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[CHEM 1120] Midterm 2
[CHEM 1120] All questions and answers provided. 95% and afterwards made the corrections so you should also receive a high mark.
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CHEM 1120 – MIDTERM 2 (39 MARKS)ANSWERS IN GREEN
Fill in the blank or multiple choice (8 marks):
1. Bond polarity can be assessed by using:
a) The ionization energy for each atom
b) The number of valence electrons for each atom
c) The atomic radius of the bonding atoms
d) The electronegativity of the bonding atoms
2. In general, when the length of the bond gets longer, the strength of the bond gets _________.
Weaker
3. Phosphorus pentachloride is an exception to Lewis theory because:
a) It forms incomplete octets
b) It forms hypercoordinate compounds
c) It has a tendency to form radical compounds
d) It exists as a gas at room temperature
4. The validity of resonance structures can be assessed by assigning _______________ to each atom.
Formal charges
5. The ionization energy of an atom is always measured in the ____________ phase. Gas
6. In covalent bonding, the electrons are __________ between two atoms. Shared
7. In general, atomic size ____________ across a period and ________ down a group. Decreases & Increases
8. Order the following compounds in order of increasing lattice energy and briefly explain why you have ordered
them this way (3 marks)
Rb2O MgO BeO LiF
LiF < RbO < MgO < BeO
Why they are ordered this way:
𝑞1𝑞2
1. Magnitude: The charge of each atom. E ∝ 𝑟
As the charge increases, the energy increases.
2. Location of the Element: As the element moves across the table, it increases. As it moves down the table, it
decreases.
9. Draw the Lewis structure for SbF5 and the dipole moment for one of the bonds. Draw a second Lewis structure
where the octet rule is obeyed and include the formal charges (4 marks).
① Hypercoordinate (expanded with too many e-) ② Obeys octet rule, meaning one F-1 is taken off.
35e- + 5e- = 40e- needed
Sb
, CHEM 1120 – MIDTERM 2 (39 MARKS)
10. Nitrogen fixation with the Haber process (shown below) is the primary method in which ammonia is produced
worldwide. Estimate the reaction enthalpy associated with this reaction (5 marks)
N2(g) + 3H2(g) → 2NH3(g) Use bond energies. Chart is provided at the back of the test package.
ΔHrxn= Bonds broken – Bonds formed
𝑘𝑗 𝑘𝑗
ΔHrxn = (946 + (3 x 436 ) + (3 x 2 x 389 )
𝑚𝑜𝑙 𝑚𝑜𝑙
ΔHrxn = (2254) + (-2334)
𝑘𝑗
ΔHrxn = -80 𝑚𝑜𝑙 EXOTHERMIC
11. Draw the Lewis structure and all possible resonance structures for OPN (P is the central atom). Assign formal
charges and determine which resonance structure is the most representative of the actual structure of the
molecule (Hint: obey the octet rule!) (5 marks)
They both have the same charges, but the oxygen on the 2nd structure has a
charge of -1 and is the most electronegative (most representative)
12. A) Briefly explain the general trend observed for the Van der Waals radius compared to the covalent radius
(3 marks) Van der Waals radius of an atom is ½ the distance between adjacent nuclei in the atomic solid. In this
type of bond, the atoms are touching but not bonded. It is the nonbonding atomic radius which determines the
distance between atoms. The covalent radius has a bonding atomic radius. In this type of bond, which is
stronger and shorter, the nuclei are closer together.
b) A scientist is trying to measure the difference in the radius for N and N-. However, she is unable to
successfully add an electron to the N atom. Briefly explain this result. 2 Ti (s) + O2 -> 2 TiO(s)
Since nitrogen has 3 single electrons in the p-orbital, it has a stable configuration and it will not readily accept
electrons (has a low electron affinity, meaning it will not attract more e-), unlike carbon or oxygen.
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