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Connor R.
UCSB General Chemistry Tutor
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Physical Chemistry
TutorMe
Question:

Would a CN ion be more stable or less stable than the a nuetral CN molecule?

Connor R.
Answer:

To determine this, the bond order for each molecule must be calculated. The easiest way to do this is to draw the MO diagram and fill in the levels according to the Hund's Rule and the Aufbau Principle. The formula for bond order is the number of electrons in antibonding MO minus the number of electrons in a bonding MO; the difference is then divided by 2. Whichever molecule has the highest bond order is more stable. In this case the CN ion is more stable than the neutral molecule.

Organic Chemistry
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Question:

The pKa of a hydrogen in cyclopentane is greater than 60. The pKa of the same hydrogen on cyclopentadiene is 15. Explain the difference in pKa between the two even though the same hydrogen is being removed?

Connor R.
Answer:

Cyclopentane is a 5 carbon ring made with only single bonds. Cyclopentadiene however is a five carbon ring with two double bonds opposite of eachother. Removing a hydrogen from either molecule results in the creation of a free lone pair, resulting in a negative charge on the molecule. In cyclopentane, this negative charge is localized to where the hydrogen was removed from. This is unstabke and therefore unlikely to occur. In cyclopentadiene, the newly created lone pair the two double bonds make the molecule meet the requirements for aromaticity, so the lone pair can delocalize an d the charge is distributed throughout the molecule. This is much more stable and therefore more likely to occur.

Chemistry
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Question:

The formation of ammonia from nitrogen and hydrogen at 273 K and one atmosphere of pressure occurs spontaneously. 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to form 2 moles of ammonia, which is gaseous. What entropy change can be predicted for this reaction?

Connor R.
Answer:

The entropy change is negative due to the loss of gas particles in the reaction. Entropy is roughly a measure of disorder in the system. The more particles a system has, the greater the potential for disorder. In the reaction that produces ammonia, 4 moles of gas are required to yield 2 moles of gas. Thusly, the reaction lowers the overall amount of gas particles in the system. The less gas particles there are, the less disorder, which results in a reduction of entropy. Therefore, the change in entropy is negative.

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