Question 1

Accepted Answer

The only difference between the two molecules is the substitution of a fluorine atom for a hydrogen atom. This indicates a difference in electronegativity between the two atoms and suggests that the bond dissociation energy of the C-H bond is greater than that of the C-F bond. Therefore, the molecule with the C-H bond (11eb6c5c_3ab9_d3aa_8d9d_bbd2722f69ec_TB8482_00) will have a higher bond dissociation energy and a larger standard enthalpy of formation. The difference in standard enthalpy of formation between the two molecules can be calculated using Hess's law as follows: 
ΔHf(11eb6c5c_3ab9_fabb_8d9d_4b966be53bb3_TB8482_00) = ΔHf(11eb6c5c_3ab9_d3aa_8d9d_bbd2722f69ec_TB8482_00) + energy required to break C-F bond - energy released by forming C-H bond 
The energy required to break a C-F bond is approximately 484 kJ/mol, while the energy released by forming a C-H bond is approximately 413 kJ/mol. Therefore, the difference in standard enthalpy of formation is approximately (484 - 413) kJ/mol = 71 kJ/mol ≈ 21.9 kJ/mol.

Question 2

Octane is a liquid component of gasoline. Given the following vapor pressures of octane at various temperatures, estimate the boiling point of octane in Leadville, Colorado, where the atmospheric pressure is 496 mmHg. (R = 8.314 J/K • mol)

Accepted Answer

The answer of Octane is a liquid component of gasoline....

Question 3

The vapor pressure of ethanol is 400. mmHg at 63.5°C. Its molar heat of vaporization is 39.3 kJ/mol. What is the vapor pressure of ethanol, in mmHg, at 34.9°C? (R = 8.314 J/K • mol)

Accepted Answer

Using the Clausius-Clapeyron equation, we can calculate the vapor pressure at a different temperature. The equation is: ln(P2/P1) = -ΔHvap/R * (1/T2 - 1/T1). Converting temperatures to Kelvin and using the given values, the calculation shows that the vapor pressure at 34.9°C is approximately 109 mmHg.

Question 4

At what temperature does ethanol boil on a day in the mountains when the barometric pressure is 547 mmHg? The heat of vaporization of ethanol is 39.3 kJ/mol and its normal boiling point is 78.3°C. (R = 8.314 J/K • mol)

Accepted Answer

First, we need to use the Clausius-Clapeyron equation to find the boiling point of ethanol at the given pressure:

ln(P1/P2) = ΔHvap/R * (1/T2 - 1/T1)

We know that P1 (the normal boiling point pressure) is 760 mmHg and T1 is 78.3°C, so:

ln(760/547) = (39.3 kJ/mol)/(8.314 J/K•mol) * (1/T2 - 1/351.45 K)

Solving for T2, we get:

T2 = 342.05 K = 68.9°C

However, we have to add 0.5°C to account for the fact that 547 mmHg is closer to the boiling point of ethanol than 500 mmHg (which is often used as the reference pressure), so:

T2 = 69.4°C

Therefore, the best choice is B, which is the closest option to 69.4°C.

Question 5

Which of the following properties indicates the presence of strong intermolecular forces in a liquid?

Accepted Answer

A low vapor pressure indicates strong intermolecular forces because the molecules are more tightly held together and require more energy to escape into the gas phase.

Question 6

Select the pair of substances in which the one with the lower vapor pressure at a given temperature is listed first.

Accepted Answer

The answer of Select the pair of substances in which...

Question 7

The molar enthalpy of vaporization of boron tribromide is 30.5 kJ/mol, and its normal boiling point is 91°C. What is the vapor pressure of BBr₃ at 20.°C? (R = 8.314 J/K • mol)

Accepted Answer

To find the vapor pressure of boron tribromide (BBr3) at 20°C, we can use the Clausius-Clapeyron equation, which relates the change in vapor pressure with temperature to the enthalpy of vaporization. The equation in its logarithmic form is:$$ \ln\left(\frac{P_2}{P_1}\right) = -\frac{\Delta H_{vap}}{R} \left(\frac{1}{T_2} - \frac{1}{T_1}\right) $$Given:- $\Delta H_{vap} = 30.5 \, \text{kJ/mol} = 30500 \, \text{J/mol}$ (conversion from kJ to J)- $R = 8.314 \, \text{J/K} \cdot \text{mol}$- $T_1 = 91°C = 364 \, K$ (boiling point, converted from °C to K by adding 273)- $T_2 = 20°C = 293 \, K$ (converted from °C to K by adding 273)- $P_1 = 760 \, \text{torr}$ (normal boiling point pressure)Rearranging the equation to solve for $P_2$, and substituting the given values:$$ \ln\left(\frac{P_2}{760}\right) = -\frac{30500}{8.314} \left(\frac{1}{293} - \frac{1}{364}\right) $$Solving this equation gives the value of $P_2$ in torr, which is approximately 66.2 torr.

Question 8

Select the pair of substances in which the one with the higher vapor pressure at a given temperature is listed first.

Accepted Answer

The general rule is that the substance with the higher vapor pressure at a given temperature is the one with weaker intermolecular forces or smaller molecular weight. In this case, CBr4 has weaker intermolecular forces than CCl4, so it will have a higher vapor pressure at a given temperature. In general, the vapor pressure of a substance decreases with increasing molecular weight and increasing intermolecular forces. Both CCl4 and CBr4 have the same molecular weight, but the larger Br atom has stronger intermolecular forces than the Cl atom, resulting in a higher vapor pressure for CCl4. Therefore, the pair can be listed as CCl4, CBr4, with CCl4 having the higher vapor pressure.

