Practice: Boiling Points and Intermolecular Forces Building Skills 1

5. Make inferences about the strength of intermolecular forces in different compounds based on their boiling points.
6. State and apply the relationships among size of molecule, amount of branching, strength of intermolecular forces, and boiling point of a compound.
Hydrocarbon Boiling Points
Hydrocarbon / Boiling Point (C)
Butane / -0.5
Decane / 174.0
Ethane / -88.6
Heptane / 98.4
Hexane / 68.7
Methane / -161.7
Nonane / 150.8
Octane / 125.7
Pentane / 36.1
Propane / -42.1

Hydrocarbon Boiling Points: Finding patterns in data

  1. Examine the chart on the right. How is the data organized?

Is this a useful way to organize data?

  1. You are searching for a trend in boiling points.

Reorganize the data to make is more useful.

Use your reorganized data to answer the following questions:

  1. Which substance(s) are gases (have already boiled) at room

temperature (22C)?

Hydrocarbon Boiling Points
Hydrocarbon / Boiling Point (C)
  1. Which substance(s) boil between room temperature and

body temperature (37C)?

  1. Define intermolecular forces:
  1. What can you infer about the intermolecular forces among decane molecules compared to those in butane?

Trends in Alkane Boiling Points:Building Skills 2

  1. Plot a graph to show the relationship between number of carbon atoms (x-axis) and boiling points (y-axis). Draw a BEST FIT LINE through the most points. Label each axis with units and use the entire graph. Label the x-axis to include the first 12 alkanes. What range will you need to make the y-axis? x-axis?
  1. Estimate the average change in boiling point when a single carbon atom and two hydrogen atoms (-CH2-) are added to an alkane chaine.
  1. The pattern of boiling points among the first ten alkanes can be used to predict boiling points for other alkanes.
  2. Using your graph, estimate the boiling points of undecane (C11H24), dodecane (C12H26), and tridecane (C13H28) by extrapolating your best fit line to the C11, C12, and C13 regions of your x-axis.

b. Compare to the actual values: undecane = 196Cdodecane = 216Ctridecane = 234C

4. A substances boiling point is related to its intermolecular forces, or attractions between molecules. What is the relationship between the number of carbon atoms in a molecule and its intermolecular forces?

Boiling Points of Alkane Isomers:Building Skills 3

Increased intermolecular forces are associated with the greater molecule-to-molecule contact possible for larger alkanes. The closer the molecules can get to each other, the higher the intermolecular forces will be.

  1. Observe the following isomer chart. How does the boiling point change as the extenet of carbon-chain branching increases?

Alkane / Condensed Formula / Boiling Point (C)
C5H12
isomers / CH3-CH2-CH2-CH2-CH3 / 36.1
CH3-CH-CH2-CH3
CH3 / 27.8
CH3
CH3 -C- CH3
CH3 / 9.5

2. Assign each of the following boiling points to the appropriate C7H16 isomer:98.4C, 92.0C, 79.2C

a. CH3-CH2-CH2CH2-CH2-CH2-CH3b. CH3-CH2-CH-CH2-CH2-CH3

CH3 CH3

c. CH3-CH2-CH2-C-CH3

CH3

3. Use the following chart to predict the boiling point of the following C8H18 isomer: CH3

CH3-CH2-CH2-C-CH2-CH3

CH3

Alkane / Condensed Formula / Boiling Point (C)
C8H18
isomers / CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3 / 125.6
CH3-CH2-CH2-CH2-CH2-CH-CH3
CH3 / 117.7
CH3
CH3 -CH- CH2-C-CH3
CH3 CH3 / 99.2

4. What general rule could summarize the trends in this activity?