Vitamin C in Fruit Juices by Redox Titration

Transcription

1 Carolina Investigations for AP * Chemistry Vitamin C in Fruit Juices by Redox Titration TEACER S MANUAL AND STUDENT GUIDE *AP and Advanced Placement Program are registered trademarks of the College Board, which was not involved in the production of, and does not endorse, this product.

2 Table of Contents Carolina Investigations for AP Chemistry Vitamin C in Fruit Juices by Redox Titration TEACER S MANUAL verview bjectives Content Standards Time Requirements Materials Safety Teaching Inquiry Background Information Guided Activity Preparation Procedure Answers to Questions in the Student Guide (and Sample Data) Inquiry Activity Preparation Procedure Student Presentations Answers to Big Idea Assessments Differentiated Instruction elpful ints Extension Activities Resources Related Products Rubric STUDENT GUIDE* Guided Activity S-1 Background S-1 Pre-Laboratory Questions S-1 Materials S-2 Procedure S-2 Data Tables S-4 Laboratory Questions S-5 Inquiry Activity S-6 Background S-6 Materials S-6 Procedure S-7 Experimental Design Template S-8 Big Idea Assessments S-10 Appendix A: Generating a Standard Curve for the Reaction of DCPIP and Vitamin C S-12 Appendix B: Sample Lab Data S-13 *Photocopy the Student Guide as needed for use in your classroom. Cover photo 2012 Jupiterimages Corporation The materials and activities in this kit meet the guidelines and academic standards of the Advanced Placement (AP) Program and have been prepared by Carolina Biological Supply Company, which bears sole responsibility for kit contents Carolina Biological Supply Company/Printed in USA. 2

3 Teacher s Manual Carolina Investigations for AP Chemistry Vitamin C in Fruit Juices by Redox Titration NTES verview Students investigate reduction oxidation (redox) titrations as they work in teams of 3 to determine the concentration of vitamin C in fruit juice. This manual includes two alternative approaches to the lab: a guided approach and a more inquiry-centered approach. With the guided approach, students first develop a standard curve, using standard solutions of ascorbic acid as a reducing agent for a redox dye. Then, by comparing their data from a juice sample to the standard curve, they determine the vitamin C concentration in the juice. The inquiry approach allows teams to plan their own experiments and investigate alternative variables such as different juices, different preparations, or even how the concentration may be affected by heating. The kit includes materials for a class of 30 working in 10 groups and lists items that are needed but not supplied. If you choose the inquiry approach, you may need to supply additional items called for in the students experimental plans. bjectives Students will develop a standard curve. calculate the concentration of vitamin C in a juice by redox titration. Content Standards This kit is appropriate for Advanced Placement high school students and addresses the following AP Chemistry concepts: Big Idea #3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons. Essential Knowledge 3.B.3: In oxidation reduction (redox) reactions, there is a net transfer of electrons. The species that loses electrons is oxidized and the species that gains electrons is reduced. Å Learning bjective 3.9. The student is able to design and/or interpret the results of an experiment involving a redox titration. Carolina Biological Supply Company/Printed in USA. 3

4 Teacher s Manual Vitamin C in Fruit Juices by Redox Titration Kit for AP Chemistry NTES Time Requirements Guided Activity R Inquiry Activity* Preparation minutes 15 minutes Activity minutes 90 minutes Presentations ~60 minutes This lab is designed to be conducted as either a guided or an inquiry activity. Follow the procedure that better fits your needs. *If the lab is conducted as an inquiry activity, the time estimates provided may vary according to students experimental procedures. Remember to allow students time to prepare their materials. Decide how best to schedule students planning for their presentations. Materials Included in the kit: 1 g 2,6-dichloroindophenol (DCPIP) 5 g ascorbic acid (vitamin C) 100 disposable pipets Teacher s Manual and reproducible Student Guide Needed, but not supplied: 1-L volumetric flask 400-mL or larger bottle with lid mL graduated cylinders 10 wash bottles filled with distilled or deionized water mL beakers mL or larger beakers to be used as waste containers marking pen(s) juice sample(s)s *Items needed but not supplied for the Inquiry Activity may vary according to your students planned experiments. Additional 50-mL beakers will be needed if multiple juice samples are tested. Safety Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as chemical splash-resistant goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. Store all chemicals in 4 Carolina Biological Supply Company/Printed in USA.

