Question 1

For the process: HNO₃(g)⇌ HNO₃(l),ΔH° is -39.04 kJ/mol and ΔS° is -111.74 J/(mol ∙ K).What is the normal boiling point of pure HNO₃? A)2.86°C B)76.2°C C)270.3°C D)349.4°C

Accepted Answer

The normal boiling point of a substance is the temperature at which its vapor pressure equals atmospheric pressure. At this point, the Gibbs free energy change (ΔG°) for the phase transition is zero. The relationship between ΔG°, ΔH°, and ΔS° is given by the equation ΔG° = ΔH° - TΔS°. Setting ΔG° to zero for the boiling point and solving for T (temperature in Kelvin) gives T = ΔH° / ΔS°. Given ΔH° = -39.04 kJ/mol = -39040 J/mol and ΔS° = -111.74 J/(mol·K), the temperature T in Kelvin is T = -39040 J/mol / -111.74 J/(mol·K) = 349.4 K. Converting this to Celsius gives 349.4 K - 273.15 = 76.2°C.

Question 2

Which of the following best explains why ΔH_vap is usually higher than ΔH_fusion? A)Vaporization occurs at high temperature. B)Vaporization involves the breaking of bonds within molecules. C)Vaporization involves the breaking of all bonds between molecules. D)Vaporization increases the entropy of molecules.

Accepted Answer

Vaporization involves the breaking of all intermolecular bonds between molecules, transitioning from a liquid to a gas state, which requires more energy than the transition from solid to liquid in fusion, where only some intermolecular bonds are broken.

Question 3

While mercury is very useful in barometers,mercury vapor is toxic.Given that mercury has a ΔH_vap of 59.11 kJ/mol and its normal boiling point is 356.7°C,calculate the vapor pressure in mm Hg at room temperature,25°C. A)2.68 × 10^-3 mm Hg B)2.99 mm Hg C)372 mm Hg D)753 mm Hg

Accepted Answer

We can use the Clausius-Clapeyron equation to calculate the vapor pressure of mercury at 25°C.

ln(P2/P1) = -(ΔHvap/R) x (1/T2 - 1/T1)

where P1 is the vapor pressure at the boiling point (taken from a steam table), P2 is the vapor pressure at the desired temperature (25°C), ΔHvap is the enthalpy of vaporization, R is the gas constant, and T1 and T2 are the boiling point and the desired temperature in Kelvin, respectively.

Converting the boiling point to Kelvin: 
T1 = 356.7°C + 273.15 = 629.85 K

Substituting the values: 
ln(P2/P1) = -(59.11 kJ/mol)/(8.314 J/mol-K) x (1/298 K - 1/629.85 K)

Solving for P2: 
P2/P1 = e^[-(59.11 kJ/mol)/(8.314 J/mol-K) x (1/298 K - 1/629.85 K)]

P2 = P1 x e^[-(59.11 kJ/mol)/(8.314 J/mol-K) x (1/298 K - 1/629.85 K)]

Using the vapor pressure of mercury at its boiling point (taken from a steam table), P1 = 1.28E+05 mm Hg

P2 = 1.28E+05 mm Hg x e^[-(59.11 kJ/mol)/(8.314 J/mol-K) x (1/298 K - 1/629.85 K)] 
P2 = 2.68E-03 mm Hg

Rounding to two significant figures: 
P2 = 2.7E-03 mm Hg 
P2 = 2.68 × 10^-3 mm Hg

Therefore, the vapor pressure of mercury at 25°C is 2.68 × 10^-3 mm Hg, which is closest to choice A.

Question 4

When a narrow diameter glass tube is inserted into a body of water,water rises in the tube and its surface inside is concave upwards.Which statement,concerning the strength of the intermolecular forces between glass and water molecules compared to those between water molecules,is accurate?
A)The forces of attraction between the glass and water are weaker than those in water.
B)The forces of attraction between the glass and water are stronger than those in water.
C)The forces of attraction between the glass and water are the same as those in water.
D)Intermolecular forces are irrelevant to this situation.

Accepted Answer

The concave meniscus formed in the glass tube indicates that the attractive forces between the glass and water molecules are stronger than the cohesive forces between water molecules alone, resulting in water being drawn up into the tube. Therefore, the answer is (B) the forces of attraction between the glass and water are stronger than those in water.

