CHE 114L: Lab 1 The Chemistry of Baking Soda

Transcription

1 CHE 114L: Lab 1 The Chemistry of Baking Soda Preface In the future, the lab instructions you receive will begin with the description of the scientific problem that will be investigated in the forthcoming lab sessions, but, in this first lab we want to discuss the overall structure of your laboratory experience in CHE 114. This lab experience may be different from what you have encountered in the past, and it may differ from your expectations. The structure and content of the labs as well as their underlying teaching methodologies have been chosen to advance the hands-on,student centered philosophy of the labs and the course that they accompany. The core of this teaching philosophy is this: that you will learn best when you are the architects of your own educational experience. It often happens that the experience of students in a college science course goes something like this 1) Students are presented in class with scientific predictions of how the material world will behave and the theory behind these predictions. 2) Students forget what you learn in #1. 3) Students are presented in lab with a detailed cook book about how to verify what they have forgotten in #1. 4) Students go through the motions and complete the cookbook but are unable to relate their lab experience to the classroom material (which they largely have forgotten). 5) Students forget what they did in #4. Later on, students are confronted with problems in their courses and on the job which would be assisted by an understanding of science, but they don t remember it, because they never understood what was going on. In some cases they find out (with considerable difficulty) what they need to know, because human beings are very good at ferreting out information when their goals are clear and when they can appreciate the importance of finding out that information. In many cases, however, people in this situation respond with despair, and consequently disparage the importance of science, because they haven t obtained at some point in their education an idea about how to approach a problem scientifically, much less the basic information about science that they need to support this approach. The purpose of this laboratory (and the course that goes with it) is to short circuit the connection between what is learned in the classroom and your subsequent experiences. This will be accomplished by presenting you with a scientific problem and asking you to investigate it and to report on your investigations. The idea is that you, the student, will acquire the necessary scientific knowledge on an as needed basis. The knowledge so acquired will mean more to you (and you will understand and retain it more effectively) because you needed it to accomplish your goal. You will not be cut adrift in this enterprise; you will get plenty of help. But the overall planning of your work will be up to you. You will often need to obtain information you need from books or from the internet. You are apt to need to perform experiments to test out your ideas. Sometimes you will spend considerable effort pursuing a false line of inquiry. That is part of the game and is all right provided that you are always learning. The particulars of the structure of the lab has been fashioned to encourage this kind of inquiry. Rather than being confined to a single three hour lab session, each laboratory unit will stretch over

2 two weeks or more organized roughly as follows: A) Session 1: Introduction to the ideas of the lab and to the lab apparatus associated with the experiments B) Sessions II and III: Performing the experiments C) Session IV: Presentation of results and discussion In most labs, you are given a brief introduction, you run the experiment, and you go home and write out a report or fill out a form and hand it in next lab. In this course you will spend an entire lab period familiarizing yourself with the theory and the experiment, and being trained to use the equipment. You will be able to plan what you will do with your lab group and come prepared to the next 2 sessions with a plan of action. Finally, you will present and discuss your results with your fellow students and, on the basis of your discussion you will write up your lab report. In addition to the structure of the lab, you will notice some distinctive features in the way in which the material is presented to you and the expectations of that your instructors have of you. In general the lab handout will consist of a single central question and numerous ancillary questions intended to direct you towards the exploration of (usually) several interrelated natural phenomena and their scientific explanations. To the extent possible, the hand out will not pre-prejudice you in a certain direction. It will not tell you, This is what you are supposed to see and here are the reasons you see it, but rather ask you, What did you see and why do you think it happened that way? You will be expected to come prepared to the labs. This means a great deal more than simply reading the lab handout. It means that you will have a fairly detailed plan of what you are going to do. For example, if you want to measure the pressure of a gas, you will be expected to have some idea of how you will do it, and what equipment you will need to do it. You will be encouraged (with supervision) to go beyond the strict definition of the experimental problem and to explore serendipitous lines of thought that diverge from the central problem posed by the lab question. When you ask questions, you may not get the kind of answer you expect. The answer may direct you to an internet site or a book, or the answer might come in the form of a question or may form the prelude of an extended discussion. Of course all of this leaves out the most important factor: you. If you do not buy into the objectives of the course, you will encounter a good deal of frustration and anguish. But if you take a serious and thoughtful approach to the task at hand, you will leave the course with an understanding of chemistry that you had not thought was possible. More important, you will have experienced firsthand the scientific process of inquiry which makes possible the technological and medical advances which we have come to take for granted. Introduction Since this is the first laboratory, there will be a large number of new techniques and instruments that you will have to familiarize yourselves with. Accordingly, this lab will run for 5 sessions. The first will consist of an introduction to the experiment and a review of the instrumentation. The next 3 will be devoted to the performance of the experiment. The last will be for your presentations of your data.

