Energy and the Solution Process (13.1.4) Flashcards
• The solubility of a substance depends on the enthalpy change and the entropy change for its solvation.
• The solubility of a substance depends on the enthalpy change and the entropy change for its solvation.
• Solvation requires breaking intermolecular bonds within the solute and the solvent and then forming new intermolecular bonds in the solution.
• Solvation requires breaking intermolecular bonds within the solute and the solvent and then forming new intermolecular bonds in the solution.
• Most solvation processes have a net increase in entropy; however, there are exceptions.
• Most solvation processes have a net increase in entropy; however, there are exceptions.
hydrophobic effect -
the ordering of water by nonpolar solutes, resulting in a decrease in entropy; the decrease in solubility of nonpolar solutes in water
Look at the diagram that compares the hydration of three different ions.
Which of the following is the correct rule for comparing the values of ΔH for different solvation events?
The larger the ion, the less the energy released by the solvation event. This is because a smaller ion bonds more strongly with H2O molecules than a larger ion does. (B)
Smaller ions have stronger bonds with water molecules than do larger ions. This is because the water molecules are closer to the central charge of the ion.
Which of the following correctly describes and explains the relationship between the values for ΔH for small ions vs. larger ions involved in hydration events?
ΔH for hydrating smaller ions is greater than ΔH for larger ions. This is because the water molecules have a stronger bond to the center charges of smaller ions. (B)
In larger molecules, the center charge and the water molecules are farther apart. Therefore, they are not as strong.
Look at the diagram for the formation of a solution from a solute and solvent.
Which of the following correctly identifies the corresponding components of this event?
Points C and F identify the solution components just prior to forming the solution. The final solution is indicated by point H. (B)
Point C identifies the solute component and Point F identifies the solvent component just prior to forming the solution. The final solution is indicated by Point H.
Nonpolar substrates are solutes that are not polar. They are very prevalent in nature (e.g., methane, ethane) and are found in the human body as nonpolar parts of proteins. Which of the following best describes how water molecules react with nonpolar substrates?
Water molecules cluster around nonpolar substrates, decreasing the entropy. As a result, the substrates do not dissolve in water. (C)
The water molecules cluster in a very orderly way. Therefore, the entropy of the system decreases. As a result of the clustering, the substrates do not dissolve in water.
Look at the diagram for the formation of a solution from a solute and solvent.
Which of the following best indicates the energy requirement(s) to break up any intermolecular forces?
Points B and E (D)
This is the best answer because both Point B and Point E are correct.
Look at the diagram for the formation of a solution from a solute and solvent.
Assuming we are not using a nonpolar solute, which of the following correctly identifies the most ordered component of the process? Also, which of the following best explains why we need to assume we are not using a nonpolar solute?
A; We assume that we are not using a nonpolar substrate because a nonpolar substrate causes a decrease in entropy. (B)
You can see from the diagram that Point A, which indicates the solute lattice, is the most ordered. In addition, we assume that we are not using a nonpolar substrate because a nonpolar substrate causes a decrease in entropy. It actually causes the solvent to be more ordered, which results in a decrease of entropy.
The value for ΔH for a solvation reaction can be measured by a calorimeter. Which statement correctly describes the value for ΔH for a solvation event?
ΔH is positive for an endothermic reaction. Heat energy has to be input as a reactant for this reaction to proceed. (A)
ΔH = enthalpy change = Hf − Hi . In an endothermic event, the value of Hf for the system is greater than Hi for the system because heat is absorbed by the system from the surroundings. Therefore, ΔH is positive. Heat energy is a reactant component of this event.
We often speak of reactions in aqueous solution. This means events that occur in water. When a solute dissolves in water, the solute is hydrated by water. Which statement best describes what happens in a hydration event?
In a hydration event, an ion is surrounded by water molecules through an exothermic process. (D)
ΔH is negative for a hydration event; it is an exothermic process.
Which of the following ultimately determines how soluble a solute is in a solvent?
both the value of ΔH and the entropy of the system (D)
Although answers A and B are partially correct, the correct answer is the value of ΔH and the entropy of the system. These two factors together determine the solubility of a solute in a solvent.
Which of the following statements about ΔH for a solvation (or hydration) event is correct?
If ΔH is positive, it costs energy to form the solution. The energy component is, arguably, a reactant, and the reaction is endothermic. (A)
If ΔH is positive, it costs energy to form the solution because the reaction is endothermic; heat energy has to be input to cause the reaction to proceed. Therefore, the energy component is, arguably, a reactant. In other words, you need heat energy, along with the solution components, to make this reaction occur.