Question 5

The vapor pressure of a pure liquid increases as the
A)average kinetic energy of the molecules in the liquid phase decreases.
B)intermolecular attractive forces increase.
C)temperature of the liquid phase decreases.
D)temperature of the liquid phase increases.

Accepted Answer

According to the Clausius-Clapeyron equation, the vapor pressure of a pure liquid increases with an increase in temperature. This is because at higher temperatures, the average kinetic energy of the molecules in the liquid phase increases, and more molecules have sufficient energy to overcome the attractive forces between them and escape into the gas phase, resulting in a higher vapor pressure. Therefore, choice D, which states that the vapor pressure of a pure liquid increases as the temperature of the liquid phase increases, is the correct choice.

Question 6

Ethyl chloride,C₂H₅Cl,is used as a local anesthetic.It works by cooling tissue as it vaporizes.The heat of vaporization is 26.4 kJ/mol.How much heat could be removed by 60.0 g of ethyl chloride? A)24.6 kJ B)1584 kJ C)2.28 kJ D)1703 kJ

Accepted Answer

First, calculate the molar mass of ethyl chloride (C2H5Cl): C (12.01 g/mol) * 2 + H (1.008 g/mol) * 5 + Cl (35.45 g/mol) = 64.51 g/mol. Then, use the formula q = m/M * ΔHvap, where m = 60.0 g, M = 64.51 g/mol, and ΔHvap = 26.4 kJ/mol. This gives q = (60.0 g / 64.51 g/mol) * 26.4 kJ/mol ≈ 24.6 kJ.

Question 7

How much heat is released when 125.0 g of steam at 100.0°C is cooled to ice at -15.0°C? The enthalpy of vaporization of water is 40.67 kJ/mol,the enthalpy of fusion for water is 6.01 kJ/mol,the molar heat capacity of liquid water is 75.4 J/(mol ∙ °C),and the molar heat capacity of ice is 36.4 J/(mol ∙ °C).
A)91.3 kJ
B)325 kJ
C)380 kJ
D)413 kJ

Accepted Answer

The answer of How much heat is released when 125.0...

Question 8

As a liquid evaporates at a temperature below its boiling point,the temperature of the liquid
A)decreases.
B)decreases at low temperatures,but increases at high temperatures.
C)increases.
D)remains unchanged.

Accepted Answer

When a liquid evaporates at a temperature below its boiling point, the evaporation causes the liquid molecules with the highest kinetic energy to escape into the gas phase. This results in a decrease in the average kinetic energy of the remaining liquid molecules, which corresponds to a decrease in temperature.

Question 9

Molecules of a liquid can pass into the vapor phase only if the
A)liquid has little surface tension.
B)molecules have sufficient kinetic energy to overcome the intermolecular forces in the liquid.
C)temperature of the liquid is near its boiling point.
D)vapor pressure of the liquid is high.

Accepted Answer

Molecules can transition from the liquid phase to the vapor phase when they possess enough kinetic energy to overcome the intermolecular forces that hold them together in the liquid state. This process is not solely dependent on factors like surface tension, the temperature being near the boiling point, or the vapor pressure being high, although these can influence the rate of evaporation.

Question 10

Which of the following is expected to have the greatest viscosity? A)C₅H₁₂ B)C₆H₁₄ C)C₅H₁₁OH D)CH₄

Accepted Answer

The presence of oxygen atoms in this compound (OH) leads to stronger intermolecular forces, which in turn results in higher viscosity.

Question 11

The magnitude of the heats of vaporization,fusion and sublimation of a substance reflect the
A)density of the substance.
B)magnitudes of the boiling and melting points of the substance.
C)strength of the covalent bonds between atoms in each molecule of the substance.
D)strength of the intermolecular forces of the substance.

Accepted Answer

The magnitude of the heats of vaporization, fusion and sublimation of a substance reflect the strength of the intermolecular forces of the substance. This is because these processes involve breaking or forming intermolecular forces between molecules. The stronger the intermolecular forces, the more energy is required to break them and therefore the higher the magnitude of these heats. The other choices, such as density, boiling and melting points, and covalent bond strength, are not directly related to these heats.

Question 12

Which of the following compounds has the highest boiling point? A)CH₃CH₂OH B)HOCH₂CH₂OH C)H₃C-O-CH₃ D)CH₃CH₂CH₂CH₃

Accepted Answer

Boiling point is related to intermolecular forces. The compound with the highest boiling point will have the strongest intermolecular forces. In this case, compound B has two hydrogen bonds, which are stronger than the dipole-dipole or London dispersion forces present in the other compounds. Therefore, B has the highest boiling point.

