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2011 Final Exam.doc

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CHEM 1001

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Final Examination December 2011CHEM 1001 A and TR BurkPage 1 of 12Part A Answer all twelve questions with a few sentences or equations 5 marks each1 State Daltons law of partial pressuresDaltons law of partial pressures states that the total pressure of a mixture of gases is the sum of their partial pressures2 How does NO act as a catalyst in the production of ozone at ground level Show the relevant reactionsgNO ONOg2g2gNOhNOO2gggOOOg2g3gSince NO is destroyed in the first step but regenerated in the second step it is acting as a catalystg3Why does a bomb calorimeter actually measure DE and not DH of a reactionIn a bomb calorimeter the volume is constant so no work is done DH is measured only under constant pressure conditions like in a coffee cup calorimeter4 State Hunds ruleHunds rule states that electrons will occupy degenerate orbitals so as to be as unpaired as possible5 Why is a substance with a large bandgap likely to be transparentIf the bandgap is large then lower energy photons such as those in the visible portion of the spectrum do not have enough energy to excite electrons from the HOMO to the LUMO These visible photons are therefore not absorbed and are transmitted by the substance causing it to be transparent clear6 Why does CO have a much lower boiling point than NO22CO is a linear molecule and is not polar and therefore experiences only dispersion forces with other 2CO molecules NO is bent and polar and therefore interacts with other NO molecules via a relatively 222strong dipoledipole force This means that it will require more thermal energy to break free from these intermolecular forces ie the boiling point will be higher7 Why is hexane CH not soluble in water614Hexane has only dispersion forces Water has Hbonds The two types of forces are very different and simply put like dissolves like These two are not alike so the solubility is very low 8Why is the freezing point depression of a concentrated solution of NaCl not as large as what is aqpredicted by DTiKbbfFreezing point depression is a colligative property and therefore depends on the amount of solute in solution In a concentrated salt solution there is a lot of ion pairing resulting in effectively fewer particles in solution hence less depression of the freezing point 9The solubility of LiSO decreases as the temperature increases Why24sFinal Examination December 2011CHEM 1001 A and TR BurkPage 2 of 12Apparently the process of dissolving LiSO is exothermic This is because hydrating the Li ion and the 242SO ion releases more energy than separating these ions from one another in the solid crystal In other 4words the hydration energy is greater than the lattice energy10What are enantiomersThese are molecules that are nonsuperimposable mirror images of one another11What is wrong with the name 134trifluorobenzene for the molecule shown below What is the correct nameNumbering beginning at the bottom we can use lower numbers 124trifluorobenzene12How do we know that CHBrCl is achiral 2To be chiral the central atom carbon in this case must be bonded to four different groups In this molecule it is bound to only three different atoms H Br and Cl so it is achiral
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