Exam I Feedback for Students

I just completed the grading for exam I and I wanted to summarize my feedback here on the blog. Hopefully this will allow you to look at these issues with your textbook in hand. I will also mention them during class- we can do some examples as a class.

Overall the average was in the mid 60s but this is because there were a few really low scores that really pulled it down. There were 6 people who scored above 90%. One person got a 98%. So- it was a manageable test. I had a good number of people in the 80 range and a few in the 70s. There were also a good chunk below 70 and this concerns me. My goal is to work hard with each of you that scored in this range so you can bring up your scores. Your homework/quiz/activity grade will help you bring your score up somewhat but you need to concentrate on performance on the tests to really do well in the class (A or B grade).

Several of you still struggle with the concepts of conversion, units and volume/density. There are a few of you that only struggled with this concept- if you had answered these questions correctly you would have been above 90% on the test. So..... keep these things in mind: Zeroes that occur before the decimal place or after the decimal place but do not follow a digit that is 1-9 are not considered significant. Therefore the number 0.00347 has only 3 significant digits. The three digits preceding the "347" are not considered significant. The number 0.03470 has 4 significant digits. The last zero is considered significant because although it falls on the right of the decimal, it follows a digit that is 1-9 and is therefore considered significant. Everybody needs to keep in mind that I consider it crucial for you to show your calculations and logic in the short answer section. I CANNOT award partial credit for an incorrect answer if you do not carefully show me your mathematics progression. I wanted problem 44 and 45 to be an easy 20 points for everyone. These were the density/mass/volume conversions and the volume conversion problems. Several of you got an answer that was on the right track but you did not receive partial credit because you did not show your calculations.
Keep in mind that while a cm3 (superscript 3) is equal to a mL, it is a dm3 (superscript 3) that is equal to a Liter. There are two ways to solve this problem (convert to dm3 or just convert from mL to Liter with the conversion 1000 mL/L) Several of you need to review your basic conversion factors- I gave you a table of all of the factors but people did not use this information correctly. I had people converting cm into meters with the conversion factor 0.01cm/m. It is opposite: 100 cm/meter. Review how these are used in word problems.
I wanted people to notice how I put a problem on the test that was an example we worked in class. Actually- this problem was in both the multiple choice section and the calculation section: You were supposed to figure out the relative atomic mass of an element based on two isotopes found in nature. There were too many people who missed this problem. I was hoping you review lecture notes and examples from class so you can get these types of problems correct.

I was very pleased with the knowledge of the class about atomic theory. The last problem was supposed to be an easy 5 points for everyone. Most people got 4/5. The two modifications to Dalton in Modern Atomic Theory are as follows:  1. Discovery of neutrons allows atoms of the same element to have different masses 2. Atoms are not the smallest building blocks of nature due to the discovery of subatomic particles. (Some of you stated that H2O and H2O2 prove that the law of definite proportions is not true and this is false. The law of definite proportions just states that compounds always contain the same ratio of elements. It doesn't mean that the ratios can't vary between compounds  (which they do only in whole number values))

Problem number 10 was a commonly missed question dealing with the law of definite proportions. The ratio of sulfur to oxygen in SO3 is 32:48.  This is obtained by multiplying the atomic mass of sulfur ( 32) by 1 (for one atom of sulfur)  (from the PT) and the atomic mass of oxygen (16) by 3 (from the PT). The mass ratio of sulfur to oxygen in any sample of SO3 will always be 32:48 by the law of definite proportions.

I'll post the key and talk more about it in class.

The Statue of Liberty, Stoichiometry and introductory chemistry.....

(Photo taken from the Introduction to Chemistry text by Bauer, 2010)



What does the statue of liberty in New York have to do with introductory chemistry? Actually, a lot. On the most basic level, the reason it was renovated in the 80s is very related to introductory chemistry. If we want to present the reactions of the statue of liberty accurately then we must consider stoichiometry and balanced equations.

The statue had copper panels on the outside and iron used in the inner framework. Over time, the copper oxidized in air to form copper (II) hydroxide, copper (II) carbonate, and copper (II) oxide while the iron formed iron (III) oxide. This created a displacement reaction situation. Copper metal was in contact with iron (III) ions and iron in contact with copper (II) ions.  Here is where the problem got really messy:

Fe (s)  +   CuO(s)   gives  Cu(s)  +   FeO(s)  (oh how I wish I could figure out subscripts and arrows in blogger.com)

According to the metal activity chart is Fe more active than copper? Yes, therefore it will displace copper's position within the copper (II) oxide compound and create an iron oxide.

This displacement further exacerbated the oxidation problem. In fact it accelerated the problem by a factor of 1000 and the statue needed to be rebuilt.

See page 179 of your text for more information.