Question 13

CFC-11 (trichlorofluoromethane,CCl₃F)has been used for many years as the working fluid in refrigerators.Given its heat of vaporization is 26.88 kJ/mol and its entropy of vaporization is 90.51 J/(mol ∙ K),what is the boiling point of CFC-11? A)-272.9°C B)0.297°C C)2.44°C D)23.8°C

Accepted Answer

The boiling point of CFC-11 can be calculated using the Clausius-Clapeyron equation, which relates the heat of vaporization ($\Delta H_{vap}$) and the entropy of vaporization ($\Delta S_{vap}$) to the boiling point ($T_b$) at 1 atm pressure. The equation in a simplified form for boiling point calculation is $T_b = \frac{\Delta H_{vap}}{\Delta S_{vap}}$. Given $\Delta H_{vap} = 26.88 \, \text{kJ/mol} = 26880 \, \text{J/mol}$ and $\Delta S_{vap} = 90.51 \, \text{J/(mol} \cdot \text{K)}$, the boiling point $T_b$ is calculated as $T_b = \frac{26880 \, \text{J/mol}}{90.51 \, \text{J/(mol} \cdot \text{K)}} = 297 \, \text{K}$. To convert to degrees Celsius, subtract 273.15 from the Kelvin temperature, resulting in $23.85^\circ C$, which rounds to $23.8^\circ C$.

Question 14

Bromine is one of only two elements that is a liquid at room temperature.Bromine has a heat of vaporization of 30.91 kJ/mol and its boiling point is 59°C.What is the entropy of vaporization for bromine?
A)-301 J/(mol ∙ K)
B)-93.1 J/(mol ∙ K)
C)10.7 J/(mol ∙ K)
D)93.1 J/(mol ∙ K)

Accepted Answer

The entropy of vaporization ($\Delta S_{vap}$) can be calculated using the formula $\Delta S_{vap} = \frac{\Delta H_{vap}}{T_{b}}$, where $\Delta H_{vap}$ is the heat of vaporization and $T_{b}$ is the boiling point in Kelvin. First, convert the boiling point from Celsius to Kelvin: $59°C + 273.15 = 332.15 K$. Then, plug the values into the formula: $\Delta S_{vap} = \frac{30.91 \times 10^3 J/mol}{332.15 K} = 93.1 J/(mol \cdot K)$.

Question 15

The normal boiling point for HBr is higher than the normal boiling point for HCl.This can be explained by
A)larger dipole-dipole forces for HBr.
B)larger dispersion forces for HBr.
C)larger hydrogen-bond forces for HBr.
D)larger dipole-dipole forces,larger dispersion forces,and larger hydrogen-bond forces for HBr.

Accepted Answer

HBr has a higher molecular weight than HCl, leading to larger dispersion forces, which increases its boiling point. Hydrogen bonding is not significant in either molecule due to the lack of a strongly electronegative atom bonded to hydrogen capable of hydrogen bonding, and while both have dipole-dipole interactions due to their polar bonds, the key difference in boiling points is attributed to the dispersion forces.

Question 16

For a particular compound,which is expected to be the largest in general?
A)the heat required to raise the temperature of one mole of the gas 10.0°C
B)the heat required to raise the temperature of one mole of the liquid 10.0°C
C)the molar heat of fusion at the normal melting point
D)the molar heat of vaporization at the normal boiling point

Accepted Answer

The molar heat of vaporization at the normal boiling point is expected to be the largest because it represents the amount of energy required to completely vaporize one mole of the substance at its boiling point. This process requires the breaking of intermolecular forces between the molecules, which requires a substantial amount of energy. The other options only represent the amount of energy required to raise the temperature or change the state of the substance, which is generally less than the energy required for vaporization.

