13.1.4 Energy and the Solution Process Flashcards Preview

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Flashcards in 13.1.4 Energy and the Solution Process Deck (12)
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1
Q

Energy and the Solution Process

A
  • 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.
  • Most solvation processes have a net increase in entropy; however, there are exceptions.
2
Q

note

A
  • The solubility of a given solute in a solvent depends on two major factors: the enthalpy change (ΔH) and the entropy change (ΔS) of the solvation process.
  • In order to dissolve a substance, the bonds between the solute particles and the intermolecular forces within the solvent must be broken. Both of these steps cost energy (ΔH > 0).
  • Energy is released as the solute and solvent come together and form new bonds with each other.
  • Whether or not the solvation process is exothermic or endothermic depends on the relative energies of the three steps of solvation.
  • When the solvent is water, the energy-releasing step in the solvation process is referred to as hydration. The enthalpy of hydration (ΔHhyd) is the energy released by the formation of a cage of water molecules around an ion or molecule.
  • The magnitude of the enthalpy of hydration for an ion depends on the charge and the size of the ion. More energy is released from the hydration of smaller ions with larger charges.
  • Li + has a more negative ΔH hyd than K + because it is smaller and the water molecules can orient themselves closer to the center of charge on the ion.
  • In general, the mixing of a solute with a solvent tends to cause an increase in entropy. There are some solvation processes, however, where there is a decrease in entropy as the solvent and solute become more ordered.
  • The solvation of ions can cause increased ordering of a solvent. This loss of entropy is especially prevalent with small, highly charged solutes.
  • Similarly, when a nonpolar solute is placed in a polar solvent such as water, the water molecules will cluster around the nonpolar molecules and increase the order of the solvent. This phenomenon is known as the hydrophobic effect. The hydration of nonpolar molecules causes a large drop in entropy, with little change in enthalpy. Thus, nonpolar molecules tend not to dissolve in water.
3
Q

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

A; We assume that we are not using a nonpolar substrate because a nonpolar substrate causes a decrease in entropy.

4
Q

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?

A

In a hydration event, an ion is surrounded by water molecules through an exothermic process.

5
Q

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?

A

ΔH is positive for an endothermic reaction. Heat energy has to be input as a reactant for this reaction to proceed

6
Q

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?

A

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.

7
Q

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?

A

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.

8
Q

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?

A

Points B and E

9
Q

Which of the following statements about ΔH for a solvation (or hydration) event is correct?

A

If ΔH is positive, it costs energy to form the solution. The energy component is, arguably, a reactant, and the reaction is endothermic.

10
Q

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?

A

Points C and F identify the solution components just prior to forming the solution. The final solution is indicated by point H.

11
Q

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?

A

Water molecules cluster around nonpolar substrates, decreasing the entropy. As a result, the substrates do not dissolve in water.

12
Q

Which of the following ultimately determines how soluble a solute is in a solvent?

A

both the value of ΔH and the entropy of the system

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