Most aspects of e-waste not regulated in U.S., Va. | Richmond Times-Dispatch

Most aspects of e-waste not regulated in U.S., Va. | Richmond Times-Dispatch

This is exactly why we need excellent chemistry education. Many of the hazards of chemicals in this waste dump are not understood. The long term decay and byproducts of waste are often not understood. It makes sense that as technology and materials change, our knowledge about them should increase. This includes not only how they function to improve our lives but how they function in their return to the ashes of the earth. "Ashes to ashes and dust to dust."  Unfortunately, in this context the route to ashes could be a problem.

where I left off.......preparing for exam I

So anyway, let me continue.......

Dalton's atomic theory was tweaked over time (as we like to say today). He originally said that atoms were the most basic unit of nature and that it was the smallest unit of matter. This is both true and not quite true today. It is true that in a chemical reaction, the smallest unit  is an atom. We look at whole number of atoms per formula unit. You cannot have a formula unit with half of an atom of any element.  However, it is not quite true in that subatomic particles exist. The smallest unit is, I think, yet to be discovered. However, the subatomic particles we study in this class are the proton, neutron and electron which I will talk about in a minute.

The other part of Dalton's theory that needed to be tweaked was his idea that all atoms of the same element have identical mass and chemical properties. This is not true because of the notion of neutrons (of which Dalton was not aware). Neutrons alter the mass of the nucleus and the overall atom. We now treat the mass of any given element on the periodic table as a representation of the relative abundance of that element in nature. 

Protons, neutrons and electrons and why they matter.....
Protons are the crux of all chemistry because they define the identity of an element. The number of protons is like someone's last name. It identifies their characteristics. If something has 6 protons it is carbon regardless of whether or not it contains neutrons or how many neutrons it contains. The number of protons defines the type element you are dealing with: other subatomic particles dictate the variety of that particular element.

Electrons are the subatomic particle that gives flavor to the element. Is it positively or negatively charged? This will depend on the ration of protons to electrons. Naturally something with 11 protons (Na) and 10 electrons will have a 1+ charge. This is very useful to understand in terms of formulating compounds that have an overall neutral charge.  We talked a bit about how JJ Thompson discovered the electron with a cathode ray tube and then Millikin came along and measured the mass and charge of the electron with the oil drop experiment. Ultimately, it became clear that the electron was exponentially smaller than the proton/neutron.  For this reason we discount its mass when calculating mass values on the periodic table.

Neutrons are their own category of subatomic particle that provide neither overall charge nor chemical identity to an element. Neutrons influence mass value only. Remember that we performed several relative mass value calculations in class to find relative atomic mass values reported on the periodic table.

The plum pudding model of the atom was eventually debunked by Rutherford who performed the gold-foil experiment to show that the nucleus of the atom contained most of the mass of the atom surrounding by a cloud of tiny electrons.

We talked about different notation systems to show number of protons, neutrons and mass value for various elements. Know the various notation systems and be able to use them on a test.

What are isotopes? Isotopes are different forms of an element that differ only in number of neutrons. Typically radioactive isotops are found in nature in small amounts and used as tools for radiometric dating and other scientific tools.

Once you understand that the periodic table is arranged by atomic number (# of protons) and you understand where the mass value comes from you are ready to start understanding how different columns/families tend to form different ions. Groups 1,2,3 tend to form 1+, 2+, 3+ charges respectively. The anions on the right side tend to form minus charges relative to how far they are located from group 8 on the periodic table. Why? Each group wants to gain the number of electrons that is identical to the noble gases. Once that magical "octet" of electrons is formed the ion is in its most stable state.

Familiarize yourself with the names of the groups in the periodic table. Know the halogens, alkali metals, alkaline earth metals and the noble gases. Know how to determine charge of an ion based on position in the periodic table.

Know your periodic table trends. Where are the metals? Where are the nonmetals? Can you identify chemical properties just by looking at position on the periodic table?

Chapter 3:
The key to chapter 3 is knowing how to classify a compound by its general appearance. For example, can you classify the following as molecular, ionic or acid?

HBr      HCl03,      NaCl,            CuO,                     Fe2O3,            NO2
Forgive me, I haven't figured out how to get a subscript to appear as a subscript in blogger. Perhaps that element of sophistication will evolve over time.

The answers are as follows:  binary acid,   polyatomic acid,    ionic,   ionic (variable charge),   ionic (variable charge), molecular

Now you use the different categories to use rules relevant to naming in that category.
Binary Acids: Start with "hydro" and add name of anion (bromine) with "ic" on the end: hydrobromic acid
polyatomic acids: Use name of polyatomic (chlorate) with "ic" and acid. chloric acid
Ionic compound (one charge): Sodium is group I and these always form +1 charge. Name all ionic compounds with "ide" ending: sodium chloride
Ionic Compound (variable charge for transition metals): Copper (name metal) (II) oxide (name charge with roman numeral and anion with "ide")
ionic compound (variable charge): Iron (name metal) (III) [charge of metal] and then "oxide" always an "ide" ending for ionic compounds
NO2: two nonmetals make a molecular compound. Use prefixes for second element (not necessary for first) nitrogen dioxide  (It would be nitrogen monoxide with only one oxygen)

That, in a nutshell is chapter 1-3 of the text!

