Final Exam Review #12 and #31

They say that all good things must come to an end, and so it seems as summer draws so near and this school year draws to a close. Yet, at the end of any school year are the dreaded final exams, late nights of studying, and hours spent reviewing all that we have learned throughout the aforementioned year. Then there are the exam reviews. Though helpful, these lengthy packets often take extended lengths of time to complete, unless there is collaboration. So lets split it up!!


How about one on colligative properties?


12) In a solution of water, what would decrease the freezing point the most: adding 1.00 mole of aluminum nitrate, adding 3.00 moles of sugar, C₆H₁₂O₆, or adding 2.00 moles of magnesium nitrate? Please explain why this freezing point depression occurs and why you selected the solute you did.

Colligative properties are those that are dependent upon the number of particles in a solution, not the type of particles. There are three main colligative properties of solutions: boiling point elevation, freezing point depression, and vapor-pressure lowering. They all have their roots in that fact that these foreign particles disturb the order and function of the native particles of the solvent.

This is a graph of the differences in the relations of pressure and temperature for pure water versus water with a dissolved solute in it.

Now lets look specifically at freezing point depression!

For a substance to freeze, the individual molecules must be able to arrange themselves in an orderly manner (to form an often crystalline structure) and bond. When foreign molecules/atoms are put into the mix however, they disrupt this orderly arrangement and make it more difficult for the particles of the solvent to make the needed bonds with each other to solidify. For the solution to freeze it must be at a lower temperature where the particles have less kinetic energy.

Recall that we are talking about colligative properties, characteristics that are dependent upon the number of solute particles in a solvent, not they type or size of these particles. Thus, as the number of solute molecules/atoms that are added to the solvent increase, the freezing point will decrease. In this manner, the answer boils down to the question of which solute, when dissolved in water, will create the largest number of specific particles:

• 1.00 mol of Al(NO₃)₃ - According to the solubility rules, aluminum nitrate is soluble and will dissociate in water yielding Al³⁺ and 3 NO₃^-, or 4.00 mol of particles. 

• 3.00 mol of sugar - Sugar is a general name for a variety of organic molecules, most commonly sucrose (C₁₂H₂₂O₁₁) and glucose (C₆H₁₂O₆).  Both of these are covalently bonded and will not dissociate. Thus, 3.00 mol of sugar will yield 3.00 mol of particles.

• 2.00 mol of Mg(NO₃)₂ - Magnesium nitrate is also soluble and will dissociate in water to yield Mg²⁺ and 2 NO₃^-.  Thus, 2.00 mol of Mg(NO₃)₂ will yield 6.00 mol of particles.

The 2.00 mol of Mg(NO3)2 will cause the largest freezing point depression.


31) Did we cover it all? Think of a topic or question from this past trimester that you think should have been covered more by this review, and respond to it.

For me, one topic, or better, on focus of a topic that I believe the review covered only briefly was a some of the special cases regarding decomposition reactions. As these cases are less common than that of acid decomposition or salt decomposition, they are often put to the side and forgotten. Here are the special cases of decomposition reactions:

• Metal halate --> metal halide + oxygen gas
• Cu(BrO₃)₂ --> CuBr₂ + 3O₂ 

• Metal peroxide --> metal oxide + oxygen gas
•  2MgO₂ --> 2MgO + O₂ 

• Metal carbonate --> metal oxide + carbon dioxide gas
• Ag₂CO₃ --> Ag₂O + CO₂   

Hopefully this can clear a few things up. Thanks!!! and Good Luck!!!

Image Citations: 
http://www.chem.queensu.ca/people/faculty/mombourquette/firstyrchem/colligative/index.htm 
http://theslolane.wordpress.com/2009/05/11/stephens-big-day/

Entire Honors Chemistry Midterm Exam Review

For anyone who would like a nice review and description of many basic topics of chemistry that I covered the first trimester, just click on the link below to download a PDF with these many topics.  It is our Midterm Exam Review that we completed before taking our test.  Thanks!!!

Honors Chemistry 2010 Midterm Exam Review

Midterm Review Question #24 - Electron Affinities

Oh, exam week!!! The wonderful weeks in each year were you review what you have learned (and possibly relearn a few things as well)!!! Well, at least our teachers diligently create thorough reviews to make the study process easier.
Here is the answer and explanation of question number 24 of our midterm review:

24) Which element on the periodic table should have the most favorable electron affinity? Which element would have the most negative electron affinity value? Explain your choices.

Essentially, these two questions are asking the same thing.  The element that has the most favorable electron affinity is actually the element that has the most negative electron affinity value.  The electron affinity value measures how much energy is released when an atom gains an electron or the amount of energy required for an atom to gain an electron.  The more negative the electron affinity value of the element is (the more favorable electron affinity), the greater the amount of energy an atom releases when it gains an electron.  Positive electron affinity values signify that for the atom to gain an electron, energy must be added to the atom (this arrangement is very unstable and when the application of energy is ceased, the atom will expel the electron and return to a lower energy level).

The image above is a 3D graph of the electron affinity values of the various elements of the periodic table.  As can be seen, chlorine has the most favorable electron affinity, just a bit more favorable than fluorine, the electronegative element.

The reason that chlorine (Cl) is the element with the highest electron affinity (the most negative value) is very similar to the reason that fluorine is the most electronegative element.  As it is in Group 17, chlorine is longing to obtain the electron configuration of the noble gas argon by gaining one electron and becoming a 1+ cation.  It has a strong attraction to electrons due to its strong positive charge in its nucleus (compared to the other elements in its period).  Furthermore, as chlorine is relatively high in the periodic table (it is in one of the lesser periods), there are fewer orbital “shells” shielding free electrons from the positive attraction of the nucleus.  Thus, a large amount of energy is released when chlorine gains an electron.

Interestingly, the reason that chlorine has a larger electron affinity than fluorine, even though fluorine is the most electronegative, is due to the fact that fluorine is a smaller atom than chlorine.  When an electron is added to fluorine, there is a greater repulsion between the electrons in the orbitals than there is with chlorine.  Thus, even though fluorine might be more electronegative than chlorine, chlorine has a more favorable electron affinity.

Attributions:
Electron Affinity 3D Graph: http://www.webelements.com/periodicity/electron_affinity/cityscape_chart.html