Question

Can you explain the pre-lap questions in detail. Thank you

Pre-lab Questions 1. Draw the Lewis structures for the following: (a) pentane and (b) 1-octanol (Please note "q" indicates the alcohol group (-OH) is on the first carbon.) 2. Determine the types(s) of intermolecular forces involved in each of the above substances. 3. Predict which liquid - pentane or 1-octanol - is more volatile. Assume the two liquids are at the Same temperature. Explain your answer. 4. Predict which liquid - pentane or 1-octanol - has the higher boiling point. Explain your answer.

Answer 1

1. The lewis structures are given below-

Lewis structure a) Pentane ни н н он , с. с . с. : с . С -н н ч н и н н . . н. н. н н Все сееееее = --с-с-с-с-н н н и н н и н н н н н н н 6) uc cicicicic: c.c:oH 1-octanal - н н н н н н н н ?н н н н и н и -e-c - - - - - c -o- и , и н н н и н и

2. The intermolecular forces between the molecules of a compound determines its physical properties. Intermolecular forces arise as a result of charge imbalance. In the alkanes, a significant electronegativity difference between carbon and hydrogen is absent. Hence the bond polarity of C-H bond is very low. Due to low bond polarity molecules have low dipole moment. The only two intermolecular forces that arises in alkanes are Vanderwaal forces of attraction and induced dipole-induced dipole type of interaction. This interactions are very weak.

Alcohols also have Van Der Waals' intermolecular attractive forces like alkanes. But, they also have an extra intermolecular force of attraction due to the presence of the hydroxyl group (O—H). This is called hydrogen bonding.
Oxygen is a highly electronegative atom and pulls the electrons in the O—H bonds towards itself. So there develops a net positive charge on the hydrogen atom and a net negative charge on the oxygen atom. Thus there is an imbalance of charge across the O-H group. The O-H group becomes polar. The net positive charge on hydrogen atom attracts the negative charge from the oxygen atom in an adjacent alcohol molecules. This permanent imbalance of charge in H-bond compared to Vander waal force results in permanent dipole moment. The hydrogen bond in alcohols is shown below. "R" represents any alkyl chain in alcohol.

6- S+ H .. - → charge imbalance in C-H bond. Strong H-tond in alrohd.

3.Volatility means the tendency of the compound to exist in vapor state than liquid. In other words, volatility refers to easy conversion of liquid state to vapor state of the compound. Volatility becomes high when intermolecular forces are weak. Alkanes are non-polar in nature and have only weak van der Waals forces. On the other hand, Alcohols can form hydrogen bonds, dipole-dipole interactions and van der Waals forces. This increases intermolecular forces and the volatility decreases.

Pentane is more volatile than 1-octanol.

4. The boiling point of alcohol and alkanes can be explained in a similar manner as that of volatility. As intermolecular forces are strong in alcohol, it requires more energy to overcome the hydrogen bonds between the alcohol molecules and to convert from liquid state to vapor state. The molecules of alcohols are tightly packed. Thus boiling point is higher. The intermolecular forces in alkanes is weak and less energy is needed to convert it from liquid state to vapor state. Hence boiling point is less.

Boiling point of pentane is less than that of 1-octanol.

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