Braindump for Exam I: What do I need to know?

So today several people stayed late after class worried about the test next week. I'm happy to see that you care about the class so much! This is an encouraging sign to me that you are serious about your work and that you will succeed in this class!

However, the question is always more geared towards this:  "How can I possibly get an A in this class? I've studied really hard and I'm still getting 6/10 on my quizzes and I don't understand the labs...." 

To you who feel this way I sympathize. It was not long ago I felt very similarly. The encouraging part of this is that it will get better. Continue to review lecture notes, try to form study groups with each other and work as many practice problems as you can from the book. Eventually the vocabular will begin to sink in and you will see the many patterns and all of the repetition that exists in the material. It is there. You just may not be able to see it yet.

So- I am leaving for a family reunion and want to give my last review for our test on Thurs on ch 1-3. I figured this would be a great blog post not to mention a great way to recruit more traffic on my site. Not only do you get 1-2 points of extra credit for a pithy comment but you get extra exam review also! What a deal.

So the crux of chapters 1-3 goes like this: (Credit to the Introductory Chemistry text by Bauer here)

Chapter 1 is focused primarily on the definition of science, explanation of chemistry and the breakdown of matter. What is chemistry and how does it differ from other forms of academic thought? The biggest difference between science/chemistry and other forms of nonscientific thought is that science uses experimentation to test out ideas. The philosophers during the middle ages were excellent at reasoning: Plato and Socrates formulated ideas from which an entire family tree of philosophers descended, however, they never tested out any of their ideas experimentally. So- while many disciplines use logic, science is special in its use of experimentation.

The definition and classification of matter is something you should review. What is the difference between matter and energy? Can you name examples of each?

Matter is subdivided into subcategories of substance vs mixture. Substances are pure in that they always have uniform composition with the same chemical and physical properties throughout. Only elements and compounds quality as pure substances. Mixtures are more tricky. A mixture is, by definition, something that contains multiple pure substances fixed in variable proportions. So, a mixture may or may not have the same composition throughout. For this reason, a subcategory has been established that differentiates between homogeneous and heterogeneous mixtures. Homogeneous mixtures (like solutions) are the same throughout while heterogeneous mixtures have a variable composition throughout. A mixture can always be separated into its different chemical components by some type of physical separation. (Filtration, evaporation, decantation, distillation, extraction, etc.)

This brings us to the discussion of states of matter. Matter can change forms without changing chemical composition. The different physical forms of matter form different states: solid, liquid and gas. These each have specific definitions and descriptions.

Energy is the ability to do work. There are all types of energies out there but the two main kinds are kinetic and potential. Within these two subdivisions are different subtypes like chemical energy, electric energy, heat energy, thermal energy, elastic energy, and many others. It is a good exercise to imagine a moving object and label the type of energy it has in various stages of movement.

Chapter 1 also covers the scientific method. The scientific method is a way of asking questions about the physical world and implementing a system of inquiry to answer those questions. Whether you hypothesize about a potential outcome or actually formulate scientific laws and theories on well-documented data, you employ the process. Review the different terms of the scientific method and familiarize yourself with them.

Chapter 1 introduces the periodic table and the classification of metals, nonmetals and metalloids. Each category embodies some similar properties of its element type. What are these properties and which elements fit into each? How would you determine this just by looking at the periodic table?

And finally conversions, significant digits, rounding, scientific notation and the entire math toolbox in chapter 1 will be critical to your success throughout this class and in the next class, Chemistry 1A. I recommend you master the entire toolbox for maximum learning.

Chapter 2 is my personal favorite out of ch 1,2 and 3 because it delves into the history of science. I personally believe our nation would be much more scientifically savvy if our education system required everybody to study the history of science in chronology with the actual content of science classes. I actually ran into someone today who knew of a program at Harvey Mudd that tried to implement such a system. Apparently it is too time consuming for students' schedules to make it work effectively. Interesting idea, however.

Chapter 2 follows the initial idea about atoms from Democritus (Greek philosopher) in 450 BC. From here, atoms were born. Dalton formulated his atomic theory based on the ideas of Proust (law of definite proportions), Lavoisier (conservation of matter) and other information to form the four tenants of atomic theory. Note that this theory is our first exposure to a scientific theory in the chemistry text. Why is it a theory? Because it provides a broad definition of nature that incorporates scientific laws (Proust's and Lavoisier's) but it goes further to suggest something that is true of all of nature. And, like any theory it can be modified and changed as further evidence becomes available.

