Understanding Concepts Taught Commonly across Science
Oxidation reduction (redox) reactions and elevation of boiling point are two key topics discussed here, but concerns occur whenever fundamental units are changed (like from imperial to metric). When teaching concepts change approach, care is needed to ensure there are no errors in revised formats.
Undergraduate texts like Vogal and Ewing taught redox reactions and by convention prior to the 1970s, authors chose reduction equations (Mx+ ---> M(x-n)+ + ne). However, the associated Nernst equation taught was in the form E = Eo + 0.0591/n*log [ox]/[red] with Eo being the standard potential measured for molar solutions. After that time, analytical chemistry texts (eg by Skoog and West or Christian) rearranged the Nernst equation to E = Eo - 0.0591/n*log [red]/[ox]. Students, if taught differently in other related subjects naturally became confused. Why the change? Likely, the change intended to unify the form of redox equations to similar equations used for other chemical reactions. In acid/base, precipitation, and complexometric reactions; all used the convention in related equilibrium equations by placing [products] over [reactants]. Teachers in different science disciplines need to agree topics are taught uniformly.
Thermodynamic laws predict that higher forms of energy progressively degrade to heat. Furthermore, heat from objects at higher temperature flows only from higher to lower. These becomes entrenched as a key for viewing normal flow of energy. To reverse the direction, work needs to be done with an external supply for additional energy, like compression for gases or by use of a heat pump. For the elevation of boiling point experiment in my undergraduate years in the 1960s, temperature increases were measured simply when a solute was added directly a flask of boiling water. Solutions of the same molarity gave the same temperature increase above pure water. However, when demonstrating this effect to physical chemistry students during a postdoc in the UK, they had a more complicated apparatus with two flasks (see figure). Pure water with a boiling point of 100oC fed steam to a second flask which contained the solute and this went up in temperature. I asked the class’s lecturer; “How does steam at 100 degrees create this higher temperature?” The reply given was an unhelpful simple comment like “That’s right!” As the 2nd flask was open to atmosphere, compression was not involved nor any input work apparent. Later when lecturing myself, I asked other teaching staff how the UK apparatus achieved the observed unexpected higher temperature. Their initial response was “That is not possible!” Now, what advantage would the more complicated two flask set up have? Possibly this was put together to eliminate loss of water as steam when only one flask is used, which would progressively increase the solute concentration by unknown amounts. In contrast when using two flasks, for the 2nd flask; steam in = steam out and the solute’s concentration is less affected. This still doesn’t explain the observed increase in temperature, which needs one to consider dynamics. When steam condenses, it releases a high latent heat (540 calories per gram of water). Steam will therefore condense in the 2nd flask containing the solute until its vapour pressure reaches a different equilibrium. The solution only reaches steam in = steam out at a higher temperature. The setup has the advantage that the concentration of solute is no longer time dependent since steam is replaced in the 2nd flask. In this way steam at 100oC makes a dissolved salt solution hotter.
References
- Christian, G. D. (2004). Analytical Chemistry (6th ed.). John Wiley & Sons.
- Ewing, G. W. (1960). Instrumental Methods of Chemical Analysis (2nd ed.). McGraw-Hill.
- Skoog, D. A. (1974). Fundamentals of analytical chemistry (2nd ed.). Saunders College Pub.
- Vogel, A. I. (1959). A text-book of Quantitative Inorganic Analysis including elementary instrumental analysis (3rd ed.). Longmans.
|
|
Dr Ray Hodges FRACI (retired Associate Professor from Monash Gippsland)
|
#Feature #ChemistryArticle