Question 17

Hydroquinone is an antioxidant that is also used as a photographic reducer and developer.The normal boiling point of hydroquinone is 310°C.Calculate the pressure at which hydroquinone will boil at 200°C given that its ΔH_vap is 73.38 kJ/mol. A)1.210 × 10^-4 mm Hg B)1.35 mm Hg C)22.5 mm Hg D)757 mm Hg

Accepted Answer

First, we need to calculate the ΔHvap of hydroquinone: ΔHvap = ΔH_vap/R = 73.38 kJ/mol / 8.314 J/mol·K = 8832.7 J/mol We can then use the Clausius-Clapeyron equation to find the pressure at 200°C: ln(P2/P1) = -ΔHvap/R * (1/T2 - 1/T1) We assume P1 = 1 atm and T1 = 310°C + 273.15 = 583.15 K T2 = 200°C + 273.15 = 473.15 K Substituting the values and solving for P2, we get: P2 = e^(-ΔHvap/R * (1/T2 - 1/T1)) * P1 = 22.5 mm Hg Therefore, the best choice is C.

Question 18

When a substance melts at its normal melting point,the sign of ΔH is ________ and the sign of ΔS of this phase change is ________.
A)+,-
B)-,+
C)+,+
D)-,-

Accepted Answer

The answer of When a substance melts at its normal...

Question 19

The normal boiling point occurs when the
A)intermolecular forces within the liquid phase are broken.
B)temperature of the pure liquid equals the external temperature.
C)vapor pressure of a pure liquid equals an external pressure of one atmosphere.
D)vapor pressure of the liquid equals the external pressure.

Accepted Answer

The normal boiling point is defined as the temperature at which the vapor pressure of a pure liquid equals the external pressure of one atmosphere. This occurs when there is enough thermal energy to overcome the intermolecular forces in the liquid, resulting in the molecules escaping into the gas phase. Therefore, choice C is the best answer.

Question 20

Which of the intermolecular forces is the most important contributor to the high surface tension shown by water?
A)dipole-dipole forces
B)dispersion forces
C)hydrogen bonding
D)ion-dipole forces

Accepted Answer

Hydrogen bonding plays the most significant role in water's surface tension as it creates strong attractions between neighboring water molecules, resulting in a cohesive force that holds the surface together. The other forces listed, such as dipole-dipole forces, dispersion forces, and ion-dipole forces, are relatively weaker and do not contribute as significantly to the high surface tension of water.

Quiz 11: Liquids, solids, and Phase Changes

For the process: HNO₃(g) ⇌ HNO₃(l) ,ΔH° is -39.04 kJ/mol and ΔS° is -111.74 J/(mol ∙ K) .What is the normal boiling point of pure HNO₃?

Which of the following best explains why ΔH_vap is usually higher than ΔH_fusion?

While mercury is very useful in barometers,mercury vapor is toxic.Given that mercury has a ΔH_vap of 59.11 kJ/mol and its normal boiling point is 356.7°C,calculate the vapor pressure in mm Hg at room temperature,25°C.

When a narrow diameter glass tube is inserted into a body of water,water rises in the tube and its surface inside is concave upwards.Which statement,concerning the strength of the intermolecular forces between glass and water molecules compared to those between water molecules,is accurate?

The vapor pressure of a pure liquid increases as the

Ethyl chloride,C₂H₅Cl,is used as a local anesthetic.It works by cooling tissue as it vaporizes.The heat of vaporization is 26.4 kJ/mol.How much heat could be removed by 60.0 g of ethyl chloride?

As a liquid evaporates at a temperature below its boiling point,the temperature of the liquid

Molecules of a liquid can pass into the vapor phase only if the

Which of the following is expected to have the greatest viscosity?

The magnitude of the heats of vaporization,fusion and sublimation of a substance reflect the

Which of the following compounds has the highest boiling point?

CFC-11 (trichlorofluoromethane,CCl₃F) has been used for many years as the working fluid in refrigerators.Given its heat of vaporization is 26.88 kJ/mol and its entropy of vaporization is 90.51 J/(mol ∙ K) ,what is the boiling point of CFC-11?

Bromine is one of only two elements that is a liquid at room temperature.Bromine has a heat of vaporization of 30.91 kJ/mol and its boiling point is 59°C.What is the entropy of vaporization for bromine?

The normal boiling point for HBr is higher than the normal boiling point for HCl.This can be explained by

For a particular compound,which is expected to be the largest in general?

Hydroquinone is an antioxidant that is also used as a photographic reducer and developer.The normal boiling point of hydroquinone is 310°C.Calculate the pressure at which hydroquinone will boil at 200°C given that its ΔH_vap is 73.38 kJ/mol.

When a substance melts at its normal melting point,the sign of ΔH is and the sign of ΔS of this phase change is .

The normal boiling point occurs when the

Which of the intermolecular forces is the most important contributor to the high surface tension shown by water?