AP 6.5 Heating Curves

How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.)

• 0.24 kJ

• 3.8 kJ

• 33 kJ

• 60. kJ

A sample of CHCl3(s) was exposed to a constant source of heat for a period of time. The graph above shows the change in the temperature of the sample as heat is added. Which of the following best describes what occurs at the particle level that makes segment D longer than segment B?

• The specific heat capacity of the liquid is significantly higher than that of the solid, because the particles in the liquid state need to absorb more thermal energy to increase their average speed.

• The specific heat capacity of the solid is significantly higher than that of the gas, because the particles in the solid state need to absorb more thermal energy to increase their average speed.

• The enthalpy of fusion is greater than the enthalpy of vaporization, because separating molecules from their bound crystalline state requires more energy than separating molecules completely from the liquid state.

• The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.

A 2.00mol sample of C2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, ΔHvap, of C2H5OH is +38.6kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram?

• +19.3kJ

• +77.2kJ

• −19.3kJ

• −77.2kJ

The graph shows the temperature of a pure substance as it is heated at a constant rate in an open vessel at 1.0 atm pressure. The substance changes from the solid to the liquid to the gas phase. The substance is at its normal freezing point at time

• t₁

• t₂

• t₃

• t₄

• t₅

The graph shows the temperature of a pure substance as it is heated at a constant rate in an open vessel at 1.0 atm pressure. The substance changes from the solid to the liquid to the gas phase. Which of the following best describes what happens to the substance between t₄ and t₅?

• The molecules are leaving the liquid phase.

• The solid and liquid phases coexist in equilibrium.

• The vapor pressure of the substance is decreasing.

• The average intermolecular distance is decreasing.

• The temperature of the substance is increasing.

The diagram above represents the melting of H2O(s). A 2.00mole sample of H2O(s) at 0°C melted, producing H2O(l) at 0°C. Based on the diagram, which of the following best describes the amount of heat required for this process and the changes that took place at the molecular level?

• 3.01 kJ of heat was absorbed to decrease the average speed of the water molecules in the liquid, which decreases the distance between molecules.

• 6.02 kJ of heat was absorbed to increase the number of hydrogen bonds between water molecules in the liquid compared to the solid.

• 12.0 kJ of heat was absorbed to decrease the polarity of the water molecules, which increases the density of the liquid compared to the solid.

• 12.0 kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.

A molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture

• is lost to the surroundings very quickly

• is used in overcoming the intermolecular attractions in the solid

• is used in breaking the bonds within the molecules of the solid

• causes the nonbonding electrons in the molecules to move to lower energy levels

• causes evaporation of the liquid, which has a cooling effect

The cooling curve above shows how the temperature of a sample varies with time as the sample goes through phase changes. The sample starts as a gas, and heat is removed at a constant rate. At which time does the sample contain the most liquid?

• t₁

• t₂

• t₃

• t₄

• t₅

Between which points is the temperature of the substance remaining constant?

• A-B only

• A-B, C-D, and E-F

• B-C only

• B-C and D-E

The melting point of the sample is

• -60 ºC

• 20 ºC

• 60 ºC

• 100 ºC