ED 545B

Reading Log #1
Prompt: What are the pros and cons of using lectures (expository teaching)?
Which of the ideas in this chapter would  I like to  incorporate in my 15-minute mini- lecture to be given in class (this semester)?

Talk Moves and Talk Formats

Talk Moves: Teacher Q’s and Statements (and silent periods) that encourage detailed student discussions (talk) and foster student cognitive engagement (stimulates cognitive thought).

• Probing: prompts Ss to extend, clarify and justify their talk
• What does that mean? Please elaborate more. What comes next? Please add more to what you just said...
• Redirecting: Asking another student to extend, clarify or add onto what the previous student had talked about.

Talk Formats: Ways of organizing Ss to facilitate talk.

• Laboratory activities, grouping Ss into teams to discuss a reading from text
• Individual Ss present findings to whole class
• discuss a video

Guided Discussions & Reflective Discussions

Guided Discussions

• teacher is the interaction leader and primary questioner

Reflective Discussions

• Initiated by the teacher, who, much like a news program moderator, functions thereafter as the discussion facilitator

What are the pros and cons of using lectures (expository teaching)?

Discussion Pros:

• Discussion, demonstrations and lectures are good for promoting scientific talk and argumentation in class.
• Promotes Scientific talk
• Allows use of new scientific vocabulary words in real life context (some science vocabulary have different meanings depending on the context, eg. Laws, Solutions, compounds...
• Discussions can promote healthy argumentation, allowing students to practice using logic, observations and evidence to support their beliefs.

Discussion Cons:

• Teacher must know content strongly to answer unpredictable Q’s from Ss.
• Discussions can become off-topic quickly, teacher must guide the discussion
• Shy students may feel reluctant to contribute, unless teacher maintains a comfortable and safe environment to speak in.

Ideas I want to use in Mini-lecture (15 min)
I want to use the Guided discussion technique and use similar questioning techniques as done in a discrepant event. I also want to use PowerPoint with embedded photos, animations and video from YouTube. Use of a video converter will be needed, and I have used www.mediaconverter.org/, http://www.convertfiles.com/.

Switching Indicators in the Acid-Base Titration Lab

The acid-base titration lab is one of my favorites in chemistry. I have performed this lab multiple times through high school and college, and every time we used Phenolphthalein as the indicator. Phenolphthalein changes from clear (acidic) to pink (basic) at pH 8.5-9.0, in the slightly basic pH range. When we mix a strong acid with strong base, like HCl and NaOH, we get a neutralization reaction resulting in a solution with a pH = 7. The resulting solution will consist of water and sodium chloride, a neutral salt, and thus the solution should have a pH = 7. Ideally, we would want to use an indicator that indicates at or near pH=7. For years  I have wondered “Why do we use an indicator that indicates at pH 8.5-9 when titrating a strong acid with a strong base? Shouldn’t we be using an indicator that indicates near pH=7?”

If phenolphthalein indicates at pH 8.5, then we actually have more OH- in our “neutral” solution than H+, and this would throw our titration calculations off. We calculate our moles of acid and moles of base using the assumption that at the equivalence point, moles of acid=moles of base. If we use phenolphthalein as an indicator, then this assumption is bad because we have more moles of base than moles of acid, due to it’s indicating range of pH 8.5-9.  

In my Honors Chemistry class, I brought this up to my CT and asked her if I could test a better indicator (Bromothymol Blue, BTB) that indicates at pH 7.2, much closer to pH 7.  BTB is yellow in acidic solutions, blue in basic solutions, and will turn green in the middle, at pH 7.2.  My CT wanted to see if we could attain better results using a different indicator, BTB. I’m not sure why we commonly use Phenolphthalein in this lab, maybe because of tradition or maybe due to cost. 

After looking up the indicator in the Flinn scientific catalog, we found the BTB to be only slightly more expensive than phenolphthalein, but not by enough o make a serious difference.

Here is some real data that I collected while comparing these two indicators.

I prepared an “unknown” solution of NaOH of 0.60M.

To prepare 1 Liter of 0.6M NaOH:

NaOH=40g/mol.

NaOH (reagent grade) pellets: 0.6 mol x 40g/mol = 24g NaOH pellets needed

Actual weight used: 24.10g

Final volume of solution: 1Liter

Actual Molarity of NaOH: 0.6025M ~ 0.60M

HCL is 0.5M

NaOH is 0.6M

M1V1=M2V2

Here are the results from the 2 indicators:

Phenolphthalein:

Trial 1: 0.531M

Trial 2: 0.525M

Trial 3: 0.529M

(We used 0.53M as the accepted value for this lab)

% Error = (0.60M – 0.53M)/(0.60M)= 11.67%

Bromothymol Blue:

Trial1: 0.603M

Trial2: 0.592M

Trial3: 0.595M

Average: 0.597M

% Error = (0.600 – 0.597)/0.600= 0.5%

The percent error shows us that using BTB gives much more accurate results in this lab. I also compared the difficulty of reaching the endpoint for each indicator, relative to each other. I found that both indicators were very similar in that a good endpoint could be reached using only ½ drop increments of acid or base, giving them similar sensitivities, which would result in no changes in procedure for the students. The green color was very sensitive and allowed for very accurate reading, similar to getting the phenolphthalein to turn very light pink.

My CT and I were very excited with the results and she is going to change the lab materials for her future titration labs. I was also very happy because I got to answer my own, age-old question of “Why do we use phenolphthalein in strong-acid/strong-base titrations?”

After researching this question, I found that it is basically a tradition. In fact, many forums about the topic suggested using BTB in this lab, and it confirmed that BTB is a better choice for this titration.