Chemistry 102 – Chapter 12 Summary and Review – Answers 1. Of the diatomic elements, which has the highest boiling point and which has the lowest boiling point? Answer: Of the diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2), all have dispersion forces. Therefore the element with the greatest total number of electrons will have the highest boiling point (iodine) and the element with the smallest total number of electrons will have the lowest boiling point (hydrogen). 2. Using the partial pressures of water given in Chapter 5, determine the molar enthalpy of vaporization of water. How close is this to the actual value? Answer: Using the partial pressure of water at 25oC (23.76 mmHg) and at 100oC (760 mmHg): P ΔHvap ⎛ 1 1 ⎞
ln 2 =
⎜ − ⎟
P1
R ⎝ T1 T2 ⎠
R × ln
ΔHvap =
P2
P1
760 mmHg
23.76 mmHg
= 42.7 kJ ⋅ mol−1
1 ⎞
⎛ 1
−
⎜
⎟
⎝ 298 K 373 K ⎠
8.314 × 10−3 kJ ⋅ mol−1 ⋅ K −1 × ln
=
⎛1 1⎞
⎜ − ⎟
⎝ T1 T2 ⎠
Comparing to the value on p. 411 − 4.7% error
3. Between dimethyl ether and ethanol, which has the highest boiling point and why? Answer: Both of these molecules have the same number of electrons, however these molecular substances do not have the same intermolecular forces. Dimethyl ether is a polar substance which therefore has dipole and dispersion forces. However, ethanol is a polar substance which has dipole, dispersion and hydrogen bonding intermolecular forces. Therefore, ethanol has the higher boiling point. 4. Draw the phase diagram for water and identify the normal boiling point and melting point, how the boiling point and melting point change as the pressure changes and the triple point. Answer: See p. 416; figure 12.30 5. Using structures show hydrogen bonding in a sample of pure ammonia. Answer: These are not shown as three dimensional, however the idea of the attraction between the molecules (of H on one molecule to the N on another molecule) is shown, albeit for only a few molecules. 6. Using the atomic radius for sodium (in chapter 8) and the density of sodium (0.971 g∙cm–3), determine how this element packs in its cubic crystalline solid. Answer: For this you are going to determine the number of atoms per unit cell: −23
3
23
⎛ 0.971 g ⎞ ⎛ 7.95 × 10 cm ⎞ ⎛ mol ⎞ ⎛ 6.022 × 10 atoms ⎞
−1
⎜
⎟
⎜
⎟ = 2.0 atoms ⋅ unit cell ⎜
⎟
⎜
⎟
3
unit cell
mol
⎝ cm ⎠ ⎝
⎠ ⎝ 22.99 g ⎠ ⎝
⎠