6.5 Notes: Energy of Phase Changes

Multiple Choice

Does water release or absorb thermal energy to become ice?

• Release

• Absorb

Possible Quiz Questions

Multiple Choice

A sample of CHCl3 (s) was exposed to a constant source of heat for a period of time. The graph shows the change in 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 a 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.

Multiple Choice

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 it's normal freezing point at what time?

• t1

• t2

• t3

• t4

Multiple Choice

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

• 19.3 kJ

• 77.2 kJ

• -19.3 kJ

• -77.2 kJ

Multiple Choice

A sample of ice at 0°C is added to 100.g of water at 33°C. The mixture is stirred gently until the temperature of the water is 0°C. All the remaining ice is quickly removed. Based on the information in the table above, the mass of ice that melted is closest to

• 0.13 g

• 0.42 g

• 1.3 g

• 42 g

Multiple Choice

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.

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