Module 8

Lesson 4.1  Monoprotic pH Curves



Key Concepts


Monoprotic pH curves are the result of a monoprotic acid reacting with a monoprotic base. In a monoprotic acid-base reaction, the acid donates one proton and the base accepts one proton.

 Monoprotic acid - acid that can donate only  one  proton
 Monoprotic base - base that can accept only  one  proton


  1. Strong Acid - Strong Base


    First, consider the characteristic curve that results when a strong base is titrated with a strong acid.


    \( \mathrm { NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) } \)


    The pH of the base solution is measured at the start of the titration - before any acid is added. Then, HCl is added slowly to NaOH until the titration is complete. At regular intervals, a pH meter is used to measure the pH changes that occur throughout the course of the experiment.



Note how slowly the pH drops until near the equivalence point. At the equivalence point, there is a very rapid change in pH, as evidenced by the steep slope.


At the start of the titration, the sample being titrated has a very high pH. By the end of the titration, the pH of the sample has dropped dramatically due to the excess amount of HCl that has been added. The midpoint of the steep slope indicates the equivalence point. At the equivalence point, chemically equivalent amounts of reactants have combined.


Next, consider the characteristic curve that results when a strong acid is titrated with a strong base. In this case, the sample being titrated has a very low pH. By the end of the titration, the pH of the sample has increased dramatically due to the excess amount of NaOH that has been added.




In general, strong acid-strong base reactions are quantitative. The pH at the equivalence point is 7.


  1.  
  2. Strong Acid - Weak Base


    Consider what happens when a weak base (NH3) reacts quantitatively with a strong acid (HCl). To write the reaction, use the following steps.


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

      \( \mathrm { H_3O^+(aq), Cl^-(aq), NH_3(aq), H_2O(l) } \)


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

      \( \mathrm { H_3O^+(aq) } \)  A
      \( \mathrm { Cl^-(aq) } \)
      \( \mathrm { NH_3(aq) } \)  B
      \( \mathrm { H_2O(l) } \)  A or B 


    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.

      \( \mathrm { H_3O^+(aq) } \)   SA
      \( \mathrm { NH_3(aq) } \)   SB


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


      \( \mathrm { H_3O^+(aq) + NH_3(aq) \rightarrow H_2O(l) + NH_4^+(aq) } \)

    5. Because this reaction is quantitative, the only acid or base present at the equivalence point (besides water) is NH4+(aq). Because the ammonium ion is a weak acid, the pH at the equivalence point will be less than 7. In general, for quantitative strong acid-weak base reactions, the pH at the equivalence point is less than 7.




      Notice that the pH drops quickly at the start of the titration, but then levels off prior to reaching the equivalence point. This is because a buffering effect is occurring as NH4+ is produced. You will learn more about buffers in the next lesson.


      Because a weak base is being titrated, the starting pH of the sample is lower than the pH of NaOH (strong base).

  1.  
  2.  
  3. Weak Acid - Strong Base


    Next, consider what happens when a weak acid (CH3COOH) reacts quantitatively with a strong base (NaOH). Again, to write the reaction, use the following steps:


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


      \( \mathrm { CH_3COOH (aq), Na^+ (aq), OH^- (aq), H_2O(l) } \)


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

      \( \mathrm { CH_3COOH (aq) } \)   A
      \( \mathrm { Na^+ (aq) } \)
      \( \mathrm { OH^- (aq) } \)   B
      \( \mathrm { H_2O(l) } \)   A or B


    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.

      \( \mathrm { CH_3COOH(aq) }\)   SA
      \( \mathrm { OH^-(aq) } \)   SB


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


      \( \mathrm { CH_3COOH (aq) + OH^- \rightarrow H_2O(l) + CH_3COO^-(aq) } \)


    5. The reaction is quantitative. At the equivalenc e point, the only acid or base present (besides water) is CH3COO-(aq). Because the acetate ion is a weak base, the pH at the equivalence point is greater than 7. In general, for quantitative weak acid-strong base reactions, the pH at the equivalence point is greater than 7.


       

      Notice that the pH rises quickly at the start of the titration, but then levels off prior to reaching the equivalence point. This is because a buffering effect occurs as CH3COO- is produced. You will learn more about buffers in the next lesson.


      Because a weak acid is being titrated, the starting pH of the sample is higher than the pH of HCl (strong acid).

Learning Tip


Steep areas of a pH curve indicate a quantitative reaction. The midpoint of the steep slope indicates the equivalence point.


Watch


 Review pages 334 to 335 and 760 to 762 in your textbook.

Check Your Understanding


Complete Practice Question 8 on page 759 and Practice Question 13 on page 763 of the textbook. Click the link below to check your answers.

Page 759 Practice Question 8

  1. around pH 9
  2. phenolphthalein (pink endpoint), thymol blue (blue endpoint)
  3. CH3COOH(aq) + OH-(aq) → CH3COO-aq) + H2O(l)
  4. The addition of titrant converts a small quantity of acetic acid into acetate ion, which has basic properties. As a result, there is a jump in pH. As titrant continues to be added, there is only a gradual change in the removal of acetic acid particles and appearance of acetate ions; therefore, the pH change is less dramatic.


Page 763 Practice Question 13

Answers in back of textbook - page 789