Thermochemistry Winter Break

Which of the following best helps to explain why the value of ΔH° for the dissolving of CaF2 in water is positive?

• Ca⁺² ions have very strong ion-ion interactions with F^- ions in the crystal lattice.

• Ca2+ ions have very strong ion-dipole interactions with water molecules in the solution.

• CaF2(s) dissolves in water to form CaF2(aq) particles.

• CaF2(s) is insoluble in water.

Some building materials contain small capsules filled with paraffin wax to improve the insulating properties of the materials. The melting and freezing of paraffin inside the capsules in the material helps to regulate temperature. Paraffin wax begins to melt at 37°C

.

(a) While the wax is melting, is the net flow of thermal energy from the wax to the surroundings or from the surroundings to the wax? Justify your answer.

Answer explanation

Thermal energy flows from the surroundings to the wax.

Melting is an endothermic process that requires the flow of thermal energy into the material for the phase change to occur.

Some building materials contain small capsules filled with paraffin wax to improve the insulating properties of the materials. The melting and freezing of paraffin inside the capsules in the material helps to regulate temperature. Paraffin wax begins to melt at 37°C

37°C.

(b) Calculate the amount of thermal energy, in kJ

kJ, that is required to melt 15.2

15.2 grams of solid paraffin wax when the temperature of the surroundings is above the melting point of paraffin. (The molar mass of paraffin is 282.62g/mol

282.62g/mol, and its molar heat of fusion is 48.78kJ/mol

48.78kJ/mol.)

Answer explanation

15.2 g × (1 mol / 282.62 g) × (48.78 kJ / 1 mol)

= 2.62 kJ

Some building materials contain small capsules filled with paraffin wax to improve the insulating properties of the materials. The melting and freezing of paraffin inside the capsules in the material helps to regulate temperature. Paraffin wax begins to melt at 37°C

37°C.

(c) When paraffin wax melts, its density (density=mass/volume ie. the amount of particles in a given space) decreases. Provide a particle-level explanation of this observation.

Answer explanation

As the solid is heated, the particles are provided enough energy to break bonds and move freely. This causes the greater average distance between particles in liquid paraffin compared to solid paraffin.

This increased distance between the moving particles causes the decrease in density.

The following questions relate to the below information.

XY₂ → X + Y₂

The equation above represents the decomposition of a compound XY₂. The diagram below shows two reaction profiles (path one and path two) for the decomposition of XY₂.

• 50 kJ of heat is transferred from the surroundings.

• 50 kJ of heat is transferred to the surroundings.

• 100 kJ of heat is transferred to the surroundings.

• 100 kJ of heat is transferred from the surroundings.

Answer explanation

The product side of the potential energy diagram is 50kJ/mol higheer than the reactant. This means energy is gained during the process. Energy must be gained from the surroundings.

K(s)+ ½ Cl₂(g) → KCl(s) ΔH° = -437kJ/mol_rxn

The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow.

Which of the values of ∆H^o for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process)?

• z only

• y and z only

• x , y , z only

• w,x ,y , and z

Answer explanation

Y is addding an electron to an atom. This is something seen in electron affinity. when electrons are added to a substance, it will release energy and e an exothermic process.

Z is exothrmic as two gas ions react to form a solid. Solids have less kinetic energy than a gas. this process will release the kinetic energy from the gas ions to form a solid compound.

In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C ?

• The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C.

• The freezing of water is an endothermic process; thus, water that freezes on the blossoms absorbs heat from the atmosphere, which in turn keeps the blossoms above 0°C.

• Water is a good thermal conductor that transfers heat from the cold air to the blossoms, keeping the blossoms from going below −2°C.

• Water on the blossoms will not freeze unless the air temperature falls significantly below −5°C.

Answer explanation

As the water freezes, heat is released, and the temperature of the blossoms will be about 0°C instead of below −2°C.

Which of the following phase changes involves the transfer of heat from the surroundings to the system?

• CH₄(g)→CH₄(l), because CH₄ molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the solid state.

• CO₂(g)→CO₂(s), because CO₂ molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the liquid state.

• H₂O(l)→H₂O(s), because H₂O

  molecules in the liquid phase must absorb energy in order to create a crystalline structure with strong intermolecular attractions in the solid state.

• NH₃(l)→NH₃(g), because NH₃

  molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.

Answer explanation

Evaporation of a liquid to form a gas is an endothermic process because energy absorbed from the surroundings is required to overcome the intermolecular forces among particles in the liquid, allowing particles to become free moving in the gas phase.

The dissolution of an ionic solute in a polar solvent can be imagined as occurring in three steps, as shown in the figure above. In step 1, the separation between ions in the solute is greatly increased, just as will occur when the solute dissolves in the polar solvent. In step 2, the polar solvent is expanded to make spaces that the ions will occupy. In the last step, the ions are inserted into the spaces in the polar solvent. Which of the following best describes the enthalpy change, ΔH, for each step?

• All three steps are exothermic.

• All three steps are endothermic.

• Steps 1 and 2 are exothermic, and the final step is endothermic.

• Steps 1 and 2 are endothermic, and the final step is exothermic.

Answer explanation

This option is correct. In steps 1 and 2, favorable intermolecular interactions are broken and this increases the enthalpy, corresponding to an endothermic process. In step 3, favorable intermolecular interactions are established, which decreases the enthalpy, corresponding to an exothermic process. (The sign of the enthalpy change for the overall process is established by the difference in magnitude between these enthalpy changes, and so the overall process can be either endothermic or exothermic.)

For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students’ observation?

• An exothermic chemical change occurred because heat flowed from the contents of the beaker to the room.

• An exothermic physical change occurred because heat flowed from the contents of the beaker and the water on the board to the room.

• An endothermic physical change occurred because the freezing of water is an endothermic process.

• An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.

Answer explanation

In this demonstration, the system can be defined the contents of the beaker. As the reaction between the two solids occurs, heat is absorbed from the surroundings, which includes the beaker and the water on the wooden board. The transfer of heat from the water on the board to the system causes the temperature of the water to decrease below its freezing point. As a result, the bottom of the beaker becomes frozen to the wood surface. Therefore, the reaction is an endothermic chemical change.