AP Chem Unit 6.5: Energy of Phase Changes

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 used in overcoming the intermolecular attractions in the solid

The following questions relate to the graph below. 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

t2

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?

−77.2kJ

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?

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

The following questions refer to the graph below, which shows the heating curve for methane, CH4. How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.)

3.8 kJ