Modeling Alkanes Lab
Black spheres – Carbon
Yellow spheres – Hydrogen
Procedure
1. Assemble a model of methane (CH4).
a) Draw the electron dot structure (This is the Structural Formula of methane)
b) Compare your model with the structural formula. Note that the angles between atoms are
not 90o, as you might think from the written formulas. This shape is called a tetrahedron.
Here’s one way to think about this tetrahedral shape. Assume that the four pairs of
electrons-all with negative charges-surrounding the carbon repel one another and stay as
far away from one another as possible. This causes the four pairs of shared electrons to
arrange themselves so that the bonds point to the corners of a tetrahedron. The angles
between the C-H bonds are each 109.5o , a value that has been verified several ways
experimentally. The angles are not 90o as they would be if methane were shaped like a
flat square. Draw the 3-d model of methane you created and include bond angles in your
drawing.
2. Assemble models of a two-carbon and a three-carbon alkane molecule. Recall that each
carbon atom in an alkane is bonded to four other atoms.
a) How many hydrogen atoms are present in the two-carbon alkane?
b) How many are present in the three-carbon alkane?
c) Draw a 3-D representation of the three-carbon alkane that reflects the angles.
3. a) Draw the structural formulas for the two- and three- carbon alkanes.
b) The molecular formulas for the first two alkanes are CH4 and C2H6. What is the molecular
formula of the third?
A shortened form of the structural formula is called a condensed formula. The condensed formula
for the 3-carbon alkane can be either CH3CH2CH3 or CH3-CH2-CH3. This gives some information
on how the atoms are arranged in the molecules and are more useful than the molecular formulas
(such as C3H8) that only show how many of each atom are present. We can use the structural,
molecular, or condensed formulas to represent alkanes. Alkanes are any molecules that contain
only carbon and hydrogen that are bonded together with only single bonds.
The general molecular formula for any alkane is CnH2n+2. If n=4 for the four-carbon alkane, then 2n
+ 2 = 10, and the formula is C4H10.
4. Predict molecular formulas for the first ten alkanes.
The names for the first ten alkanes are listed at the end of this document. These names are
composed of a root, followed by –ane (designating an alkane). The root refers to the number of
carbon atoms in the backbone carbon chain. To a chemist, meth- means one carbon atom, ethmeans 2 and so on. Write the names for the first ten alkanes next to the formulas.
5. Write structural formulas for butane and pentane.
6. a) Name the alkanes with these formulas:
1)
2)
CH3CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH2CH3
b) Write shorter molecular formulas for the two alkanes listed in Question 6a.
7. Write a molecular formula for an alkane containing 25 carbon atoms.
8. Name the alkane having a molar mass of
a) 30 g/mol
b) 58 g/mol
9. Write out the condensed formula for:
a) butane
b) pentane
Names of the first 10 alkanes:
# of Carbon atoms
Names
1
Methane
2
Ethane
3
Propane
4
Butane
5
Pentane
6
Hexane
7
Heptane
8
Octane
9
Nonane
10
Decane