Tuesday, May 17, 2011

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!!!

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