I will post more as another entry.

law of definite proportions

As I was preparing yet another lecture for my introductory chemistry class I realized something: this class is all about grasping the law of definite proportions. Once the fundamental concept of relationships in chemical quantities is realized and accepted, students can propel themselves into much bigger and sophisticated equations and chemical concepts.

What is the law of definite proportions? Let's go to my friend Wikipedia for a brief description. (I realize some readers may have an aversion to Wikipedia but I go there when my textbook is otherwise preoccupied with another instructor as it is currently.) Wikipedia is generally correct although I will admit there are some QA issues with the site.

"In chemistry, the law of definite proportions and also the elements, sometimes called Proust's Law, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example, oxygen makes up ⁸/₉ of the mass of any sample of pure water, while hydrogen makes up the remaining ¹/₉ of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.^"




This little paragraph nicely describes my overall definition of the law of definite proportions.

This scientific law was incorporated into Dalton's Atomic Theory in the early 1800s. Now it probably just blends in as a fundamental tenet of atomic theory, I doubt most people know it originated with Proust.

While Atomic Theory (especially modern atomic theory) is important for introductory chemistry, the class can be defined at a more fundamental level as applicable to the law of definite proportions.

Why? Here are the reasons: The law of definite proportions explains why the following are true.

1. The relationships between amu mass/molecule is the same as the number of grams in one mole of something. A strange and difficult relationship to swallow especially when it would seem experimentally impossible to prove this. I'm not really sure how they did it. (If anybody reads this and knows please fill me in)

2. The manipulation of numbers between grams, moles and particles depends on the relationships in #1 (previous bullet). Without these relationships, the theoretical yield of a chemical reaction cannot be calculated and compared with the actual experimental value. Hence, without this concept there is no percent yield calculation.

4. While the rest of atomic theory is critical to chemistry it does not fundamentally affect calculations (for which we grade the students). 
5. None of the other tenets of the theory allow for such simple yet profound manipulation of the experimental data to give predictions, results and information!



Here is the Wikipedia definition for law of multiple proportions:
If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.^[4]
For example carbon oxide: CO and CO₂, 100 grams of carbon may react with 133 grams of oxygen to produce carbon monoxide, or with 266 grams of oxygen to produce carbon dioxide. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 ≈ 2:1, a ratio of small whole numbers. The Law of Multiple Proportions, is just what the name suggests, the law of multiple proportions of one constant element within differing compounds sharing the same type of chemical bonding.

Last semester our learning objectives were to:
1. Calculate and predict theoretical yield in a chemical equation. Use this with experimental data to calculate percent yield.
2. Use Lewis theory to build molecules and predict shapes. Use shapes to predict molecular behavior.
3. Predict behavior of atoms based on periodic table trends

#1 is by far the most important components of this class.

Ideas about learning: Jeanne Baxtresser and flute performance

Recently I've been listening to the former principal flute player of the New York Philharmonic on my ipod while I take a morning walk. She recorded all of the major orchestral flute solos with a commentary about each one. A virtuoso, a genius, an accomplished artist. She is all of these.

One of the solos is by Prokofiev from Peter and the Wolf. As I listened to it perpetually in awe of Ms. Baxtresser's abilities I realized something. Peter and the Wolf was my very first piano book when I was five. Newly graduated from the clapping classes of babies I had progressed into simple melodies of the treble clef. Peter and the Wolf was the introductory piano reduction my early teacher had chosen.

How is it that the difficult music demonstrated by the virtuoso Jeanne Baxtresser could also have been my initial baby food as a tiny pianist? This is the concept of levels that I'm learning in my chemistry teaching.

Everything has levels. The challenge is to find the level that relates best with the audience to whom you are addressing. In the case of flute/piano music, there is no way a five-year-old could relate to the difficult double-tonguing passages of the full-fledged Prokofiev piece. No way. However, the simple melodies are appropriate for learning how to read music. 

Perhaps I am pointing out something that is incredibly obvious. However, on some level it must not be obvious to everyone. I think this is why so many people find introductory science classes so very difficult and boring. We are not presenting students with material that they are equipped to grasp at an appropriate level. We throw Jeanne Baxtresser's virtuosity at them without ever having fed them the piano reductions that so simply and beautifully embody the melody on the five black keys of the treble clef of the piano.

For this reason, context and culture is really critical to the betterment of science in society. People must understand the concepts of chemistry and physics in terms of how it makes sense for their everyday life. After they understand it in everyday life perhaps it might motivate them to memorize chemical mechanisms, difficult vocabulary and other mundane flashcard activities. (I promote these to the hilt.)