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exom question ~ The Exam Video
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JOHN J. ALEXANDE; University of Cincinnati cin~~nnati.0~~5~1
Expected Answer
John R. Luoma and Susan M. Yochum Cleveland State University Cleveland, OH 44115
Chemistry is a dynamic discipline because it focuses on substances changing into new substances. It is much more exciting for students when they actually see a chemical reaction occur than when they simply read a written description about a chemical process. We address this issue by incorporating the "exam video" into our general chemistry exams. Each exam video shows a chemical reaction. Questions relating to that reaction are on the exam. Each hour exam and final exam have at least one exam video. The videos are made by the authors with a VHS camcorder. Due to the editing capabilities of the camcorder, an exam video can be produced in about 30 min. The videos include title, introductory, and end screens, a s well as sound. A separate tape is used for each exam video. A V C W cart is placed in the back of the classroom. (It is advantageous to use a separate VCR for each tape.) When ready to answer the exam video questions, a student goes to the VCR and plays the video tape. Students are permitted to play the tape as many times as needed. This method of presenting questions about processes on exams appeals to the present generation of students. As one of our students wrote in a survey we conducted, "We are the TV generation!" This method also stimulates and challenges the students to develop their observational skills. The following are two examples of the exam video. Video 1: Zinc + Acid/Evolution of Hz Scene 1
Students see approximately 75 mL of 3 M HC1 being poured into a 250-mL Erlenmeyer flask. Next, several pieces of mossy zinc are dropped into the flask. The flask is swirled. A 9-in. balloon is placed over the mouth of the flask. Filming continues until the balloon is filled with HZ. (The reaction is conducted in a fume hood.) Scene 2: The Big Bang!
The balloon filled with Hz from the previous scene is removed from the flask and tied to an iron ring attached to a ring stand. A lighted splint comes in contact with the balloon, and the resulting explosion produces a loud noise. (An explosion shield in the fume hood is placed in front of the balloon.) Questions
Half-reaction F (v) ~0.76 Zn(s) + 2nZ+(aq) + 2e0.00 2e- + 2Hi(aq) + Hdg) +0.76=Ecell Zn(s)+ 2Hi(aq) + Zf+*(aq)+ H2(g) Since gel, > 0 and AGRn = -2FE&1, then AG,.< 0 and the reaction can be spontaneous. 1
Scene 2 1. Write a balanced chemical reaction that describes the observed explosion! Expected Answer 2Hz(g)+ O,(g) + flame + 2H20(g)+ energy
1
2. Use the thermodynamic tables to support your observation. (Use the reaction above.) Answer (Students are given AfQ and SP tables.) Step 1:Calculate and find that AH&, = 483.7 kJ Step 2: Calculate and find that ASP, = 0-,08876 kJ/K Step 3: Calculate AGR, from mrn - TASP, AGP, = 483.7 W - ((298.15K)(4.08876WK)) A G L = 457.2 W Thus, the reaction is potentially spontaneous. Video 2: Sodium +Water The video begins with a shot of a labeled jar containing a large piece (about 500 g) of Na immersed in toluene. The Na is removed and placed in a glass dish. A small piece (a square centimeter) of Na is cut. A 600-mL beaker containing 400 mL of HzO is displayed. Afew drops of thymol blue indicator are added to the beaker. The indicator causes the Hz0 to turn yellow. The cut piece of Na is added to the beaker. As the reaction proceeds, the solution turns blue, a pop and a yellow spark are produced, and condensed steam is seen. Questions
1. Write a balanced net ionic chemical reaction for the process that causes the color change of the aqueous media. Identify the species that causes the color change. Expected Answer
Scene I 1. Write a balanced net ionic chemical reaction that describes the chemical process you have observed.
2Nais) + 2Hz0(1)+ 2Nai(aq) + 20K(aq)+ H2(g)+ energy
The presence of OH-(aq) causes the indicator to turn hlue.
J Expected Answer + Hg(g) Zn(s) + 2Hi(aq) + znZ+(aq) I
2. Write a balanced chemical equation that describes the process that produces the spark! I
2. Use reduction potentials to support your observations. (Students are given a table of standard reduction potentials.) 932
Journal of Chemical Education
Expected Answer 2Hz(g)+ OzW + 2HzO(g)+ energy
