Constructivist Planning of a Chemistry Lesson – Acid and Base

Title of lesson: Acids and Bases

Level: 3^rd year Chemistry

Lesson plan:

I. Goals

The student will be able to:

• Define the terms acid and base
• Explain what makes an acid/base strong or weak
• Calculate the pH or pOH for a given system
• Calculate acid or base concentrations based on titration data

II. Overview

Acids and bases are an important part of our day-to-day lives. The term acid rain comes up frequently when discussing environmental concerns. Acids are defined as being electron pair acceptors, frequently meaning that they donate an H₃O⁺ ion. A base is defined as being an electron pair donor, frequently meaning that they accept H₃O⁺ or donate OH^-. pH is defined as being the negative log of the hydrogen ion concentration [H₃O⁺]. pOH is defined as being the negative log of the hydroxide ion concentration of the solution [OH^-]. The product of the [H₃O⁺] and [OH^-] concentrations is the K_w=1.0 x 10^-14. The more an acid or base dissociates in an aqueous solution, the stronger it is considered to be. For example, HCl is considered to be a strong acid, and NaOH is considered to be a strong base.

Students will be investigating acids and bases using pH tests, titrations, class discussions, lecture and reading.

III. Resources, materials and supplies needed

• pH testing equipment

litmus paper

colorimetric indicators

pH probes and CBLs - one per group of students

• Textbook or other resource with course level appropriate information on acids and bases.

• Burets ­ one per group of students

• Erlenmeyer flasks - two per group of students

• Colorimetric indicators or pH probes and CBLs ­ one dropper bottle of indicator per group, or one pH probe and CBL per group of students

• Standard acid and base solutions (known concentrations) 0.5 M HCl and NaOH solutions 75 ml of each per group of students

• Unknown acid and base solutions 0.75 or 1M concentrations of HCl and NaOH solutions 15 ml of each per group of students

• Titration curves for strong, weak, and polyprotic acids, titrated with a base.

Engagement

Student behaviors / activities:

Using litmus paper, pH indicators, and/or pH probes determine the pH for various household chemicals, tap water (especially in Austin, the water is quite basic), and deionized water.

Answer questions of why this relates to acids and bases.

Create a concept map about acids and bases.

Brainstorm what makes acids and bases strong or weak.

Predict where household products (bleach, vinegar, solution of baking soda, carbonated beverages) would fall on the pH scale.

Teacher behaviors / activities:

Demonstrate technique for using various testing methods.

Observe students while they are performing the experiments. Answer any questions. Make sure students follow lab safety rules.

Facilitate discussions of acids and bases.

Clarify misconceptions about acids and bases.Ask students why two samples of water have a different pH.

Draw pH scale on board.

Ask how the pH of something can be changed.

.

Exploration

Student behaviors / activities:

Experiment with pH by measuring the pH of water before and after adding a drop of acid or base to it.

Graph the relationship between the amount of acid/base added to the pH.

Look at titration curves of strong acids with strong bases, weak bases, and the titration curves of polyprotic acids.

Teacher behaviors / activities:

Monitor students while doing the experiment.

Compile data on the board.

Answer any student questions. Ask students to compare the graphs they generated with the titration curves. How are they related?

Explanation

Student behaviors / activities:

Discuss findings from the exploration.

Hypothesize the relationship between the graphs generated to titration curves.

Discuss how pH relates to acid/ base concentration.

Read about acids/bases and discuss as a class their misconceptions before the reading.

Ask questions.

Teacher behaviors / activities:

Present information on what defines an acid or a base. Discuss what makes an acid/base strong or weak. Relate this concept to solubility.

Explain pH/pOH scale, and how pH/pOH is determined ([H₃O⁺] ion concentration and [OH^-] ion concentration).

Demonstrate pH/pOH calculations.

Review basic log rules.

Elaboration

Student behaviors / activities:

Titrate a strong acid solution with a base. Titrate a strong base with an acid. Determine the concentrations of these solutions.

Teacher behaviors / activities:

Discuss neutralization reactions.

Explain titrations and why they are used.

Demonstrate titration technique.

Demonstrate titration calculations.

Evaluation

Student behaviors / activities:

Generate a basic lab report about the titration experiment.

As a class discuss the findings of the comparisons of titration curves.

Take a quiz about acids and bases, where they must identify something as being an acid or base, determine the pH of a solution, and determine the concentration of an acid or base solution from titration data.

Teacher behaviors / activities:

Facilitate class discussion, and help the students relate titration curves, and the titration process to the topic of acids and bases in general.

Ø Reflection: what was the most difficult aspect of planning? How is planning a constructivist activity different from other types of lesson?

Planning allows teachers to personalize the curriculum and allows them to anticipate instructional needs in advance so materials can be gathered and organized. It provides a script that directs interactions with students and provides a form of psychological or emotional security which reduces the normal anxiety associated with teachers. But planning a constructivist activity is different from other types of lesson and presents some difficulties. It needs much more time to search and think about many types of questions that involve students and lead them to reach the right answers and to achieve the goal of the lesson. Furthermore, these different types of questions should fulfill the hierarchical different levels in the Bloom’s Taxonomy (cognitive domain). In addition, Teacher-learning contexts change and teachers' behavior must change accordingly. The basic problem for teachers is, therefore, to acknowledge that there is no one best way to behave, and then to learn to make decisions in such ways that their behaviors are continually appropriate to the dynamic, moment to moment complexity of the classroom. Thinking and preparing activities that motivate and involve students in a way to lead them to construct their own understanding is not easy at all. It needs lots of materials other than just a textbook to prepare such sort of activities and concrete examples. One needs to prepare graphs, pictures and many others such as the ones listed above in my lesson plan, in order to illustrate clearly the content of the lesson. Finally, the teacher should find a way to know if they students understand the material and their role and the most important thing is that they reach the goal of the lesson.