Lesson 12: Acid-Base Titrations

_{Corresponding Textbook Chapter - 8.7}

Lesson Instructions

- Review the learning goals and success criteria for Lesson 12: Acid-Base Titrations.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to …

- Determine the pH of the solution formed in a neutralization reaction.
- Determine the pH of the solution formed in a neutralization reaction that has not yet reached equivalence.

Success Criteria

I am able to …

- Calculate the pH at equivalence for titrations involving strong acids and strong bases.
- Calculate the pH at equivalence for titrations involving weak acids and strong bases.
- Calculate the pH at equivalence for titrations involving weak bases and strong acids.
- Calculate the pH for titrations that are not at equivalence.
- Draw or examine a titration curve and identify the equivalence point and endpoint.

Minds-On

- You were first introduced to titrations in Grade 11 Chemistry, SCH3U. In the field of chemistry, there are many commercial applications that rely on titrations to determine unknown concentrations of solutions. Here are some examples!

Acid Rain and Wastewater Analysis

- Environmental chemists analyze samples of contaminated rainwater or snow and titrate them to determine their pH.
- Wastewater can become contaminated, and the necessary filtration and treatment solutions need to be developed. Titrations can be used to measure pollutants in wastewater, such as ammonia.

Nutrition and Food

- Titration can be used to analyze properties of different foods. For example, the acidity of orange juice can be determined. There will be certain quality control concerns associated with acidity, and titration can be used to help identify those levels.

The Making of Wine

In the wine industry, where quality and hence chemical precision are paramount, titration is used to determine acidity in multiple ways and also to evaluate sulfur dioxide content. Although advanced techniques such as gas chromatography or liquid chromatography are usually unavailable to most wineries for practical reasons, titration is nevertheless critical to ensure the consistency of products sold.

Source: https://sciencing.com/use-titration-5855864.html

Figure 1. Titrations are commonly used. to assess acidity of wine for quality control purposes.

Figure 2. This is an automated titrator that you will see in post-secondary and commercial applications.

Figure 3. A typical titration set up with a burette, filled with the titrant, over the erlenmeyer flask, filled with the analyte. We can see a colour change of pink indicating that phenolphthalein was used as the indicator.

Action

What is Titration?

- Recall from Grade 11 Chemistry (SCH3U), that titrations are used to determine the unknown concentration of an acid or a base.

Some key terminology regarding titrations …

Equivalence Point

- The equivalence point is the point where the acid and base are equal in moles.
- On a titration curve, it is the midpoint on the steep rise.

Endpoint

The endpoint occurs when the indicator changes colour.
As the pH changes rapidly near the equivalence point, the difference between end point and equivalence point is negligible.

Figure 4. Diagram showing the set-up for a titration. The burette is filled with the titrant. The concentration and the volume of the titrant is known. The analyte, inside of the erlenmeyer flask, is unknown. However, the volume of the analyte is known. An indicator, such as phenolphthalein, is added to the analyte in the erlenmeyer flask to signify that a change in pH has occurred. The equation C1V1=C2V2 is commonly used to calculate the unknown concentration of acid or base.

Titration Curve

- The diagram on the right is an example of a titration curve.
- In this example, the pH of a solution starts off around 1 (very acidic). As the strong base, NaOH, is added, the pH stays relatively acidic.
- However, the pH rapidly changes as the equivalence point is approached.
- As the pH changes rapidly near the equivalence point, the difference between end point and equivalence point is negligible.

Figure 5. Titration curve showing the addition of a strong base (NaOH) to an unknown strong acid. The pH transitions from acidic (~1) to basic (~14).

Titrations Involving Strong Acids and Strong Bases

Figure 6. Here is an example of a titration curve between a strong acid (HCl) and a strong base (NaOH).

The equivalence point of the titration is the point at which exactly enough titrant has been added to react with all of the substance being titrated with no titrant left over.

In other words, at the equivalence point, the number of moles of titrant added corresponds exactly to the number of moles of substance being titrated according to the reaction stoichiometry.

In an acid-base titration, there is a 1:1 acid:base stoichiometry, so the equivalence point is the point where the moles of titrant added equals the moles of substance initially in the solution being titrated.

Example 1.

a) What volume of 0.100 M sodium hydroxide (NaOH) is required to reach the equivalence point when titrating 50.0 mL of 0.200 mol/L nitric acid (HNO₃)?

