Formatting Part 2 Handout 4

Evaluations Ties the different parts of the lab together: purpose, observations, calculation results. Where the author draws conclusions based on the data. Analyzes how error affects results

Types of evaluation summaries Results statement Results statement + error Full write up (won’t do this in this class)

Results statement Short Links purpose to observations to results Supports the purpose; if the purpose were posed as a question, did your results answer that question? Can include procedure

Example Part I: Purpose, results In this lab, I performed and observed five reactions involving copper metal or copper compounds. I explored the concept of conservation of mass by trying to recover as much copper as possible at the end of the experiment. If mass is truly conserved, I should have obtained 100% of my copper metal mass back. I realize though that the experiment has many places where I could lose copper ions, and therefore copper metal. (B) I recovered 70.1 % (m/m) of copper metal. This was not quite the 100% I was looking for, but I realize as I did the experiment, there were many places where I could have lost copper ions.

Example Part 2 Observations and conclusions During this lab, I had the opportunity to observe reactions that were discussed in chapter 4: precipitation, acid-base neutralizations, and redox. I observed that copper metal is oxidized by concentrated nitric acid. The acid reacts with the metal to produce a noxious brown gas, nitrogen monoxide, copper (II) ions, and water. The copper metal lost electrons and was oxidized while the nitrogen in the nitric acid gained electrons and was reduced. The copper(II) ions turned the water blue. Adding base to this solution neutralized the excess acid and made copper(II) hydroxide, a blue gel precipitate. I was interested in the precipitation reaction, because it removed most of the copper ions from solution. Most hydroxide compounds are insoluble in water. Once this solid was isolated, it was dehydrated upon heating to yield a black solid, copper(II) oxide. Copper(II) oxide, a base and a black solid, was neutralized with sulfuric acid to give aqueous copper(II) sulfate, a light blue solution. The copper(II) ions were reduced with elemental magnesium to yield elemental copper. Copper metal is less active than magnesium metal, so it makes sense that the magnesium metal would give up electrons to the copper ions to form copper metal. A side reaction was the formation of hydrogen gas. Magnesium metal is more active than hydrogen, so reacting it with an acid yields hydrogen gas.

Error Evaluation Contains the results statement, AND discusses how error affected the results. For this class, I ask you to pick 1 or 2 errors and discuss how that might affect your results.

Errors in Exp. 2 Experiment 2 had many places where errors would affect the results. Let’s look at some of them

Reading volume The biggest place where an error can occur is the volume. The graduated cylinders are not very precise. Small quantities of air could be trapped around the metal which would make the volume larger than it actually is.

How will this affect density? The density is inversely proportional to the volume. If the volume is bigger than it should be, the density will be smaller than it should be. If I calculated a density of Al to be 3.0g/mL, the error is probably in the volume reading.

How will this affect the predicted volume? In exp. 2, I use the density calculated for the first metal to predict the volume of the second metal. The predicted and measured volumes should be close. IF they are not, it is probably for two reasons: 1) I am using a density that has error, and 2) I measured the volume with a tool that has poor precision for the second piece of metal

Laboratory Assignment for Handout 4-Results Statements and Error Analysis. You have finished Exp. 2 After you finish the lab, think about how you might write a results statement and error analysis for the lab. You will find this handout under Exp. 2 under the laboratory heading

YOU ARE NOT WRITING AN EVALUATION In Laboratory assignment for handout 4, you will evaluate pre-written evaluations. You are not required to write an evaluation for exp. 2.

Perfect? Close to perfect? Close to yucky? None of these evaluations are perfect. Some are better than others. Think about what you did in the lab. Which one of these would you be proud to turn in. Why? Why not for the others?