9/23/13 Weekly Reflection

This week, we focused on stoichiometry, going more in depth and expanding it to more practical uses. We dealt with limiting and excess reactants and also examined empirical formulas and how to convert them to molecular formulas given the necessary mass percents and formula weights. We also looked at yield in the form of theoretical, actual and percent yield problems.The limiting reactant is the reactant that is completely consumed in the reaction - the one that limits the number of times the reaction can occur. The excess reactant is the reactant that is not completely consumed in the reaction - the one that is in excess, that is left over after the reaction is complete. Empirical formulas are formulas that express the number of atoms in relation to one another, with no regard paid to absolute quantities or structures of compounds. Molecular formulas are formulas that represent the number of atoms in a compound. Limiting and excess reactants are very important in stoichiometry, especially when one is actually conducting an experiment given only the quantities of reactant and the chemical reaction. Empirical formulas have thus far only been really important when converting mass percents to molecular formulas, but I'm sure they will have other uses further down the road. In an effort to hone our knowledge of stoichiometry and calculations involving stoichiometry and empirical formulas, we did a series of worksheets and discussed them in class. I found a website that covers this information as well. We also did a couple of other sets of worksheets and then presented our work (both successes and failures) to the class for discussion and analysis.

While I understand most of what was discussed during this week, seeing as most of stoichiometry is just simple calculations, I have been getting really strange answers for the more difficult problems that convert mass percents to molecular formulas. I think most of my problem is with rounding - more when to round and how far, given that we often have to round two or three decimal places off to get the whole numbers we wanted. I found this power point extremely helpful, though. I loved how 'common sense' this section was. It followed a natural progression from masses and mass percents to moles of a substance, then using moles to find an overall ratio, and on to the molecular formula. I tried hard to participate a lot in class this week, especially with presenting the information, even though I'm not good at it. I can only get better from here. I understood this section really well, which does not suprise me, considering the simplicity of this section. I have a few questions that may extend this, although I'm not sure how well they will do so. What is a structural formula and what does it tell you about the structure of a molecule? What other uses are there for an empirical formula?Other slightly less related questions: What determines whether or not two substances will react? Are the materials such as glass used in test tubes potentially dangerous if a chemical reaction is accidentally induced? Can the results to a chemical reaction that hasn't been done before be predicted and if so, how?  I will be puzzling over these and the relationships between various different molecules, especially relationships in hundreds of unknown chemical reactions, for a while.