt solution when hydrogen chloride is passed through the solution.

Ans: NaCl ionizes as:

NaCl → Na+ + Cl

HCl ionizes as:

HCl → H+ + Cl

When HCl gas is passed into saturated solution of sodium chloride, the concentration of common Cl ion is increased. So, product of [Na+] and [Cl+]  ion becomes greater than solubility product of NaCl and gets precipitation occurs.

 8. What is Ostwald’s dilution law? What are its limitation?

Ans: It states that “The extent of ionization of a weak electrolyte increases with increase in dilution”.

If an electrolyte having degree of ionization α  is dissolved in v litre of solution, then the degree od ionization can be calculated as:

$$\{alpha}$  = \frac{\{Number of moles ionized }} {[\total number of moles}]}$$

Limitations of Ostwald’s dilution law:

– This law is applicable only for weak electrolyte.

– This law doesn’t hold true for concentrated solution.

 9. What is common ion effect ?

Ans: When a strong electrolyte is a added to the solution of a weak electrolyte having common ion, the ionization of weak electrolyte is highly suppressed. This phenomenon is known as common ion effect. The ionization of AgCl is decreased when HCl gas is passed through it.

10. Explain the relation between pH and pOH.

Ans: Relation between pH or pOH.

It is experimentally found that the ionic product of water (Kw) is given by:

Kw = [H+][OH] = 10-14 at 25<sup>o</sup>

Taking log on both sides,

log{[H+][OH]} = log[10-14]

or, log[H+] + log [OH] = -14

Multiplying both sides by minus sign,

– log[H+] – log [OH] = 14

pH + pOH = 14

This is the required relation.

11. Differentiate between solubility product and ionic product.

  Solubility product     Ionic product
Solubility product of a sparingly soluble electrolyte is defined as the product of its ions in its saturated condition at 25oC. When the temperature of solubility is greater or less than 25oC, then the product of its ion is called ionic product.
It has constant value for an electrolyte at constant temperature. It’s value varies with the change in concentration.

12. All Bronsted bases are also Lewis base but all Lewis bases are not Bronsted bases. Explain.

Ans: Lewis bases generally contain one or more lone pair of electrons and therefore they can also accept proton. Thus, all Lewis bases are also Bronsted bases. On the other hand, Bronsted acids are those which can give proton. But HCl, H2SO4 cannot accept pair of electron. So, all Bronsted acids are not Lewis acids

13. What is buffer solution? Mention its types.

Ans: The solution which can resist the change in pH on addition of small amount of strong acid or strong base is called buffer solution. Two types of buffer solution are:

  1. Acidic buffer
  2. Basic buffer

 

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