Module 8

Lesson 2.4  Five-Step Method for Predicting Acid-Base Reactions



Key Concepts


Reaction systems often contain several different acids and bases. Theoretically, then, there are many possible acid-base reactions that may occur to a greater or lesser extent. However, the evidence indicates that one reaction predominates over the others. The predominant acid-base reaction in a reaction system is the one that involves a proton transfer from the strongest acid to the strongest base. In this lesson, you will learn how to predict predominant acid-base reactions using the five-step method summarized on page 731 of your textbook.

  1. List all entities initially present as they exist in aqueous solution.


    The following chart summarizes the way in which various substances should be considered when predicting acid-base reactions in aqueous solutions.


     Listing for Substance

    		 Reason
      All 6 strong acids are written as H3O+(aq).
     Strong acids ionize 100% in aqueous solution.  Therefore, there are no un-ionized acid particles.

     Strong bases (hydroxides) are listed as  OH-(aq).
     Ionic hydroxide compounds will dissociate 100% to  produce OH-(aq).


     Ionic compounds are listed as  dissociated ions.

    		 Ionic compounds dissociate to form ions in water.
     
     Molecular substances are written in    their molecular form. This includes  weak acids and weak bases.

    		 The extent of reaction with water is uncertain;  therefore, substances are listed in a un-ionized  form.
     Metallic ions may be omitted from  consideration as possible reactants.  
     Metallic ions do not have a proton to donate. Nor do  they demonstrate the ability to accept a proton  during a chemical reaction.


  2. Using the Bronsted-Lowry definition, identify and label all possible aqueous acids and bases.


  3. Identify the strongest acid (SA) and the strongest base (SB) present, using the table of Relative Strengths of Aqueous Acids and Bases. These will be the reactants.


  4. Write an equation showing a transfer of one proton from the strongest acid to the strongest base, and predict the products. The products will be the conjugates located beside the SA and SB on the table of Relative Strength of Acids and Bases.


  5. Predict the approximate position of the equilibrium. The relative position of the strongest acid and the strongest base on an acid-base table can be used to determine the equilibrium position.


    Reactants / Position of Reactants Description of Equilibrium Position Symbol Used
    Hydronium and hydroxide
     quantitative

    		→
    Acid above base  Products favoured (greater than 50%)

    Base above acid  Reactants favoured
     (less than 50%)


 Read pages 728-731 in your textbook.



Watch

 


Check Your Understanding

Complete Practice Questions 9, 10, 12, 14 and  15 on page 731 of the textbook. Check your answers by clicking the link below.

Page 731 Practice Question 9

Step 1: \( \mathrm { HF(aq), Na^+(aq), SO_4^{2-}, H_2O(l) } \)
Step 2:
\( \mathrm { \overset{A}{HF(aq)},~Na^+(aq),~\underset{B}{SO_4^{2-}(aq)},~\overset{A}{\underset{B}{H_2O(l)}} } \)
Step 3:


Strongest acid: \( \mathrm { HF(aq) } \)

Strongest base: \( \mathrm { SO_4^{2-}(aq) } \)
Step 4: \( \mathrm { HF(aq) + SO_4^{2-}(aq) \leftrightharpoons F^-(aq) + HSO_4^-(aq) } \)
Step 5: \( \mathrm { HF(aq) + SO_4^{2-}(aq) \overset{<50\%}{\leftrightharpoons} F^-(aq) + HSO_4^-(aq) } \)

The predominant reaction is equilibrium favouring the reactants.


Page 731 Practice Question 10

Step 1: \( \mathrm { H_3O^+(aq), ~ClO_4^-aq),~ Na^+(aq),~ OH^-aq), H_2O(l) } \)
Step 2:

Step 3:


Strongest acid: \( \mathrm { H_3O^+(aq) } \)

Strongest base: \( \mathrm { OH^-aq } \)
Step 4: \( \mathrm { H_3O^+(aq) + OH^-(aq) \leftrightharpoons H_2O(l) + H_2O(l) } \)
Step 5: \( \mathrm { H_3O^+(aq) + OH^-(aq) \rightarrow H_2O(l) + H_2O(l) } \)

Question specifies that the predominant reaction is a quantitative reaction.


Page 731 Practice Question 12

Step 1: \( \mathrm { NH_4^{+}(aq), ~Cl^-(aq),~ Na^+(aq), ~NO_2^{-}aq),~ H_2O(l) } \)
Step 2:

Step 3:


Strongest acid: \( \mathrm { NH_4^+(aq) } \)

Strongest base: \( \mathrm { NO_2^-(aq) } \)
Step 4: \( \mathrm { NH_4^{+}(aq) + NO_2^{-}(aq) \leftrightharpoons NH_3(aq) + HNO_2(aq) } \)
Step 5: \( \mathrm { NH_4^{+}(aq) + NO_2^{-}(aq) \overset{<50\%}{\leftrightharpoons} NH_3(aq) + HNO_2(aq) } \)

The predominant reaction is equilibrium favouring the reactants.



Page 731 Practice Question 14

Step 1:

\( \mathrm { Na^+(aq),~ HSO_4^-(aq),~ Na^+(aq),~ OH^-(aq),~ H_2O(l) } \)

Step 2:


\( \mathrm { Na^+(aq),~ \overset{A}{\underset{B}{HS}}O_4^-(aq),~ Na^+(aq),~ \underset{B}{OH}^-(aq),~ \overset{A}{\underset{B}{H_2O}}(l) } \)

Step 3:


Strongest acid: \( \mathrm { HSO_4^-(aq) } \)

Strongest base: \( \mathrm { OH^-(aq) } \)
Step 4: \( \mathrm { HSO_4^{-}(aq) + OH^-(aq) \leftrightharpoons SO_4^{2-}(aq) + H_2O(l) } \)
Step 5: \( \mathrm { HSO_4^{-}(aq) + OH^-(aq) \leftrightharpoons SO_4^{2-}(aq) + H_2O(l) } \)  equilibrium favours products



Page 731 Practice Question 15

Step 1,2:

Step 3:


Strongest acid: hydronium

Strongest base: water

Step 4: \( \mathrm { H_3O^+(aq) + H_2O(l) \leftrightharpoons H_2O(l) + H_3O^+(aq) } \)
Step 5: \( \mathrm { H_3O^+(aq) + H_2O(l) \leftrightharpoons H_2O(l) + H_3O^+(aq) } \)

The predominant reaction is no change; reactants and products are identical. Therefore, ammonium nitrate fertilizer added to water cannot be used to neutralize HCl(aq).