Chemistry 1046 – Thermodynamics of Solutions

Flashcards covering the thermodynamics of solution formation, enthalpy and entropy changes, intermolecular forces, and the aqueous solubility behavior of ethanol, dimethyl ether, and propane.

What are examples of a gas-in-gas solution and a solid-in-solid solution (alloy)?

Air is a gas-in-gas solution (nitrogen is the solvent; oxygen and other gases are solutes), and bronze is a solid-in-solid solution (copper is the solvent; tin is the solute).

Thermodynamically, what is required to break chemical bonds or intermolecular interactions?

Breaking chemical bonds or intermolecular interactions always requires the input of energy (an endothermic process).

Thermodynamically, what occurs when new chemical bonds or intermolecular interactions are formed?

Forming new chemical bonds or intermolecular interactions always leads to the release of energy (an exothermic process).

How is an exothermic reaction defined in terms of bond energetics and system energy?

An exothermic reaction occurs when the interactions formed are stronger than those broken, resulting in a net release of energy ($\Delta H < 0$) where the system loses energy.

How is an endothermic reaction defined in terms of bond energetics and system energy?

An endothermic reaction occurs when the interactions formed are weaker than those broken, resulting in a net absorption of energy ($\Delta H > 0$) where the system gains energy.

How is entropy ($S$) defined, and what is the natural tendency of systems regarding entropy?

Entropy can be defined as the number of possible arrangements (microstates) for a given state; the natural tendency of systems is toward an increased number of arrangements (higher entropy).

What occurs in Step 1 of solution formation, and what is its enthalpy sign?

Step 1 involves separating solute particles from one another; it is always endothermic ($\Delta H_{\text{solute}} > 0$) because energy is needed to overcome solute-solute intermolecular forces.

What occurs in Step 2 of solution formation, and what is its enthalpy sign?

Step 2 involves separating solvent molecules to make room for solute particles; it is always endothermic ($\Delta H_{\text{solvent}} > 0$) because energy is needed to overcome solvent-solvent intermolecular forces.

What occurs in Step 3 of solution formation, and what is its enthalpy sign?

Step 3 occurs when solute and solvent particles mix; it is always exothermic ($\Delta H_{\text{mix}} < 0$) because energy is released when solute-solvent intermolecular forces form.

According to Hess' Law, what formula represents the overall enthalpy of solution ($\Delta H_{\text{soln}}$)?

$$\Delta H_{\text{soln}} = \Delta H_{\text{solute}} + \Delta H_{\text{solvent}} + \Delta H_{\text{mix}}$$

When $\Delta H_{\text{soln}} \approx 0$, what is the primary thermodynamic driving force for solution formation?

Solution formation is driven primarily by the increasing entropy upon mixing ($\Delta S > 0$).

What two thermodynamic factors drive an exothermic solution process ($\Delta H_{\text{soln}} < 0$)?

An exothermic solution process is driven by both lower potential energy ($\Delta H < 0$) and greater entropy ($\Delta S > 0$).

Under what condition can an endothermic solution process ($\Delta H_{\text{soln}} > 0$) still occur spontaneously?

It can occur spontaneously as long as $\Delta H_{\text{soln}}$ is not too large, specifically when $T\Delta S > \Delta H$, allowing $\Delta G$ to remain negative.

What general rule of thumb predicts solubility based on intermolecular forces?

"Like dissolves like"—polar solvents dissolve polar or ionic substances, while non-polar solvents dissolve non-polar substances.

What temperature change occurs when ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) dissolves in water, and what does this indicate about $\Delta H$?

The solution temperature decreases, which indicates that the dissolution process is endothermic ($\Delta H > 0$).

What are the signs of $\Delta H$, $\Delta S$, and $\Delta G$ for ethanol dissolving in water?

$\Delta H > 0$ (endothermic), $\Delta S > 0$ (increased particle disorder upon mixing), and $\Delta G < 0$ (spontaneous solution formation).

What hybridization and local molecular geometry does the oxygen atom exhibit in ethanol?

The oxygen atom exhibits $sp^3$ hybridization with a bent molecular shape and bond angles near $109.5^\circ$.

What is the strongest intermolecular interaction that forms between ethanol and water molecules?

Hydrogen bonding (ethanol can both donate and accept hydrogen bonds with water).

What temperature change occurs when dimethyl ether ($\text{CH}_3\text{OCH}_3$) dissolves in water, and what does this indicate about $\Delta H$?

The solution temperature increases, which indicates that the dissolution process is exothermic ($\Delta H < 0$).

What are the signs of $\Delta H$, $\Delta S$, and $\Delta G$ for dimethyl ether dissolving in water?

$\Delta H < 0$ (heat released), $\Delta S > 0$ (mixing disorder), and $\Delta G < 0$ (thermodynamically favorable / spontaneous).

How does dimethyl ether participate in hydrogen bonding with water compared to ethanol?

Dimethyl ether can only accept hydrogen bonds from water through its ether oxygen atom ($-\text{O}-$), whereas ethanol can both donate and accept hydrogen bonds.

What happens to the temperature and enthalpy when propane ($\text{CH}_3\text{CH}_2\text{CH}_3$) is added to water?

The temperature remains constant, meaning the enthalpy change is approximately zero ($\Delta H_{\text{soln}} \approx 0$).

What is the hybridization and geometry of all carbon atoms in propane?

All carbon atoms exhibit $sp^3$ hybridization with tetrahedral geometry and bond angles near $109.5^\circ$.

What is the hydrophobic effect and its consequence on system entropy when propane is in water?

The hydrophobic effect causes water molecules to order themselves around non-polar solute molecules, decreasing system entropy ($\Delta S < 0$).

Why is propane insoluble in water from a thermodynamic standpoint?

Because $\Delta H \approx 0$ and $\Delta S < 0$ due to water cage ordering, the Gibbs free energy change is positive ($\Delta G > 0$), making dissolution non-spontaneous.

What is the strongest intermolecular interaction present between propane and water?

Dipole-induced dipole forces (between polar water and non-polar propane).

What is the ranking of propane, dimethyl ether, and ethanol in order of increasing aqueous solubility?

$$\text{propane} < \text{dimethyl ether} < \text{ethanol}$$

Why is ethanol more soluble in water than dimethyl ether?

Ethanol possesses a polar hydroxyl group ($-\text{OH}$) capable of both donating and accepting hydrogen bonds, making it miscible in all proportions, whereas dimethyl ether can only accept hydrogen bonds.

What thermodynamic equation defines $\Delta G$ and determines whether solution formation is spontaneous?

$\Delta G = \Delta H - T\Delta S$ (spontaneous dissolution strictly requires $\Delta G < 0$).

What are the molecular shape and approximate bond angle of a water molecule ($\text{H}_2\text{O}$)?

Water has a bent molecular shape with bond angles of approximately $104.5^\circ$.

What structural criteria must be met for hydrogen bonding to occur?

A hydrogen atom must be directly bonded to a highly electronegative atom: nitrogen ($\text{N}$), oxygen ($\text{O}$), or fluorine ($\text{F}$).