Name:______Intermolecular Forces: Applying What You Know
PART 1: INTERMOLECULAR FORCES IN LIQUIDS AND GASES.
Molecules attract each other, and the force of attraction increases rapidly as the intermolecular distance decreases. In a liquid, the molecules are very close to one another and are constantly moving and colliding. When a liquid evaporates, molecules in the liquid must overcome these intermolecular attractive forces and break free into the gas phase, where on average the molecules are very far apart. For example, when water evaporates, rapidly moving H2O molecules at the surface of the liquid pull away from neighboring H2O molecules and enter the gas phase.
CRITICAL THINKING QUESTIONS:
1) When water evaporates, are any bonds between the H atoms and O atoms within a molecule broken?
2) On average, are the intermolecular forces stronger in liquid water or in gaseous water? Explain. Name:______PART 2: INTERMOLECULAR FORCES AND BOILING POINTS.
To a large extent, the boiling point of a liquid is determined by the strength of the intermolecular interactions in the liquid. These interactions are largely determined b y the structure of individual molecules.
Alkane MW bp (°C) Ketone MW bp (°C) Alcohol MW bp (°C) (amu) (amu) (amu)
CH3CH2CH3 44.1 -42.1 CH3CCH3O 58.1 56.2 CH3CH2CH2OH 60.1 97.4 propane acetone 1-propanol
CH3(CH2)2C 58.1 -0.5 CH3CCH2CH3O 72.1 79.6 CH3(CH2)2CH2OH 74.1 117.0
H3 2-butanone 1-butanol butane
CH3(CH2)3C 72.2 36.1 CH3C(CH2)2CH30 86.1 102.0 CH3(CH2)3CH2OH 88.2 137.0
H3 2-pentanone 1-pentanol pentane
CH3(CH2)4C 86.2 69.0 CH3C(CH2)3CH3O 100 128.0 CH3(CH2)4CH2OH 74.0 158.0
H3 2-hexanone 1-hexanol hexane
CH3(CH2)8C 142 174 CH3C(CH2)7CH3O 156 210.0 CH3(CH2)8CH2OH 158 229.0
H3 2-decanone 1-decanol decane
MW= Molecular Weight Alkanes are hydrocarbons containing only C and H and have all single bonds. Ketones contain a C=O group. Alcohols contain an O-H group.
CRITICAL THINKING QUESTIONS
3) Recall that the electronegativity of C and H are roughly the same, but that O has a significantly higher electronegativity. For each type of compound (alkane, ketone, alcohol) predict whether or not the compound is expected to be polar or nonpolar.
4) For each type of compound below, indicate how the boiling point changes as the molecular weight of the compound increases: a) alkane b) ketone c) alcohol
5) Based on your answers to question 4, how do the intermolecular forces between molecules change as the molecular weight increases?
6) Find an alkane, a ketone and an alcohol with roughly the same size. Rank these compounds in terms of relative boiling points. Name:______
7) Repeat question 6 with a different set of compounds.
8) Using complete sentences, describe any general pattern that you can identify about the relative boiling points of alkanes, ketones and alcohols of roughly equal size.
9) Rank the three types of compounds in terms of their relative strength of intermolecular interaction, for molecules of roughly the same size.
10) Based on the data provided on the pervious page, does the presence of a dipole moment in a molecule tend to increase or decrease the strength of intermolecular interactions? Explain your reasoning.
11) What is the difference between intramolecular bonds and intermolecular forces?
12) Rank these intermolecular forces in terms of the typical relative strengths: hydrogen bonding; dipole-dipole, and London dispersion forces.
13) In the alkanes: what types of intermolecular forces are present? Which is the strongest?
14) In the Ketone: what types of intermolecular forces are present? Which is the strongest?
15) In the alcohols: what types of intermolecular forces are present? Which is the strongest?
16) In terms of intermolecular forces, why does the boiling point of a particular type of compound (for example, an alkane) increase as the molecular weight increases? Name:______
17) Both cis-1,2,- dichloroethylene and trans-1,2,-dichloroethylene pictured below have the same molecular formula:
C2H2Cl2. However, the cis- compound is polar, while the trans- compound is non polar. One of these chemicals has a boiling point of 60.3°C and the other has a boiling point of 47.5°C. Which compound has which boiling point?
18) Rank each of the following groups of substances in order of increasing boiling point, and explain your reasoning:
a) NH3, He, CH3F, CH4
b) CH3Br, Ne, CH3OH, CH3CN
c) CH4, SiH4, GeH4, SnH4
19) Fluoromethane, CH3F, and methanol,CH3OH, have approximately the same molecular weight. However, the boiling point of CH3OH is 65.15°C, whereas the boiling point of CH3F is almost 100 degrees lower, -78.4°C. Explain.
20) Which has a higher boiling point, ethylene glycol or ethanol? Why? Both are pictured below.