Chapter 7.14 - Ionization of Acids and Bases

In the previous section, we saw Lewis acids and bases. In this section, we will see ionization of acids and bases

• We saw three theories about acids and bases:
   ♦ The Arrhenius theory
   ♦ The Brönsted-Lowry theory
   ♦ The Lewis theory
• The first two theories are simple and can be applied in most practical situations
• This is because, in most practical situations, acids and bases will be reacting with water
• We have seen that, both Arrhenius and Brönsted-Lowry theory help us to write accurate chemical equations of such reactions with water

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• Now we will see how the two theories help us to define strong acids and bases
• Let us first consider the Arrhenius theory. It can be written in 2 steps:
1. Acids like HCl, HNO₃ etc., dissociate almost completely when added to water
    ♦ HCl(aq) + H₂O(l) ⇌ H₃O⁺(aq) + Cl^-(aq)
    ♦ HNO₃ + H₂O ⇌ H₃O⁺(aq) + NO₃^-(aq)
(Recall that, the H⁺ ions combine with H₂O molecules to form H₃O⁺ ions)
• Since they dissociate almost completely, we get a large concentration of H⁺ ions
◼  So according to Arrhenius theory, such acids are strong acids
2. Similar is the case with bases like LiOH and NaOH. They dissociate almost completely
• Since they dissociate almost completely, we get a large concentration of OH^- ions
◼  So according to Arrhenius theory, such bases are strong bases

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Next, we will see how the Brönsted-Lowry theory helps us to define strong acids and bases. It can be written in 7 steps:
1. Acids like HCl, HNO₃ etc., dissociate almost completely when added to water
• Since they dissociate almost completely, we get a large concentration of H+ ions
• This is same as:
   ♦ They donate a large number of protons
   ♦ In other words, such acids are good proton donors
◼  So according to Brönsted-Lowry theory, such acids are strong acids
2. Similar is the case with bases like LiOH and NaOH. They dissociate almost completely
• Since they dissociate almost completely, we get a large concentration of OH^- ions
• OH^- ions accept H⁺ to become water
   ♦ If more OH^- ions are available, more H⁺ can be accepted
   ♦ This is same as:
         ✰ These bases can accept a large number of protons
• In other words, such bases are good proton acceptors
◼  So according to Brönsted-Lowry theory, such bases are strong bases

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Let us see how the Brönsted-Lowry theory can be used to analyze ionic equilibrium. It can be written in 6 steps
1. We have seen that, ionization reaction is a reversible reaction
    ♦ Both forward and backward reactions are taking place
    ♦ With the passage of time, the reaction will be moving towards equilibrium
2. So a question arises:
• At equilibrium, which of the following two conditions will be obtained?
    ♦ A condition in which there is a greater concentration of products
    ♦ A condition in which there is a greater concentration of reactants
3. We can make this question clearer using an example. It can be written in steps:
(i) Consider the reaction:
HX(aq) + H₂O(l) ⇌ H₃O⁺(aq) + X^-(aq)
(ii) At equilibrium which of the following two conditions will be obtained?
    ♦ A condition in which there is a greater concentration of H₃O⁺ and X^-
    ♦ A condition in which there is a greater concentration of HX and H₂O
4. At equilibrium,
• If there is a greater concentration of H₃O⁺ and X^-, We can say:
    ♦ Forward reaction is favored
    ♦ HX undergoes almost complete dissociation
• If there is a greater concentration of HX and H₂O, We can say:
    ♦ Backward reaction is favored
    ♦ HX undergoes very low dissociation
5. Let us write a summary. It can be written in two steps:
(i) If the forward reaction is favored,
    ♦ At equilibrium, there will be a large concentration of H₃O⁺ and X^-
    ♦ There will be only very low concentration of HX
    ♦ Then HX can be considered as a strong acid
(ii) If the backward reaction is favored,
    ♦ At equilibrium, there will be a large concentration of HX and H₂O
    ♦ There will be only very low concentration of H₃O⁺ and X^-
    ♦ Then HX can be considered as a weak acid
6. Recall that, Brönsted-Lowry theory gives us information about conjugate acids and bases also. Let us apply it to our present case. It can be written in 4 steps:
(i) We know how to identify conjugate acids and bases in a reaction
• For our present case, they can be written along with the chemical equation as shown below:

    ♦ So the [acid, conjugate base] pair is: [HX,X^-]
    ♦ Also, the [base, conjugate acid] pair is: [H₂O,H₃O⁺]
(ii) So we have two pairs:
    ♦ [acid, conjugate base], which is: [HX,X^-]
    ♦ [base, conjugate acid], which is: [H₂O,H₃O⁺]
• We have to analyze each pair when the forward reaction is favored
• We have to analyze each pair when the backward reaction is favored
(iii) First we will analyze the pairs when the forward reaction is favored:
◼ Consider the first pair [acid, conjugate base], which is: [HX,X^-]
    ♦ HX will dissociate completely
    ♦ The backward reaction will be slow
        ✰ X^- will be reluctant to accept protons
        ✰ That means X- is a weak base
• Thus in the pair [HX,X^-]:
    ♦ The acid HX is strong
    ♦ The conjugate base X^- is weak
◼ Consider the second pair [base, conjugate acid], which is: [H₂O,H₃O⁺]
    ♦ H₂O will dissociate completely
    ♦ The backward reaction will be slow
        ✰ H₃O⁺ will be reluctant to donate protons to X^-
        ✰ That means H₃O⁺ is a weak acid
• Thus in the pair [H₂O,H₃O⁺]:
    ♦ The base H₂O is strong
    ♦ The conjugate acid H₃O⁺ is weak
The two cases are shown in the fig.7.18 below:

Fig.7.18

(iv) Next we will analyze the pairs when the backward reaction is favored:
◼ Consider the first pair [acid, conjugate base], which is: [HX,X^-]
    ♦ HX will be reluctant to dissociate
    ♦ The backward reaction will be fast
        ✰ X^- will readily accept protons from H₃O⁺
        ✰ That means X^- is a strong base
• Thus in the pair [HX,X^-]:
    ♦ The acid HX is weak
    ♦ The conjugate base X^- is strong
◼ Consider the second pair [base, conjugate acid], which is: [H₂O,H₃O⁺]
    ♦ H₂O will be reluctant to dissociate
    ♦ The backward reaction will be fast
        ✰ H₃O⁺ will readily donate protons to X^-
        ✰ That means H₃O⁺ is a strong acid
• Thus in the pair [H₂O,H₃O⁺]:
    ♦ The base H₂O is weak
    ♦ The conjugate acid H₃O⁺ is strong
The two cases are shown in the fig.7.19 below:

Fig.7.19

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In the next section, we will see ionization constant of water and the pH scale


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