5 Vitamin C in Fruit Juices by Redox Titration Kit for AP Chemistry Teacher s Manual accordance with their chemical compatibilities in approved storage areas. Upon completion of the lab, properly dispose of chemicals according to federal and local regulations. NTES The powder DCPIP and its solutions will stain skin or clothing. Take care when using this chemical. Teaching Inquiry Since the National Research Council published the National Science Education Standards in 1996, the inquiry approach to science education has become recognized as a method that actively engages students in a learning process that results in a greater mastery of scientific concepts. The findings of the National Research Council support the evidence that an inquiry approach to education helps students gain an in-depth understanding of science by building upon previous knowledge (Inquiry and the National Science Education Standards 2000). Students become empowered, taking responsibility for their learning by conducting inquiry investigations and communicating their discoveries. Inquiry activities encourage students to explore questions in a scientific way. In structured inquiry, the question is supplied to the student. A systematic procedure for exploring the question and reaching a conclusion may also be provided. In open inquiry, the student directs the entire scientific investigation, determining the question to explore, the materials to use, the procedure to follow, and the methods used to analyze data. Current models of inquiry in science instruction derive from the work of several researchers, including Schwab (1962), erron (1971), and Rezba, Auldridge, and Rhea (1999). Carolina s Approach Carolina Investigations for AP Chemistry offer an inquiry approach to scientific instruction. Teachers may choose to perform a lab as either a Guided Activity, in which students investigate a pre-determined question using an established procedure, or as an Inquiry Activity, using student-directed investigations and presentations. Both approaches are described in each kit manual. With each kit, teachers decide which approach better suits their situation and the needs of their students and fulfills the requirements of the AP Chemistry curriculum. In the inquiry approach, student teams choose a question to investigate and design a procedure to answer the question. After they secure teacher approval, students proceed with their experiment. Teachers assist by providing any extra materials needed, ensuring safe laboratory practice, and asking questions that encourage critical thought about the teams progress and findings. Pre-laboratory questions in the Guided Activity test students prior knowledge. Big Idea Assessments administered after the activities provide practice for the AP Chemistry exam s free-response questions and help students internalize and effectively communicate the scientific concepts addressed by the laboratory activities. Carolina Biological Supply Company/Printed in USA. 5

6 Teacher s Manual Vitamin C in Fruit Juices by Redox Titration Kit for AP Chemistry NTES Background Information See the Student Guide for additional information about vitamin C. Note the Prior Knowledge section that follows. If need be, perform a run-through of the laboratory activity so that you will be able to troubleshoot. A titration is one technique used to determine the concentration of a particular substance in solution. A solution of known concentration, called the titrant, reacts with the solution of unknown concentration, called the analyte. A specific titrant is selected that is known to react completely and quickly with the analyte, and with known stoichiometry. The equivalence point describes the state at which all of the analyte has reacted with all of the titrant and no reagent exists in excess. The experimental estimate of the equivalence point is called the end point. ne type of titration is a reduction oxidation (redox) titration. In redox reactions, electrons are transferred from the reducing agent to the oxidizing agent. xidation refers to the loss of electrons; reduction is the gain of electrons. In a redox reaction, the reducing agent is oxidized while the oxidizing agent is reduced. The color change observed in this laboratory experiment is caused by the reduction of 2,6-dichloroindophenol (DCPIP) as shown in the following reaction. N (Blue ) e (colorless) Ascorbic acid is oxidized to dehydroascorbic acid as shown in the following reaction. N e Vitamin C is an essential compound synthesized by plants and most animals. umans and some others obtain vitamin C from their food. Citrus fruits, strawberries, and pineapples are rich in vitamin C, as are sweet peppers, cabbage, broccoli, and spinach. The peels of citrus fruits contain even more vitamin C than the juice. Vitamin C is water soluble, and it is easily lost when fruits and vegetables are boiled. Also because it is water soluble, large amounts of vitamin C in the body are excreted in urine. This strong reducing agent is quickly degraded in the presence of air; therefore, the concentration of vitamin C can be altered by the freshness and storage of the sample. 6 Carolina Biological Supply Company/Printed in USA.