1. Module 7 Intro

1.17. Page 3

Lesson 3

Module 7—Chemical Analysis

Read

 

The unique shape of a titration curve demonstrates that important changes occur. Interpreting the different parts of a titration curve is an important skill. Read the first four paragraphs of “Interpreting Titration pH Curves” on pages 334 and 335 of the textbook.

 

Self-Check

 

SC 1. Retrieve the extra copy of the titration curve you saved to your course folder. Find the equivalence point and equivalence point pH for each titration curve. Using your labelled graphs, state the equivalence point pH and volume of titrant required to reach the equivalence point for each titration. Save your answer for use later in this lesson.

 

Check your work.

Self-Check Answer

 

SC 1.

 

 

 

Titration

Equivalence Point pH

Volume of Titrant to Reach Equivalence Point

HCl(aq) vs. NaOH(aq)

6.75

15.2 mL

CH3COOH(aq) vs. NaOH(aq)

8.75

4.0 mL

 

Read

 

Read the rest of “Interpreting Titration pH Curves” on page 335 of your textbook.

 

You will note that a titration curve conveys a great deal of information about the chemical reaction that occurred to produce this data.

 

A graph of a titration is shown. The graph has 16 horizontal lines and 19 vertical lines. Further description is provided below. The x-axis is labelled Volume of 0 decimal 1 0 0 NaOH aqueous added in units of millilitres. Numbers start at 0 and increase by 5 at every second vertical line ending at 45. A label appears on the graph that reads: The x-axis is the volume of titrant added to the sample. The y-axis is labelled pH. Numbers start at 0 and increase by 1 for each horizontal line to a maximum of 15. A label appears on the graph that reads: The y-axis is the pH of the sample as the titration progresses. The title for the graph shown is Titration curve for titrating 15.0 mL of 0 decimal 100 mol/L HCl(aq) with 0 decimal 100 mol/L NaOH(aq). A label is added around the text “15.0 mL of 0 decimal 100 mol/L HCl(aq)” indicating that the information that goes here is the volume and concentration of the sample in the flask. A label is added to the text: “0 decimal 100 mol/L NaOH(aq)” indicating that the information that goes here is the concentration of the titrant (in the burette).

 

Axes Labels: The x-axis represents the volume of titrant added to the sample (usually in mL). The y-axis represents the pH of the sample as the titration progresses.

 

Title: The title identifies the reactants.

 

A very important piece of information on the graph is the equivalence point. The equivalence point represents the end of the reaction and is located at the midpoint of the steep middle region. While the titration graph for a strong monoprotic acid reacting with a strong monoprotic base will have an equivalence point pH of 7, this is not necessarily the case for weak acids or weak bases being titrated.

 

The volume of titrant required to reach the equivalence point is the value used to complete calculations like those you have completed in the previous lessons.

 

The titration data and the completed titration curve are shown here for comparison.

 

A two column table containing the data from a titration is shown. The left hand column is titled Volume of zero decimal one zero zero aqueous NaOH. The right hand column is titled pH. The data for each row, read as volume and its corresponding pH, is: Row 1: 0 and 1 decimal 0, Row 2: 2 decimal five and 1 decimal 2, Row 3: 5 decimal zero, 1 decimal 3, Row 4: Seven decimal five, one decimal five, Row 5: Ten decimal zero, 1 decimal 7, Row 6: 12 decimal 5, 2 decimal zero, Row 7: 15 decimal zero, 7 decimal 0, Row 8: 17 decimal 5, 11 decimal 9, Row 9: 20 decimal 0, 12 decimal 1, Row 10: 22 decimal 5, 12 decimal 3, Row 11: 25 decimal 0, 12 decimal 4, Row 12: 27 decimal 5, 12 decimal 5, Row 13: 30 decimal zero, 12 decimal 5

 

A graph of a titration is shown. The axes of the graph are identical to the ones described previously in this lesson. The titration data from the table preceding is plotted. Refer to these descriptions for further information.

 

Selecting Indicators for a Titration Curve

 

If you titrate HCl(aq) with NaOH(aq)—two colourless solutions—without using an indicator, there will be no colour change to signify that the equivalence point has been reached. Indicators respond to changes in pH and provide a dramatic colour change to signify the endpoint of a titration. But with so many indicators to choose from, which one is best?

 

Read “Choosing Acid-Base Indicators for Titration” on page 336 of your textbook.

 

A good rule of thumb for selecting an indicator is that the midpoint pH of the range of colour change is equal to the equivalence point pH.

 

Self-Check

 

SC 2. Retrieve the copy of the titration curve that you used in SC 1. Use the information provided in the “Acid-Base Indicators” table in the Chemistry Data Booklet to identify suitable indicators that will signify the endpoint for the titration shown. Then indicate the colour change that would signify the endpoint of the titration.

 

Check your work.
Self-Check Answer

 

SC 2.

 

Titration

Suitable Indicator

pH of Indicator

Colour Change at Endpoint

HCl(aq) vs. NaOH(aq)

bromothymol blue
phenol red

6–7.6

 

6–8.0

green (intermediate between yellow to blue)

orange (intermediate between yellow and red)

CH3COOH(aq) vs. NaOH(aq)

thymol blue

phenolphthalein

8–9.6

 

8–10.0

green (intermediate between yellow to blue)

pink
 
Module 7: Lesson 3 Assignment

 

Retrieve the Module 7: Lesson 3 Assignment you saved earlier in this lesson. Complete questions 2, 3, and 4. Save a copy of your assignment to your course folder.