CEM 141 Chapter 3 Test Questions
and Answers
Which will have a higher BP: CH4 or C2H6? - Answer- C2H6 because the molecule is
bigger and therefore has more LDFs, requiring more energy to break them
What info is missing from a Lewis Structure? - Answer- 3D
What is the same in isomers? - Answer- 1. chemical formula
2. number of each type of atoms
3. look for the longest chain and where the double bonds and triple bond is located is
how you determine if two molecules are isomers
What is different in isomers? - Answer- 1. connectivity between atoms
2. different properties
How many different isomers can you draw for the formula C5H12? (different
arrangement of atoms) - Answer- 3 (shortest C chain you could make was 3)
What is the generic formula for a hydrocarbon containing "n" carbon atoms? - Answer-
1. CnH2n+2
a. can only use formula if there are no double bonds, triple bonds, or rings
b. Ex. What is the formula for a linear hydrocarbon with 7 C atoms?
i. C7H16
Alkenes - Answer- 1. hydrocarbons containing double bonds
2. In C2H4- Double Bonds
a. C is sp2 hybridized
b. contain C-C double bond
i. one sigma bond (sp2-sp2 overlap)
ii. one pi bond (sideways p-p overlap)
c. restricted rotation around the double bond because pi bonds don't rotate freely
In C2H4, what would happen if the C atoms rotated relative to each other? - Answer- It
would break the bond (requires energy)
What would you have to do to rotate around a pi bond? - Answer- 1. put in energy
a. pi bonds restrict rotation because of the electron overlap both above and below the
plane of the atoms
, Similarities between LDF and Covalent Bond - Answer- they have similar causes:
electrostatic attraction of the electron of one atom to the nucleus of the other
Differences between LDF and Covalent Bond - Answer- 1. magnitude of attraction
2. how the electrons are arranged in the new species formed by the interaction
Why do bonds form between atoms? - Answer- because valence electrons are attracted
to the nuclei of other atoms
Covalent Bonds - Answer- 1. valence electrons from one atom become attracted to the
nucleus of the other atom
2. each nucleus is attracting both electrons
3. when bonds form energy is released to surrounding
What is bond length? - Answer- the most stable distance between atoms (lowest PE)
Molecular Orbital Theory - Answer- 1. atomic orbitals can combine constructively to
form a molecular orbital of lower energy (in phase)
2. atomic orbitals can combine destructively to form a molecular orbital of higher energy
(out of phase)
3. combine n atomic orbitals--> n molecular orbitals
4. Bonding and Anti-bonding orbitals
a. both are in the same place of space, just at different energies
Bonding Orbitals - Answer- 1. of lower energy than the atomic orbitals (typically)-->
stabilizing
a. electrons in bonding orbitals make the species more stable
Anti-bonding Orbitals - Answer- 1. of higher energy than the atomic orbitals and
destabilizing
a. electrons in anti-bonding orbitals make the species less stable
Why do 2 H atoms form a bond while 2 He atoms don't? - Answer- 1. In H2: 2 electrons
in a bonding orbital make 1 bond (H2 has one bond between 2 H atoms)
2. In He: 2 electrons in a bonding MO is stabilizing, but 2 electrons in an anti-bonding
MO cancel out stabilization (so no bonds between He atoms)
Bonding in H and He - Answer- 1. In hydrogen-hydrogen, only the bonding MO is
occupied, leading to a net stabilization of the interaction
a. to break the bond, enough energy has to be added to raise an electron to the anti-
bonding orbital
2. In helium-helium, both bonding and anti-bonding molecular orbitals are occupied.
Because their energies (negative for bonding orbital and positive for anti-bonding
orbital) are equal and opposite, there is no net stabilization for interaction
and Answers
Which will have a higher BP: CH4 or C2H6? - Answer- C2H6 because the molecule is
bigger and therefore has more LDFs, requiring more energy to break them
What info is missing from a Lewis Structure? - Answer- 3D
What is the same in isomers? - Answer- 1. chemical formula
2. number of each type of atoms
3. look for the longest chain and where the double bonds and triple bond is located is
how you determine if two molecules are isomers
What is different in isomers? - Answer- 1. connectivity between atoms
2. different properties
How many different isomers can you draw for the formula C5H12? (different
arrangement of atoms) - Answer- 3 (shortest C chain you could make was 3)
What is the generic formula for a hydrocarbon containing "n" carbon atoms? - Answer-
1. CnH2n+2
a. can only use formula if there are no double bonds, triple bonds, or rings
b. Ex. What is the formula for a linear hydrocarbon with 7 C atoms?
i. C7H16
Alkenes - Answer- 1. hydrocarbons containing double bonds
2. In C2H4- Double Bonds
a. C is sp2 hybridized
b. contain C-C double bond
i. one sigma bond (sp2-sp2 overlap)
ii. one pi bond (sideways p-p overlap)
c. restricted rotation around the double bond because pi bonds don't rotate freely
In C2H4, what would happen if the C atoms rotated relative to each other? - Answer- It
would break the bond (requires energy)
What would you have to do to rotate around a pi bond? - Answer- 1. put in energy
a. pi bonds restrict rotation because of the electron overlap both above and below the
plane of the atoms
, Similarities between LDF and Covalent Bond - Answer- they have similar causes:
electrostatic attraction of the electron of one atom to the nucleus of the other
Differences between LDF and Covalent Bond - Answer- 1. magnitude of attraction
2. how the electrons are arranged in the new species formed by the interaction
Why do bonds form between atoms? - Answer- because valence electrons are attracted
to the nuclei of other atoms
Covalent Bonds - Answer- 1. valence electrons from one atom become attracted to the
nucleus of the other atom
2. each nucleus is attracting both electrons
3. when bonds form energy is released to surrounding
What is bond length? - Answer- the most stable distance between atoms (lowest PE)
Molecular Orbital Theory - Answer- 1. atomic orbitals can combine constructively to
form a molecular orbital of lower energy (in phase)
2. atomic orbitals can combine destructively to form a molecular orbital of higher energy
(out of phase)
3. combine n atomic orbitals--> n molecular orbitals
4. Bonding and Anti-bonding orbitals
a. both are in the same place of space, just at different energies
Bonding Orbitals - Answer- 1. of lower energy than the atomic orbitals (typically)-->
stabilizing
a. electrons in bonding orbitals make the species more stable
Anti-bonding Orbitals - Answer- 1. of higher energy than the atomic orbitals and
destabilizing
a. electrons in anti-bonding orbitals make the species less stable
Why do 2 H atoms form a bond while 2 He atoms don't? - Answer- 1. In H2: 2 electrons
in a bonding orbital make 1 bond (H2 has one bond between 2 H atoms)
2. In He: 2 electrons in a bonding MO is stabilizing, but 2 electrons in an anti-bonding
MO cancel out stabilization (so no bonds between He atoms)
Bonding in H and He - Answer- 1. In hydrogen-hydrogen, only the bonding MO is
occupied, leading to a net stabilization of the interaction
a. to break the bond, enough energy has to be added to raise an electron to the anti-
bonding orbital
2. In helium-helium, both bonding and anti-bonding molecular orbitals are occupied.
Because their energies (negative for bonding orbital and positive for anti-bonding
orbital) are equal and opposite, there is no net stabilization for interaction
