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Tutor profile: Alexis C.

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Alexis C.
Organic Chemistry Postdoc with 9 Years of Teaching Experience
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Questions

Subject: Organic Chemistry

TutorMe
Question:

The rate of the substitution reaction of chlorocyclohexane with sodium cyanide in ethanol increases when a small amount of sodium iodide is added to the solution. Explain this acceleration of rate.

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Alexis C.
Answer:

The iodide ion (from NaI) is both an excellent nucleophile AND leaving group. Substitution on chlorocyclohexane is faster with the iodide ion than cyanide (from NaCN). Substitution by cyanide is much faster with iodocyclohexane, so two reactions involving the iodide ion replace the single, slower reaction (substitution of chlorocyclohexane with cyanide anion), resulting in an overall rate increase.

Subject: Chemistry

TutorMe
Question:

The dipole moments of hydrobromic acid HBr and methyl iodide, CH3I, are 0.80 D and 1.62 D, respectively. a) Which of these substances will have the greatest dipole-dipole attractions among its molecules? b) Which of these substances will have the greatest London dispersion attractions? c) The boiling points of HBr and CH3I are 207.1 K and 315.6 K, respectively. Which substance has the greatest overall attractive forces?

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Alexis C.
Answer:

a) Dipole-dipole attractions increase in magnitude as the dipole moment of the molecule increases. Since methyl iodide has a greater dipole moment, the methyl iodide molecules attract each other by stronger dipole-dipole forces than HBr molecules do. b) The more massive molecule generally has the stronger dispersion attractions, therefore since methyl iodide (152.0 amu) is larger than HBr (80.9 amu), methyl iodide has stronger dispersion forces. c) Because methyl iodide has a higher boiling point, this indicates that more energy is required to overcome attractive forces between methyl iodide molecules. Thus, the total intermolecular attractions are stronger for methyl iodide.

Subject: Basic Chemistry

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Question:

(On uncertainty and measurement) What is the difference between 8.0 and 8.00 g?

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Alexis C.
Answer:

In the two measurements, the number of significant figures are different. Significant figures are all digits of a measured quantity, including the uncertain one (the last digit). The value 8.0 has two significant figures, while 8.00 has three. Since 8.0 has less significant figures, it implies that it also has more uncertainty. A mass of 8.0 g indicates that the mass is between 7.9 and 8.1 g (8.0 +/- 0.1 g). A measurement of 8.00 g indicates that the mass is between 7.99 and 8.01 g (8.00 +/- 0.01 g).

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