Unit 3 Exam Prep

Question 1

What is the equation for Bond Order(BO)?

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Answer 1

Bond Order = (Bonding e^- - Antibonding e^-)/2.

If BO > 0, molecule is stable.

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Question 2

What makes a molecule paramagnetic?

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Answer 2

Unpaired electrons

(e.g., O₂ has 2 unpaired e ^- in pi*2p).

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Question 3

Does 2s-2p mixing occur in O₂?

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Answer 3

No

Only in B, C, N (Not O, F, Ne)

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Question 4

Rank IMFs from strongest to weakest

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Answer 4

H-bonding > dipole-dipole > london dispersion

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Question 5

Why does CH₃Br have a higher bp than CH₃F?

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Answer 5

London dispersion forces dominate in CH₃Br

(more electrons)

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Question 6

What causes high surface tension in water?

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Answer 6

String H-bonding

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Question 7

What equation is used for phase changes?

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Answer 7

q = n*ΔH_fus/vap

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Question 8

Why does boiling point decrease at high elevation?

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Answer 8

Lower atmospheric pressure –> VP = P_atm at lower T.

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Question 9

How do you calculate heat for cooling water (25°C –> -10°C)?

(The equations only)

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Answer 9

1. Cool water: q₁ = m * c_water * ΔT
2. Freeze: q₂ = n * ΔH_fus
3. Cool ice: q₃ = m * c_ice * ΔT

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Question 10

How many atoms are in FCC, BCC, and SC unit cells?

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Answer 10

FCC = 4, BCC = 2, SC = 1

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Question 11

What’s the packing efficiency of FCC?

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Answer 11

74%

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Question 12

For BCC, how is atomic radius (R) related to edge length (a)?

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Answer 12

a = 4R/√3

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Question 13

What is molality(m)?

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Answer 13

m = moles solute/ kg solvent

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Question 14

What is the van’t Hoff factor (i) for CaCl₂?

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Answer 14

~3 (Ca²⁺ + 2Cl^-).

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Question 15

How much NaCl raises bp of 500g water by 1.0°C?

(Do the calculation)

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Answer 15

ΔT_b = i * K_b * m –> 1.0 = 2 * 0.512 * m
m = 0.977 mol/kg –> 28.5 g NaCl.

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Question 16

Is F₂^2- stable?

(Calculate BO)

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Answer 16

BO = 0.5

(yes, barely).

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Question 17

Which has higher bp: NH₃ or CH₄?

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Answer 17

NH₃

H-bonding > London dispersion

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Question 18

What’s the density of a BCC metal (a = 300 pm, M = 50 g/mol)?

(Calculate the density)

Answer 18

Volume = (3 * 10^-8cm)³ = 2.7 * 10^-23cm³
Mass = 2 atoms * 50/(6.022 * 10²³) = 1.66 * 10^-22g
Density = (1.66 * 10^-22)/(2.7 * 10^-23) = 6.15 g/cm³