Page 2 (ii) released (endothermic, ΔH is a negative value) (i) Solubility of a solute decreases with increasing temperature if ΔHsoln is negative (exothermic) (ii) Solubility of a solute increases with increasing temperature if ΔHsoln is positive (endothermic) (i) Solubility of a gaseous solute increases if its partial pressure increases. (ii) Solubility of a gaseous solute decreases if its partial pressure decreases. We can prepare a saturated aqueous solution by using the known solubility of a solute in a specified amount of water at a specified temperature. The solubility of some solutes in water is shown in the table below1.
For a saturated aqueous solution in equilibrium with gaseous solute, gas particles dissolved in water are in equilibrium with undissolved gas particles above the solution and we can write:
For a saturated aqueous solution in equilibrium with a molecular (covalent) solute which is a solid, molecules of solute dissolved in water are in equilibrium with undissolved molecules of solute and we can write:
For a saturated aqueous solution in equilibrium with an ionic solid solute, undissolved ions making up the lattice are in equilibrium with dissolved ions in solution and we can write:
Note that, at constant temperature and pressure, we can NOT increase the concentration of a saturated solution by adding more solute! Adding more CuSO4(s) to saturated CuSO4(aq) at constant temperature will NOT increase the concentration of CuSO4(aq) because the water has already dissolved as much CuSO4 as it can. Adding more NH3(g) to saturated NH3(aq) at constant temperature and pressure will NOT increase the concentration of NH3(aq) because the water has already dissolved as much of the NH3(g) as it can. It is possible to force more solute to dissolve in a given amount of solvent, if you change the conditions, that is, if you disturb the equilibrium .....
Do you know this? Join AUS-e-TUTE! Play the game now! Let's see what happens when we change either of these conditions. For a gaseous solute in equilibrium with its saturated aqueous solution, we can write solute(g) + H2O(l) solute(aq) By Le Chatelier's Principle, increasing the partial pressure of a gaseous solute will favour the side of the equation with the fewest gas molecules in order to minimise the effect of the increased pressure, that is, the equilibrium position will shift to the right and the concentration of solution will increase according to Henry's Law. Similarly, decreasing the partial pressure of the gaseous solute will force the equilibrium position to shift to the left to increase the number of gas molecules and hence the pressure, so solute molecules escape from the solution phase and enter the gaseous solute phase and the concentration of the solution decreases. In the school laboratory, increasing the pressure on a solid or liquid solute in equilbrium with its aqueous solution will have no noticeable effect. Changing TemperatureChanging the temperature of a saturated solution will result in more, or less, of the solute being dissolved depending on whether the process is endothermic or exothermic.When a solute dissolves in a solvent to form a solution, energy is either absorbed or released. If heat is released when the solute dissolves:
By Le Chatelier's Principle, if we add more heat to this system, the equilibrium position will shift to the left to minimise the effect of the change by consuming more heat, which will result in the concentration of the solution decreasing as more solute is formed on the reactant side of the equation. Similarly, if the system is cooled, that is, heat is removed from the system, the equilibrium position will shift to the right to minimise the effect of the change by producing more heat, so the concentration of the solution increases as more solute on the left hand side of the equation dissolves in the solvent. If heat is absorbed when the solute dissolves:
By Le Chatelier's Principle, if we add more heat to this system, the equilibrium position will shift to the right to minimise the effect of the change by consuming more heat, which will result in the concentration of the solution increasing as more solute is dissolved in solvent. Similarly, if the system is cooled, that is, heat is removed from the system, the equilibrium position will shift to the left to minimise the effect of the change by producing more heat, so the concentration of the solution decreases as more solute particles leave the solution phase. This means that in order to understand the impact of a change of temperature on the concentration of a specified saturated solution, we need to know whether the dissolving process is endothermic or exothermic.
Do you understand this? Join AUS-e-TUTE! Take the test now! Applying Le Chatelier's Principle, we can see that
For a saturated aqueous solution in equilibrium with its solute, we can write:
If we increase the temperature of the system, the equilibrium position will shift to the left according to Le Chatelier's Principle in order to minimise the effect of the change by consuming some of this energy. This means that the concentration of the solution will decrease and the amount of undissolved solute will increase. Another way to say this is to say that the solubility of the solute decreases with increasing temperature. Similarly, if we were to decrease the temperature of the system, the equilibrium position will shift to the right according to Le Chatelier's Principle in order to minimise the effect of the change by producing more heat. This means that the concentration of the solution will increase and the amount of undissolved solute will decrease. Another way to say this is to say that the solubility of the solute increases with decreasing temperature.
For example, consider a saturated aqueous solution of ammonia gas in equilibrium with gaseous ammonia at 25oC:
If we increase the temperature of the system while maintaining constant pressure, the equilibrium position will shift to the left by Le Chatelier's Principle so that the concentration of the aqueous ammonia solution will decrease and the amount of ammonia gas above the solution will increase. Endothermic process: ΔHhyd is positiveIf a solute absorbs energy from the surroundings when it dissolves in a solvent, the process is endothermic, energy is a reactant, and the heat (enthalpy) of solution is positive.For a saturated aqueous solution in equilibrium with its solute, we can write:
If we increase the temperature of the system, the equilibrium position will shift to the right according to Le Chatelier's Principle in order to minimise the effect of the change by consuming more energy. This means that the concentration of the solution will increase and the amount of undissolved solute will decrease. Another way to say this is to say that the solubility of the solute increases with increasing temperature. Similarly, if we were to decrease the temperature of the system, the equilibrium position will shift to the left according to Le Chatelier's Principle in order to minimise the effect of the change by producing more heat. This means that the concentration of the solution will decrease and the amount of undissolved solute will increase. Another way to say this is to say that the solubility of the solute decreases with decreasing temperature.
For example, consider a saturated aqueous solution of ammonium nitrate in equilibrium with solid ammonium nitrate at 25oC:
If we increase the temperature of the system, the equilibrium position will shift to the right by Le Chatelier's Principle so that the concentration of the aqueous ammonium nitrate solution will increase and the amount of undissolved solid ammonium chloride will decrease.
Can you apply this? Join AUS-e-TUTE! Take the exam now! 1It is assumed that gaseous solutes are acting as Ideal Gases, that is, there is no chemical reaction between the gaseous solute and the solvent. 2The values given are for solutions at infinite dilution which is the enthalpy change when 1 mole of solute in its standard state is dissolved in an infinite amount of water. 3These graphs are only descriptive in order to show general trends. The solubility curve for an given solute is much more likely to be a curve than a straight line. |