AP Chemistry Question of the Day --26

Welcome back to my blog. Here comes the answer to question 25 I posted yesterday. The correct answer is C. 

First of all, we have to know that this question is asking about trend in physical properties, boiling point particularly in this case. Then we need to determine what kind of forces is in play. Since both compounds are covalent molecules. It is the strength of intermolecular force that decides the trend in boiling point. Both compounds are non polar compounds. The only intermolecular force in non polar molecules is London Dispersion Force. The strength of the London Dispersion Force is dependent on the molecular weight and the complexity of the compound. In the same series, the higher the molecular weight, the stronger the intermolecular force. When the molecular weight are comparable,  the more complex the molecular structure, the stronger the London Dispersion Force. Thus, the two molecules in question have almost the same molecular weight, nonane has a longer carbon chain, which means a more complex structure will have higher boiling point.

Answer A and B are not correct because during the precess of evaporation, no covalent bond is broken.   Boiling point has no direct relationship with the polarity and strength of the covalent bond. Answer D is not correct because when a compound has higher boiling point, the intermolecular force is stronger which held the molecules closer, not further apart.

Following is Question 26 of the series. 

26.

Based on the data in the tables above, which of the following statements provides the best prediction for the boiling point of NaCl?

(A) NaCl will have a lower boiling point than NaF because the coulombic attractions are weaker in NaCl than in NaF.

(B) NaCl will have a boiling point between that of NaF and MgO because the covalent character of the bonds in NaCl is intermediate between that of MgO and NaF.

(C) NaCl will have a higher boiling point than MgO because the ions are spaced farther apart in NaCl.

(D) NaCl will have a higher boiling point than MgO because the energy required to transfer electrons from the anion to the cation is larger in NaCl than in MgO.

I will post the answer and explanation tomorrow. 

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