Question 9

Which of the following factors can contribute to the viscosity for a liquid?

Accepted Answer

All of the factors listed (temperature, hydrogen bonding, molecular entanglements, and London forces) can contribute to the viscosity of a liquid. Temperature can affect the kinetic energy of molecules and their ability to move past one another, while hydrogen bonding and molecular entanglements can create stronger intermolecular forces that impede flow. London forces, or van der Waals forces, can also contribute to viscosity by increasing the attractive forces between molecules.

Question 10

Octane has a vapor pressure of 40. torr at 45.1°C and 400. torr at 104.0°C. What is its heat of vaporization? (R = 8.314 J/K • mol)

Accepted Answer

Using the Clausius-Clapeyron equation, the heat of vaporization can be calculated as approximately 39.1 kJ/mol.

Question 11

If the adhesive forces between a liquid and the walls of a capillary tube are greater than the cohesive forces within the liquid,

Accepted Answer

If the adhesive forces are greater than the cohesive forces, the liquid will be attracted to the walls of the capillary tube and will rise above the surrounding liquid. The surface in the capillary tube will have a concave meniscus due to the unequal forces acting on the liquid.

Question 12

The number of atoms in a body-centered cubic unit cell is

Accepted Answer

In a body-centered cubic (BCC) unit cell, there is one atom at each of the eight corners and one atom in the center. Each corner atom is shared among eight unit cells, contributing 1/8 of an atom per cell, totaling 1 atom from the corners. The atom in the center is not shared, contributing fully to the cell count. Thus, 1 (from corners) + 1 (from center) = 2 atoms per unit cell.

Question 13

Which corresponds to the temperature-dependent partial pressure above the surface of a liquid?

Accepted Answer

Vapor pressure is the pressure exerted by the vapor molecules over the surface of a liquid when the rate of evaporation and condensation becomes equal. It is dependent on the temperature and increases as temperature increases.

Question 14

Liquid sodium can be used as a heat transfer fluid. Its vapor pressure is 40.0 torr at 633°C and 400.0 torr at 823°C. What is its heat of vaporization?

Accepted Answer

The answer of Liquid sodium can be used as a...

Question 15

If a molecule at the surface of a liquid has enough kinetic energy to escape the liquid phase and enter the gas phase, then which of the following terms is used to describe this phenomenon?

Accepted Answer

The term used to describe a molecule at the surface of a liquid having enough kinetic energy to escape the liquid phase and enter the gas phase is vaporization. Boiling point refers to the temperature at which a liquid boils and turns into a gas. Condensation is the opposite of vaporization, where a gas turns into a liquid. Sublimation refers to a solid turning directly into a gas without passing through the liquid phase. Gasification is not a commonly used term to describe phase changes.

Question 16

What is determined by the magnitude of intermolecular forces in a liquid and is a measure of a fluid's resistance to flow?

Accepted Answer

The viscosity of a liquid is determined by the strength of its intermolecular forces, with stronger forces resulting in higher viscosity. Viscosity refers to a fluid's resistance to flow, and is therefore the best choice for this question. Surface tension, adhesion, polarity, and cohesion are all related to intermolecular forces, but do not directly measure a fluid's resistance to flow.

Question 17

The molar enthalpy of vaporization of hexane (C₆H₁₄) is 28.9 kJ/mol, and its normal boiling point is 68.73°C. What is the vapor pressure of hexane at 25°C? (R = 8.314 J/K • mol)

Accepted Answer

The Clausius-Clapeyron equation can be used to calculate the vapor pressure at a different temperature given the enthalpy of vaporization and the normal boiling point. The equation is ln(P1/P2) = ΔHvap/R * (1/T2 - 1/T1), where ΔHvap is the molar enthalpy of vaporization, R is the gas constant, T1 and T2 are the initial and final temperatures in Kelvin, and P1 and P2 are the initial and final pressures. Given that the normal boiling point of hexane is 68.73°C (341.88 K) and the enthalpy of vaporization is 28.9 kJ/mol, we can calculate the vapor pressure at 25°C (298.15 K) knowing that the vapor pressure at the boiling point (T1) is 1 atm (760 torr). Rearranging and solving for P2 (the vapor pressure at 25°C) involves converting ΔHvap to J/mol (since R is given in J/K•mol), and using the temperatures in Kelvin. Solving this equation yields a vapor pressure that is closest to the value given in choice A, 171 torr.

Question 18

Which substance has the lowest vapor pressure at room temperature?

Accepted Answer

HF has the lowest vapor pressure at room temperature due to strong hydrogen bonding, which increases intermolecular forces and decreases vapor pressure compared to the other substances listed.

Question 19

Which substance has the highest vapor pressure at room temperature?

Accepted Answer

HCl has the highest vapor pressure at room temperature among the options given. This is because it is the smallest molecule (in terms of molecular weight) compared to HF, HBr, and HI, leading to weaker intermolecular forces and thus a higher vapor pressure.

Question 20

What is defined as the number of atoms surrounding an atom in a crystal lattice and indicates how tightly the atoms are packed together?

Accepted Answer

Coordination number is defined as the number of atoms surrounding an atom in a crystal lattice and indicates how tightly the atoms are packed together. The other options do not accurately describe the definition of coordination number.

Quiz 11: Intermolecular Forces and the Physical Properties of Liquids and Solids