Investigating Acids and Bases: Strong vs Weak

Linda Soderlund, Eden Prairie High School, Eden Prairie, MN, based on an activity we have done for so long that the original source is unknown.
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Summary

In this chemistry lab activity, students will apply what they learned from titrating and graphing a strong acid (HCl) with a strong base (NaOH) to a titration involving a strong base and a weak acid.
Before performing the titration students will be asked to predict how the end point of the titration will shift. They will collect data and graph it in the same way they did in the earlier titration, determine the end point, and note how it shifted. Finally they will be asked to apply their understanding of the definitions of "strong" and "weak" as applied to acids and bases to explain the observed shift.

Learning Goals

Concepts & content:
2. Predicting results (hypothesis formation)
3. Interpreting graphs (data analysis)
4. Explaining results in terms of the extent of ionization differences between "strong" and "weak" acids and bases
Skills
1. Safe laboratory procedure/behavior
2. Observing, measuring, recording data
3. Graphing data
4. Using a buret
5. Using a pH meter or electronic pH probe
Key concepts:
1. When an acid(aq) is mixed with a base (aq), the products are water(l) and a dissolved salt (aq).
2. Titration is a process of slowly adding base (or acid) to acid (or base) to reach an end point (indicator color change).
3. The graph of pH vs mL titrant added produces a characteristic S-shaped curve.
Vocabulary:
1. Acid
2. Base
3. Strong acid/base
4. Weak acid/base
5. Titration
6. Indicator (phenolphthalein)
7. End point

Context for Use

This activity is designed for high school chemistry classes that are not accelerated or enriched. It is to be used after students have studied the properties of acids and bases as well as the differences between strong acids/bases and weak acids/bases. It is an extension of standard labs that have students generate a pH curve for strong acid vs strong base without going into the concept of polyprotic acids. In fact, our text does not go into weak acids/bases at all, which is the reason I added this extension. The activity fits into the study of acids/bases before students learn how titration data is used to determine an unknown concentration. It is an investigation of the pH curve and an assessment of student's understanding of strong vs weak. Burets should be available because they are more accurate than counting drops. Electronic probeware would be nice, but not necessary. Hand drawn graphs work satisfactorily. Adding this extension should not require much extra time because students use the same equipment and materials as they did for the initial lab. This is definitely an inside activity.

Description and Teaching Materials

(Background) My students watch an old ChemStudy video, "Acid-Base Indicators" before we do the initial lab to construct a standard pH curve. They come away from the video thinking that phenolphthalein would NOT be a useful indicator of pH=7 (because phenolphthalein changes at about pH=9). So the initial lab is introduced as a way to evaluate the indicator. Students add 0.15 M NaOH in 0.5 mL increments to 0.10 M HCl with 2 drops phenolphalein, measuring and recording the pH after each addition. They mark on the data table when the color changes and continue to add base until the pH is above 12. The color change does not occur at the exact equivalence point, so students graph their data, pH vs mL base added. We discuss why the end point is pH=7 and then students compare the mL base added at the end point to the mL base added when the color changed. Careful students find that the two volumes are very close to each other, and through discussion we conclude that the indicator changed color at pH=7, with minimal error because the slope of the curve is so vertical. Most will accept that we can use phenolphthalein as an accurate indicator for pH=7. Students use standard 50-mL burets for the base and titrate into a 400mL beaker containing 10.0 mL acid and 50.0 mL water so they can insert a pH meter and stirring rod. We have not used electronic probeware yet. We have been doing this initial activity for so long that the origin is unknown.

One purpose of the extension is to give students additional practice using the burets. Another purpose is to evaluate students' understanding of the relative ionization levels in strong and weak acids. So I will ask students to predict how the end point of a titration will change, or shift, if we use vinegar (a weak acid) instead of HCl. Then students perform the same lab using vinegar, graph their data, and note the position of the end point when the same volume of base has been added as in the initial activity. Finally students will write an explanation of the results. I expect them to realize that the weak acid will not use up all of the OH- ions provided by the strong base, so the end point has a pH in the basic range.

Closure involves having students pose additional safe and testable questions such as: Would the end point shift into the acid range if a weak base and strong acid were used?; What will be the end point of a titration using NaOH and a diprotic acid such as H2SO4? Lab handout, "pH Titration Curve" (Acrobat (PDF) 13kB Aug24 07)

Teaching Notes and Tips

I think that adding this simple extension will add an element of inquiry to the course as we teach it at EPHS. It is a first step - making small modifications to the "cookbook" labs we already do in order to add inquiry components. We use a lot of lecture in an effort to cover all the concepts and can't agree about what to omit in order to provide greater depth.

My students have done a simple titration using medicine dropper pipettes before doing this one, so they know what phenolphthalein looks like in an acid and in a base. I show them that tap water is basic and remind them to rinse all their glassware with distilled water before and after using it. Teachers with large classes and/or unruly students may want to fill the burets instead of having students do it. And of course safety glasses and aprons should be worn.

Assessment

Students will hand in an informal lab report that includes a prediction, data table, graph, and analysis. The analysis will include a description of whether the data supports the prediction or not and an explanation of the end point shift showing understanding of what is happening on a microscopic level between H3O+ and OH- ions.

Standards

9-12.I.B.1 - design & complete an investigation
9-12.I.B.3 - mathematics & models
9-12.II.B.5 - combining acids & bases

References and Resources