Heat of phase changes and dissolution Knowledge-Based Practice

Questions

1. 1.

   Why does the temperature of a pure crystalline solid remain constant while it is melting, even though heat is continuously supplied?

   • A) The absorbed thermal energy is converted into chemical energy by breaking intramolecular covalent bonds.
   • B) The absorbed thermal energy is used to overcome intermolecular forces holding the lattice together rather than increasing particle kinetic energy.
   • C) The rate of heat loss to the surroundings spontaneously balances the rate of heat absorption from the source.
   • D) The specific heat capacity of the substance drops to zero during the phase transition, preventing temperature change.
2. 2.

   When solid sodium hydroxide dissolves in water, the temperature of the resulting solution increases significantly. Which statement correctly explains why this dissolution process is exothermic?

   • A) The energy required to separate the ions from the crystal lattice is greater than the energy released when the ions are hydrated.
   • B) The total energy released during the hydration of the ions is greater than the energy absorbed to overcome the lattice energy of the solid.
   • C) Hydration of gaseous ions is an endothermic process that releases net thermal energy to the surroundings.
   • D) Breaking the ionic bonds in the solute releases thermal energy, while hydrating the ions requires an input of heat.
3. 3.

   Steam at 100 °C causes significantly more severe thermal burns than an equal mass of liquid water at 100 °C. Which statement best explains this difference in terms of thermochemical principles?

   • A) Steam possesses a much higher specific heat capacity than liquid water, allowing it to retain more thermal energy.
   • B) Steam releases a substantial amount of latent heat of condensation as it transitions into liquid water at 100 °C.
   • C) Water molecules in steam have a higher average kinetic energy than molecules in liquid water at the same temperature.
   • D) Steam undergoes an endothermic phase change upon contact with the skin, accelerating thermal transfer.
4. 4.

   The molar heat of vaporization of water (40.7 kJ/mol) is significantly greater than its molar heat of fusion (6.01 kJ/mol). Which statement correctly accounts for this difference?

   • A) Melting requires breaking the covalent O–H bonds within the molecules, whereas vaporization only overcomes weaker dispersion forces.
   • B) Vaporization requires completely separating molecules by overcoming all intermolecular hydrogen bonds, whereas melting only partially disrupts the hydrogen-bonding network.
   • C) Liquid water has a lower kinetic energy than solid ice, requiring a greater input of energy during the transition to the gaseous state.
   • D) Water expands significantly upon freezing, so the enthalpy of fusion reflects work done against atmospheric pressure rather than intermolecular attraction.
5. 5.

   When solid ammonium nitrate dissolves in water, the temperature of the mixture decreases. Which statement correctly explains why this dissolution process is endothermic?

   • A) The hydration energy released by the ions is greater than the lattice energy of the solid salt.
   • B) Strong covalent bonds within the nitrate polyatomic ions are completely broken upon dissolution.
   • C) The lattice energy required to separate the ions exceeds the total hydration energy released as the ions are solvated.
   • D) Water molecules absorb heat exclusively to undergo a phase change from liquid to gas during mixing.

Answers and step-by-step solutions appear after you finish the test.