3 It is well known that baking soda (sodium bicarbonate) evolves gas when vinegar (acetic acid) is added to it. Your objective in this experiment is to examine the chemistry of baking soda. In the process you will become familiar with the use of electronic balances, pressure transducers and the time-resolved collection of experimental data. In addition, you will learn a bit about how computers can be used to measure physical quantities and store and analyze experimental data. Your objectives will include the following: A) Study and record the behavior of bicarbonate of soda when it is subjected to acids of various strengths; B) Devise a method to determine the weight lost by bicarbonate of soda when treated by acid, and use this information to determine the percentage of carbon in the compound C) Study the time dependence of the evolution of the gas from the reaction. D) Study the behavior of gases: specifically how does volume depend on pressure and how does pressure depend on temperature? Here are some pointers and some items for you to think about: Item A) We have rated the acidity of the acids for you below in order of decreasing acidity: Concentrated sulfuric Dilute Sulfuric Vinegar Cream of Tartar 7-Up But how do you know that we are telling you the truth? We have told you that carbon dioxide is emitted from the reaction of bicarb with vinegar. How could you confirm that independently? What s up with cream of tartar? You see it occasionally as a recipe ingredient. What is it and why do you use it? Bicarbonate is sometimes called baking soda. What os the difference between baking soda and baking powder? Does bicarbonate evolve gas no matter what you mix it with? Item B) This is the central question of the experiment. How do you go about figuring out how much of something is in a chemical? If I asked you to figure out what percent of a bag of groceries was meat, could you figure it out? Conceptually there is no difference between that question and the one in this lab. Experimentally, however, the question posed in the lab is harder.

4 Item C) Figure out a way to get a graph that shows you how fast the gas is emitted as a function of time. This is similar to what we asked you to do in part A, except part A is qualitative, whereas this is quantitative. Item D) It isn t giving anything away to tell you that the pressure transducer will be useful here. (It might be useful in other areas of the experiment as well.) What are the units used in measuring pressure? Are they the same as the units used in measuring blood pressure? General Comments One question that might occur to you is how much of each chemical should I use? This is a good question but the answer depends on precisely what kind of experiment you will be performing. There is no one correct quantity but there are a few general principles to keep in mind: The amounts must fit your measuring apparatus. So, if you are measuring mass,, the mass must be within the capacity of the scale you are using. Likewise, if you are measuring pressure, you don t want to pop the plunger off the syringe or raise the pressure above the maximum tolerated by the pressure transducer. You must use enough reagent for the reaction to go to completion. By this I mean that if you are trying to find something quantitative out about baking soda, you have to use enough vinegar (for example) to use up all the baking soda. Some of these you can figure out by trial and error. For others you might want to check with your instructor. Experimental Equipment Spot Plates for working with small quantities of chemicals Eyedroppers Beakers, flasks and other laboratory glassware O Haus electronic balances interfaced to Apple Macintosh computers Pressure transducers Syringes for measuring volume Safety Goggles Chemical Reagents Baking soda Baking powder Cream of tartar Dilute sulfuric acid (0.1 Molar) Concentrated sulfuric acid (1.0 Molar) Vinegar 7-Up

5 Your Lab Report Again, because this is the first lab, we want to spend a few words in discussing the format of your lab report. Before we even mention that topic, however, we want to discuss the basis of your report: the lab notebook. Each of you personally (not each working group) must keep your own lab notebook into which you enter all data. The notebook must be bound (wire wound, ring binders, tear out papers are not acceptable.). The easiest and cheapest solution is a marble copybook, which you can purchase for under a dollar at most supermarkets and drug stores at this time of year. It is never acceptable to take down your data onto loose pieces of paper. If you write something out in error, just cross it out. The most important thing about your notebook is that it be accurate. if it looks disorderly, that s OK, just so you can retrieve your data. You should write down your numerical measurements and your qualitative observations. You should date your book and clearly explain to yourself what you were doing when you entered the data. This will come in handy later on when you are trying to make sense out of your data. The only exception to the lab notebook rule is the case in which your data is in the form of hardcopy taken from a computer. Each individual must submit a report which he or she has written individually. Any deviation from this is considered to be plagiarism and will be dealt with in accord with the colleges regulations on academic integrity. You share your data, and you can work together on your calculations, but you must write up your results individually. Your report should be divided into four parts: Introduction Experimental Results Discussion Introduction This section should provide an overview of the lab. It should answer the question: What am I trying to accomplish, and how does this relate to broader scientific questions? Experimental What equipment did you use? What techniques did you use? Don t simply provide me with a list; rather you should provide me with the context within which you utilized these techniques. Results Report both quantitative and qualitative results here. What did you observe? Under what conditions did you observe it? If you have numerical data, it is often useful to organize it using tables. If you did any calculations, this is the place to explain them, perhaps by showing a sample calculation. Discussion Discuss the implications of your data here. What conclusions can you draw from your

6 experiments? Do these conclusions agree or disagree with your expectations. Are they contradicted or confirmed by other published studies? You should also try to discuss the reliability of your data. Are there valid critiques of your technique? Are there assumptions you had to make? Were there some experiments that gave unusual results or contradicted the bulk of your work? How does your study relate to the larger world of science? Did you confirm one or more basic scientific principles? Your Presentation Each group will be required to present their results to the rest of the lab. You must each take turns in the role of the principle presenter. Your presentation will be a 5 minute, lecture style talk given to the other lab members. You may use whatever AV aids that you consider pertinent and helpful. You should present and discuss your results and the be forthright about any doubts and shortcomings you have about your work. In essence, what you are doing is giving a précis of your anticipated results and discussion sections. You should be conversational in your presentation but also professional. You should be prepared to field questions from the floor and you should deal with them graciously. You should be respectful in your demeanor and behavior when others in the group are giving their presentations. Due Date Lab reports will be due one week following the presentation day for a given lab. Lab reports for the last lab will be due on the last day of exam week.