Answer the following questions in terms of principles of chemical bonding and intermolecular forces. — Intermolecular Forces Chemistry Question
Problem Context
Answer the following questions in terms of principles of chemical bonding and intermolecular forces. In each explanation where a comparison is to be made, a complete answer must include a discussion of both substances. The following complete Lewis electron-dot diagrams may be useful in answering parts of this question.
[VISUAL]
Model Answer
Both molecules are nonpolar, and the only intermolecular forces in each are London dispersion forces. However, pentane is larger than propane and has a more extensive electron cloud that can be involved in a greater number of London interactions, leading to stronger intermolecular attractions overall. Thus it takes a higher temperature for pentane molecules to have enough kinetic energy (on average) to overcome their stronger intermolecular attractions, thus pentane has the higher boiling point.
One point is earned for recognizing that pentane and propane both have only LDF’s.
One point is earned for recognizing that pentane has a larger electron cloud with greater IMF’s and higher BP (reference to state of matter).
Model Answer
Propane molecules are nonpolar and only interact with one another via London dispersion forces. Methanol molecules are polar and hydrogen bonds (as well as London forces) can form among them. Because hydrogen bonds are stronger than London forces, methanol has greater intermolecular attractions. Thus it takes a higher temperature for methanol molecules to have enough kinetic energy (on average) to overcome their stronger intermolecular attractions, thus methanol has the higher boiling point.
One point is earned for recognizing that methanol has hydrogen-bonding IMF’s and propane has LDF’s.
One point is earned for recognizing that hydrogen-bonding results in a greater IMF causing methanol to be a liquid.
Model Answer
sp3
One point is earned for the correct hybridization. || sp2
One point is earned for the correct hybridization.
Model Answer
One point is earned for the correct diagram (all electron pairs must be included).
Expected structure: H3C-CH2-COOH showing all single bonds, the C=O double bond, and two lone pairs on each of the two oxygen atoms (all octets complete).
Model Answer
Resonance structures can be drawn for the methanoate anion, with one carbon-oxygen bond as a single bond and the other as a double bond (and the opposite in the other structure). The electrons are shared equally between the two carbon-oxygen bonds in the methanoate anion, resulting in two bonds with the same length.
One point is earned for the correct explanation. || The two identical carbon-oxygen bonds in the methanoate anion each have a bond order of 1.5. The carbon-oxygen bond in methanol is a single bond, and the carbon-oxygen bond in methanal is a double bond. Single bonds between atoms of the same two elements are longer than double bonds, and a bond with a bond order of 1.5 would have a bond length between that of a single bond and that of a double bond.
One point is earned for the correct explanation comparing the single, double and resonating bonds of the respective molecules.