Promoting Chemistry Insight:
oxidation numbers, formal charges, molecular polarities,
climate change, pH, the checking of calculation results
and the correcting of errors.

For a newer, more extensive and interactive version of this site, please visit:  http://murov.info/oxidation.htm

Steve Murov, Professor Emeritus of Chemistry, murovs@mjc.edu

            This paper is the second of two chemistry education articles that discuss the promotion of the use of insight by chemistry students.  The first article[1] entitled Promoting Insight: Atomic Mass, focused on atomic mass and the information that can be gleaned from its value.  This valuable insight is not commonly discussed in texts but provides useful insight into the isotopes of each element and is worthy of students’ attention.  This paper continues on with the theme that students can often gain valuable insight into chemistry concepts by thinking a little beyond the box.  Applications to oxidation numbers, formal charges, molecular polarities, climate change, pH, the checking of calculation results and the correcting of errors are presented. John Packer, et. al[2] and David DeWit[3] have previously presented examples on some of these topics. 

             Oxidation numbers.  Oxidation numbers are useful as a bookkeeping tool to track electrons and determine if reactions are redox in nature.  While students are often asked to calculate the oxidation states of the central atom in polyatomic ions such as permanganate, chromate, chlorate, perchlorate and nitrate, it should be pointed out to students that the resulting large and perhaps unexpected oxidation numbers should have led to the expectation of strong oxidizing ability.[4]  The high oxidation state and/or positive oxidation states for electronegative elements does not guarantee that the ion will be a good oxidizing agent but just the suspicion that it could be gives the student potentially valuable information and a better feel for chemistry.  Exceptions such as perchlorate can be used to explore thermodynamic vs kinetic reasons for the apparent lack of oxidizing ability.[5]  Hydrogen peroxide, hydrides and Fe₃O₄ are further examples in which unusual oxidation numbers are good predictors of extraordinary behavior.

             Formal charges.  Formal charges are also useful for anticipating unusual chemical behavior.  Most texts suggest the use of formal charges to prioritize resonance structures and to choose between different isomeric structures.[6]  DeWit does point out that formal charges can be used to predict that carbon monoxide should be more reactive than nitrogen.[3]  The formal charge of +1 on the central oxygen of ozone also should lead to expectations of unusual chemistry. Unpaired electrons in compounds are a property where instructors commonly do alert students to the probability of high reactivity.  However, despite the common in depth discussion in texts of the dimerization of NO₂, the reasons for this reaction not going virtually to completion are not usually presented.  Packer, et. al., explain that the two adjacent +1 formal charges on the nitrogens of N₂O₄ make this observation understandable.[2] 

             Molecular polarity.  Understanding the polarity of molecules can go a long way towards understanding and predicting solubility and the outcomes of organic reactions.  The polarities of iodine and CO₂ can be determined from Lewis structures and the prediction made that their solubilities should be low in water.  Ask students to predict the solubility of iodine in water and then hold up a glass bottle containing a saturated iodine solution and ask students if the prediction was correct.  Because the solution is dark amber, they begin to doubt their predictive abilities but this leads to a discussion of the concentration of iodine needed to make the solution dark amber and the realization that the prediction was valid.

            Climate change and pH.  Thinking about the magnitudes of concentrations can also provide valuable insight.  Most people including those with strong opinions on both sides of the global climate debate cannot name the three most abundant gases in dry air.  Many name carbon dioxide among the top three.  It is difficult to understand how people can be so opinionated on global climate change if they do not even know that carbon dioxide ranks number 4 but far down in concentration value from nitrogen, oxygen and argon.  Because carbon dioxide is a minor constituent of the air, human activity has increased its concentration from 280 ppm to over 390 ppm.[7]  Even if people notice the units of ppm, most do not notice that the value is low.  While the consequences of this change on the global climate are difficult to predict with high certainty, understanding that human activity has changed the content of the atmosphere does help when trying to explain the reason for concern. 

            pH is another value that does not receive sufficient attention with regard to absolute values.  When it is stated that neutral aqueous solutions have a pH of 7, how many students think about how low the acid and base concentrations are in neutral water.  Students should be asked how much the addition of 1 drop of 1 M acid or base to 1 L of pure water will change the pH of the water.  Alternatively, ask students how much sodium chloride is needed to prepare 1 L of 1x10^-7 M NaCl and whether the resulting quantity can even be weighed.  It is important that students understand the limitations of balances.  Considering the previous discussion on the low concentration of carbon dioxide in the air, it is also interesting to ask why the pH of freshly distilled water eventually drops below 7.  Strongly related to this concept is the observation that the pH of the oceans has decreased about 0.1 pH unit as a result of the atmospheric carbon dioxide increase and is threatening ocean life and the sustainability of coral reefs.[8]  Thus it is possible to argue that because of ocean threats, air and water pollution, wars etc., even without consideration of climate change consequences, fossil fuels need to be phased out.

It is also important for students to think about the high end of practical concentrations.  Most students have a real problem when asked to calculate the concentration of pure water and most will not have thought about the reason why acids generally do not and cannot exceed about 20 M.

Checking and correcting calculations.  For final topics, the importance of reflecting on the results of calculations and learning from mistakes needs to be emphasized to students.  Too often, students obtain results and then move on without questioning the logic of the result and its implications.  As a carefully selected example, when students are asked to calculate the mass of an antimony atom, some will come out with a result of 7.3x10²⁵g because of the inverted use of Avogadro’s number.  Making this mistake is ok but it is not ok to not notice that the answer is unreasonable as it is close to the mass of the moon.  Calculations should always be followed by the question:  Does the answer make sense?  For example, values of atomic and molecular mass have boundary values and answers out of the logical range should lead to rechecking of the problem.  The simple technique of using insight to determine if the result makes sense can frequently lead to a discovery of a calculation error.  As instructors, we are also often too lax when it comes to encouraging students to learn from their mistakes.  Students need to be encouraged and perhaps required to correct missed problems on tests.  Students have a tendency to merely look at the score and not learn from their mistakes.  Probably more important than a tool for assessing progress in a course, tests should function as learning instruments.  Insight gained from missed problems can go a long way toward making the student more competent in chemistry.  When students realize they have acquired insight in the field of chemistry, they will become the positive learners that you will enjoy teaching in your classes.

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