CLAS – Chem 1C – Chapter 16 (part 1) – Key

1. Indicate the types of forces present for each of the following substances:a. CCl4 ⇒ intra = covalent; inter = London dispersion forcesb. HF ⇒ intra = covalent; inter = hydrogen bonding, dipole-dipole, London dispersion forcesc. NH4Br ⇒ intra = ionic, covalent; inter = N/Ad. F2 ⇒intra = covalent; inter = London dispersion forcese. CH3OH ⇒ intra = covalent; inter = hydrogen bonding, dipole-dipole, London dispersion forcesf. Ar ⇒ intra = N/A; inter = London dispersion forcesg. H2S ⇒ intra = covalent; inter = dipole-dipole, London dispersion forcesh. KOH ⇒ intra = ionic, covalent; inter = N/Ai. BH3 ⇒ intra = covalent; inter = London dispersion forcesj. H2CO⇒ intra = covalent; inter = dipole-dipole, London dispersion forces

2. Using intermolecular forces predict the following:a. highest melting point F2 or Br2

b. highest melting point HF or HClc. highest boiling point HOCH2CH2OH or CH3CH2OHd. highest boiling point CH3CH3 or CH3CH2CH3

e. highest vapor pressure CH3CH2OH or CH3OCH3

f. highest vapor pressure CCl4 or CH2Cl2

g. highest freezing point MgO or H2Oh. highest surface tension C14H30 or C24H50

3. Consider the following phase diagrams for water and carbon dioxide respectively – which phase is the densest for each substance? for water liquid phase is the densest whereas for carbon dioxide the solid phase is the densest How does pressure affect the MP and BP for each substance? If the slope of the curve is positive there is a direct proportionality between T and P and vice versa for negative slopes

4. Pure compound Z has a triple point at 18 ° C and 72 torr, a normal melting point at 21 °C, and a normal boiling point at 87 °C. Which of the following statements regarding compound Z is/are correct?a. The density of the solid is greater than that of the liquid.b. Sublimation occurs if starting with a solid at a constant temperature of 17 °C the pressure is decreased until a

phase change occurs. c. Condensation occurs if the temperature is decreased from 55 °C to 13 °C at a constant pressure of 1.00 atm.

5. The enthalpy of vaporization for water is 44 kJ/mol. What is the boiling point if the atmospheric pressure were 0.33 atm?

ln (P2P1 ) = - ΔH vap

R (1T 2– 1

T 1 )⇒ ln (0.33 atm1 atm ) = - 44,000J/mol

8.314 J/molK (1T 2– 1

373K ) ⇒ T2 = 346 K or 73

°C

6. How much heat is required to take 10 g of ice from -31 °C to vapor at 155 °C if the pressure is hel constant at 1 atm? (Csolid = 2.03 J/g°C , Cliquid = 4.18 J/g°C , Cgas = 2.02 J/g°C, ΔHfus = 6.01 kJ/mol, and ΔHvap = 40.7 kJ/mol)This is a 5 step process => 3 changes in temperature (q=mC Δ T) and 2 phase changes (q=n ΔH)q1=(10g)(2.03 J/g°C)(+31°C) = 629.3J or 0.629kJq2=(10g/18g/mol)(6.01kJ/mol) = 3.34kJq3=(10g)(4.18 J/g°C)(+100°C) = 4180J or 4.18kJq4=(10g/18g/mol)(40.7 kJ/mol) = 22.6kJq5=(10g)(2.02 J/g°C)(+55°C) = 1.11kJqtotal = 31.9 kJ

7. Determine the final temperature if a 25 g cube of ice at -7 °C is placed in 180 g of water at 64 °C and allowed to come to equilibrium. (Csolid = 2.03 J/g°C , Cliquid = 4.18 J/g°C , Cgas = 2.02 J/g°C, ΔHfus = 6.01 kJ/mol, and ΔHvap = 40.7 kJ/mol)Heat is transferred from the hot water into the cold water ⇒ q total = 0

0 = mCΔT(warm water) + mCsolid Δ T(ice cube) + nHfus (ice cube) + mCliquidΔ T(ice cube) 0 = (180g)(4.18J/g°C)(Tf-64°C) + (25g)(2.03J/g°C)(7°C)+(25g/18g/mol)(6010J/mol)+(25g)(4.18J/g°C)(Tf-0°C)

Tf = 46°C