Introduction to Acids and Bases

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Objective

Students will be able to differentiate between acids and bases through performing an inquiry lab and taking notes.

Big Idea

Acids and Bases have characteristic properties such as pH, reactivity with metals, conductivity, color change with litmus paper, and color change with phenolphthalein.

Introduction

In this lesson students are introduced to acids and bases through performing an activity.

  • This lesson does not align with any specific performance expectations with the Next Generation Science Standards; however, I feel that understanding acids and bases is an important part of the chemistry curriculum.  This is content that has historically been covered and students taking chemistry in college are expected to have already seen this content.
  • This lesson aligns with the Next Generation Science and Engineering Practice 3: Planning and Carrying Out Investigations.  It does so because students are performing a lab activity.
  • This lesson also aligns with the Next Generation Science and Engineering Practice 4: Analyzing and Interpreting Data.  It does so because students use the data gathered in the lab to come up with generalizations about acids and bases.

For this lesson there are several resources needed at each lab station:

  1. At station #1 you need four beakers (250-400mL), acetic acid, HCl, NaOH, ammmonia, parafilm, a conductivity tester for each solution, rinse water, and a conductivity probe.
  2. At station #2 you need containers of red litmus paper, blue litmus paper, tweezers, paper towel, a beaker labeled waste, a dropper bottle of acetic acid, a dropper bottle of HCl, a dropper bottle of NaOH, and a dropper bottle of ammonia.
  3. At station #3 you need a well plate labeled with ammonia, HCl, NaOH, and acetic acid, phenolphthalein, a dropper bottle of acetic acid, a dropper bottle of HCl, a dropper bottle of NaOH, and a dropper bottle of ammonia.
  4. At station #4 you need four beakers or flasks, acetic acid, HCl, NaOH, ammonia, parafilm, pH meters, water, and a pH probe.
  5. At station #5 you need tweezers, a beaker labeled waster, zinc metal, a well plate labeled with ammonia, NaOH, HCl, and acetic acid, a dropper bottle of acetic acid, a dropper bottle of HCl, a dropper bottle of NaOH, and a dropper bottle of ammonia.
  6. At station #6 you need a thermometer or temperature probe, a dropper bottle of NaOH, a dropper bottle of HCl, small test tubes, and a small test tube rack.
  7. At station #7 you need a pH chart with pH, H+, OH-, and pOH labeled.

Explore

45 minutes

In this section of the lesson students are performing their acids and bases intro lab with their partners. 

Before students begin the lab I go over what they will be doing in the lab. 

  • I give them several minutes to skim the lab and I then briefly discuss what they will be doing at each station. 
  • I tell them that when they get to each station they should first read the background information on their lab paper.  They will then perform the experiment as indicated on the paper at the station (I place papers at each station before class which I print from the acid base intro lab station information paper). 
  • I instruct them that as they perform each experiment they should record information about acids and bases. 
  • Finally, I tell them that when they are done that they will come up with a generalization about acids/bases based on data from their tests.
  • I then briefly go over what students will be doing at each station:
    • At station 1 students are testing conductivity.
    • At station 2 students are testing blue and red litmus paper
    • At station 3 students are testing phenolphthalein.
    • At station 4 students are testing pH
    • At station 5 students are testing reactivity with metals
    • At station 6 students are testing what happens when acids and bases react
    • At station 7 students are studying a pH, pOH, H+, and OH- scale.
  • Finally I divide students into cooperative groups and assign each group a station to start at.  This is a picture of the cooperative groups I use in my classroom.  For more details on how I do this in my classroom see my reflection on cooperative groups in my Density Lesson.

I give students 4 minutes per station and then have them rotate. 

  • I time them using my TimerTools application. 
  • I tell students that they should switch roles as they rotate stations so whomever was Primary Investigator is now Post Doc, Post Doc become Graduate Student, Graduate Student becomes Undergraduate Student, and then finally Undergraduate Student becomes the Primary Investigator.

As students work I walk around to ensure that they stay on task. 

  • This is a movie showing students working on the labs and my checking for understanding as they are working.
  • For the first few stations I make sure to stop students and tell them to come up with generalizations of acids vs. bases. 
  • The station which students have the hardest time is station #7 where some get confused as to how to explain the differences between acids and bases.  I help them out with the example that acids have a lower pH than bases, etc.

As students complete their labs I have them turn into my basket.  I then grade their labs using this rubric.

The most difficult part of the lab for students is coming up with generalizations about acids and bases at each station.  It is in their nature to just summarize what they saw, but my goal is for them to come up with a generalization for each station. For example at station 1 they should have that both acids and bases conduct electricity and for station 2 they should have that acids make blue litmus paper turn red while bases turn red litmus paper to blue.

Here are two student examples which show how they made observations and then generalizations for each station:

Student example # 1:  In this first example, which is characteristic of many of the labs that got turned in, the student did not make complete generalizations about acids and bases in stations 1, 2, and 6.   For station #1 the student just said that they all conduct light but did not specify acids and bases.  For station #2 the student gave specifics about the four substances and did not generalize about acids and bases.  For station #6 the student just said that the temperature increases but again was not explaining her though fully in terms of acids and bases.

Student example #2: In this second example, the student did a great job of coming up with generalizations about acids and bases for each station and could be used as a key when you do the lab with your class.

Explain

30 minutes

In this section of the lesson I build on the introduction to Acids and Bases from the lab by having students take notes.  They take notes on their notes graphic organizer while I present information on the PowerPoint.

  • I begin with having students write down properties of acids and base which I show on slides 2 and 3.  Many of these are properties that they just discovered in their lab, so I make sure to refer to the lab when I present them.
  • Next I go into the arrhenius definition of acids and bases on slides 4 and 5.
  • I also teach students the bronsted-lowry definition of acids and bases on slide 6.  I make sure to show students how a proton is just a hydrogen that lost an electrons.  I do this by drawing a bohr model for hydrogen on the board and then showing how if we take away an electron all we have is a proton.
  • Students then do examples of conjugate acid and base pairs on slides 7-12.
  • This is a movie of my explaining to students how to figure out the conjugate acid-base pairs.
  • This is a copy of one student's completed notes.

 

 

Evaluate

To evaluate students' learning in this lesson I have them complete a homework assignment.  I pass out the homework and let them know that they are responsible for completing the assignment at home.  I then stamp the homework for completion at the following class and review using the answer key

Some of the common mistakes by students includes figuring out the conjugate acid/base pairs and writing the conjugate acid or base.  I make sure that as I am reviewing the homework that I go over how to figure out the partners (with the only difference being an H+) and also go over how you can differentiate the acid from the base (acid is the proton donor so has the extra H+).