Solution Equilibrium

Solution Equilibrium
The Three-Stage Solution Process

  • The formation of a solution can be divided into three distinct stages:

Stage 1:

  • The process begins with solvent particles surrounding solute particles.

  • During this stage, the amount of solid solute decreases over time as it begins to dissolve.

Stage 2:

  • As dissolution continues, the concentration of solute particles increases to a point where:

    • They collide with each other, leading to some of the solute precipitating out of the solution.

    • However, the rate of dissolution still exceeds the rate of recrystallization, indicating that more solute is going into solution than is leaving it in solid form.

Stage 3:

  • At this final stage, a dynamic equilibrium is reached:

    • The rate of dissolution is equal to the rate of recrystallization.

    • In mathematical terms, this can be represented as:

      extRateextdissolution=extRateextrecrystallizationext{Rate}_ ext{dissolution} = ext{Rate}_ ext{recrystallization}

    • The solution at this point is considered to be in a state of dynamic equilibrium.

Example of Sodium Chloride (NaCl) Solution Process

  • Initially, when sodium chloride (NaCl) is added to water:

    • Solid NaCl dissolves into sodium (Na$^+$) and chloride (Cl$^-$) ions:

      extNaCl(s)→extNa+(aq)+extCl−(aq)ext{NaCl}(s) \rightarrow ext{Na}^+(aq) + ext{Cl}^-(aq)

  • As the solution's concentration rises, some Na$^+$ and Cl$^-$ ions will begin to stop dissolving and to recrystallize back into solid NaCl:

    • The dynamic equilibrium process is summarized in a stepwise manner:

    • (a) Initial conditions - rate of dissolution is greater than the rate of recrystallization.

    • (b) During the dissolving process, concentrations shift until equilibrium is established.

    • (c) At dynamic equilibrium - rates of dissolution and recrystallization are equal.

Saturation

  • A saturated solution is defined as:

    • The solution that contains the maximum possible amount of dissolved solute at a given temperature.

    • In saturation:

    • The solute and solvent are in dynamic equilibrium, so the concentration of solute remains constant.

    • If more solute is added to a saturated solution, it will not dissolve, and the concentration will not change.

  • An unsaturated solution:

    • Contains less solute than what can be dissolved at that temperature, indicating that dynamic equilibrium has not yet been reached.

    • The concentration is below the saturation level.

Supersaturation

  • The concept of supersaturation arises from:

    • Conditions changing, such as temperature or pressure, which affect saturation concentration.

    • A solution is termed supersaturated if it contains more solute than its saturation point.

  • Creation of a supersaturated solution can occur through:

    • Heating a saturated solution to increase solubility, adding more solute at this higher temperature, and then cooling it back down.

  • Note:

    • Supersaturated solutions are typically unstable. Excess solute may precipitate out quickly from the solution.

  • Example: Adding a piece of solid sodium acetate to a supersaturated sodium acetate solution will trigger crystallization of the excess solute.

TopHat Question Summary

  1. Question regarding saturation:

    • When saturated, which is NOT true?

      • A. The rates of solute molecules going in and out of solution are equal.

      • B. Net rate of dissolution of solute is zero.

      • C. Net rate of precipitation of solute is zero.

      • D. Solution concentration depends on the amount of precipitated solute present.

  2. Question regarding copper(II) chloride:

    • A saturated solution at 40

      °C\text{°C} is heated to 80

      °C\text{°C} to dissolve more solute; what type of solution is this?

      • A. Unsaturated

      • B. Saturated

      • C. Supersaturated

Temperature and Solubility of Solids/Liquids

  • Solubility curves represent maximum solubility at given temperatures:

    • Above the line indicates supersaturation.

    • Below the line indicates unsaturation.

  • General observation:

    • The solubility of most solid solutes in water increases as temperature increases, with exceptions (e.g., cerium(III) sulfate, Ce$2$(SO$4$)$_3$).

Enthalpy of Solution

  • The relationship between temperature and solubility is impacted by the enthalpy of solution principles:

    • If \Delta H_{\text{solution}} < 0 (exothermic process), decreasing the temperature will increase solubility.

    • If \Delta H_{\text{solution}} > 0 (endothermic process), increasing the temperature will boost solubility.

Factors Affecting the Solubility of Gases

  • Characteristics of gas solubility:

    • Gas solubility does not require overcoming solute-solute attractions, making the enthalpy changes always negative (exothermic).

    • Result: Gas solubility tends to decrease as temperature rises.

    • Example: Carbon dioxide (CO$_2$) escapes from warm soda much faster than from cold soda.

Pressure and Gas Solubility

  • Pressure Effects:

    • Pressure has negligible effects on the solubility of solids and liquids. However, it plays significant role in gas solubility.

  • Henry's Law:

    • States that gas solubility increases with pressure.

    • The equation for Henry's Law:

    • Let:

      • SS = solubility of the gas (in mol/L)

      • kHk_H = Henry's law constant (in M/atm, dependent on solute, solvent, and temperature)

      • PgasP_{\text{gas}} = partial pressure of the gas

Practice Questions

  • Calculate the concentration of CO$_2$ in a soft drink with a partial pressure of 380 torr at 25

    °C\text{°C}:

    • Henry's law constant for CO$_2$ at 25

      °C\text{°C} is 3.4 × 10$^{-2}$ mol/L·atm.

    • Density of the solution is 1.00 g/mL.

Additional TopHat Question

  • Given a system in equilibrium, if the pressure is decreased from 1 atm to 0.66 atm, a diagram comparison will indicate the new equilibrium state after restoration.