Watch the following video solution to example 1. Please copy this solution into your notes.

Example 2.

b) What is the pH at the equivalence point?

Watch the following video solution to example 2. Please copy this solution into your notes.

Example 3.

c) What was the pH of nitric acid (HNO₃) before any base was added?

Watch the following video solution to example 3. Please copy this solution into your notes.

Example 4.

d) Calculate the pH of the solution after 50.0 mL of 0.100 M sodium hydroxide (NaOH) is added to 50.0 mL of 0.200 M nitric acid (HNO₃).

Watch the following video solution to example 4. Please copy this solution into your notes.

Titration of a Weak Acid with a Strong Base

- Recall, the anion from the salt produced will react with water.

Figure 7. Here is an example of a titration curve for the reaction between a strong base (NaOH) and a weak acid.

Here, 0.100 M NaOH is being added to 50.0 mL of 0.100 M acetic acid.

There are three major differences between this curve (in blue) and the one we saw before (in black):

1. The weak-acid solution has a higher initial pH.
2. The pH rises more rapidly at the start, but less rapidly near the equivalence point.
3. The pH at the equivalence point does not equal 7.00.

POINT OF EMPHASIS : The equivalence point for a weak acid-strong base titration has a pH > 7.00

For a strong acid-weak base or weak acid-strong base titration, the pH will change rapidly at the very beginning and then have a gradual slope until near the equivalence point. The gradual slope results from a buffer solution being produced by the addition of the strong acid or base, which resists rapid change in pH until the added acid or base exceeds the buffer's capacity and the rapid pH change occurs near the equivalence point.

Example 5.

100. mL of 0.100 mol/L cyanic acid (HCN) is titrated with 0.100 mol/L potassium hydroxide (KOH). Calculate the pH at equivalence.

Watch the following video solution to example 5. Please copy this solution into your notes.

Titration of a Weak Base with a Strong Acid

- Recall, the cation from the salt produced will react with water.

Figure 8. Here is an example of a titration curve for the titration between a weak base (ammonia) and a strong acid (HCl). The weak base has a basic pH of ~13 and is lowered to a pH of ~2 once the equivalence point has been reached.

Here, 0.100 M HCl is being added to 50.0 mL of 0.100 M ammonia solution.

As in the weak acid-strong base titration, there are three major differences between this curve (in blue) and a strong base-strong acid one (in black): (Note that the strong base-strong acid titration curve is identical to the strong acid-strong base titration, but flipped vertically.)

The weak-acid solution has a lower initial pH.
The pH drops more rapidly at the start, but less rapidly near the equivalence point.
The pH at the equivalence point does not equal 7.00.

POINT OF EMPHASIS : The equivalence point for a weak base-strong acid titration has a pH < 7.00.

Example 6.

20. mL of 0.20 mol/L ammonia (NH₃) is titrated against 0.20 mol/L hydrochloric acid (HCl). Calculate the pH at equivalence.

Watch the following video solution to example 6. Please copy this solution into your notes.

Calculating the pH NOT at the Equivalence Point

Example 7.

A 35.0 mL solution of 0.20 mol/L cyanic acid (HCN) is titrated with 50.0 mL of 0.10 mol/L potassium hydroxide (KOH). Calculate the pH of the solution.

Watch the following video solution to example 7. Please copy this solution into your notes.

Performing a Titration - Improving Your Lab Technique!

In Grade 11 Chemistry, SCH3U, you completed your first set of titration experiments. Please watch the following video from Carolina Biological to review how titration experiments are conducted in the lab!

This lesson is now complete. Consolidate your learning by completing the self-check below and the assigned practice questions.

Self-Check

How prepared am I to start my homework? Can I ...

- Determine the pH of the solution formed in a neutralization reaction?
  - - Calculate the pH at equivalence for titrations involving strong acids and strong bases.
    - Calculate the pH at equivalence for titrations involving weak acids and strong bases.
    - Calculate the pH at equivalence for titrations involving weak bases and strong acids.
    - Calculate the pH for titrations that are not at equivalence.
- Draw or examine a titration curve and identify the equivalence point and endpoint.

Practice

Using your textbook, complete the following questions:

- p547 #1,2
- p554 #1,2
- p557 #1-6, 8

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