Solubility Equilibrium

Transcription

1 2016 Ksp note.notebook Solubility Equilibrium Learning Goals: to understand what happens when a compound dissolves in water to calculate the extent of dissolution...the molar solubility to calculate the solubility product constant, Ksp, given molar solubilities SOLUBILITY EQUILIBRIUM Solubility equilibrium is based on the assumption that solids dissolve in water to give the basic particles from which they are formed. Molecular solids dissolve to give individual aqueous molecules. Ionic solids dissociate to give their respective positive and negative ions: The ions formed from the dissociation of ionic solids can carry an electrical current. Salt solutions, therefore, are good conductors of electricity. Molecular solids, however, do not dissociate in water to give ions, so no electrical current can be carried. Solubility The ratio of the maximum amount of solute to the volume of solvent in which this solute can dissolve. Generally expressed in two ways: grams of solute per 100 g of water grams per Litre moles of solute per Liter of solution g/100g g/l mol/l 1

2 Solubility of Salts Soluble Ions: Dissolve Do NOT form a solid precipitate. alkali metal ions and ammonium ion: Li +, Na +, K + +, NH 4 1 acetate ion: C 2 H 3 O 2 1 nitrate ion: NO 3 halide ions (X 1 ): Cl, Br, I (Exceptions: AgX, HgX, and PbX 2 are insoluble) sulfate ion: SO 2 4 (Exceptions: SrSO 4, BaSO 4, and PbSO 4 are insoluble; Ag 2 SO 4, CaSO 4, and Hg 2 SO 4 are slightly soluble) Insoluble Ions: Do NOT Dissolve Do form a solid precipitate. 2 carbonate ion: CO 3 2 chromate ion: CrO 4 3 phosphate ion: PO 4 sulfide ion: S 2 (Exceptions: CaS, SrS, and BaS are soluble) hydroxide ion: OH (Exceptions: Sr(OH) 2 and Ba(OH) 2 are soluble; Ca(OH) 2 is slightly soluble) A salt is considered soluble if it dissolves in water to give a solution with a concentration of at least 0.1 M at room temperature. A salt is considered insoluble if the concentration of an aqueous solution is less than M at room temperature. Salts with solubilities between M and 0.1 M are considered to be slightly soluble. Equilibrium of Salts Salts that have extremely low solubilities dissociate in water according to the principles of equilibrium. For example, the reaction for the dissociation of the salt AgCl is: The reverse reaction for the dissolving of the salt would be the precipitation of the ions to form a solid: The system has reached equilibrium when the rate at which AgCl dissolves is equal to the rate at which AgCl precipitates. Saturated solution Contains the maximum concentration of ions that can exist in equilibrium with the solid salt at a given temperature. The equilibrium reaction for the dissociation of AgCl is: 2

3 At first, the only process occurring is the dissolving of the salt the dissociation of the salt into its ions. However, soon the ions floating in the water begin to collide with the salt crystal and are pulled back in to the salt. (precipitation) Eventually the rate of dissociation is equal to the rate of precipitation. The solution is now saturated. It has reached equilibrium. Solubility Equilibrium: Dissociation = Precipitation In a saturated solution, there is no change in amount of solid precipitate at the bottom of the beaker. Concentration of the solution is constant. The rate at which the salt is dissolving into solution equals the rate of precipitation. 3

4 Solubility Product Equilibrium Constant (Ksp) The product of the equilibrium concentrations of the ions in a saturated solution of a salt. Each concentration is raised to the power of the respective coefficient of ion in the balanced equation. The solid reactant is not written because its concentration effectively remains constant. Consider the reaction for the dissociation of CaF 2 in water: The solubility product equilibrium constant for this reaction would be the product of the concentration of Ca +2 ion and the concentration of the F 1 ion raised to the second power (squared): The solubility product is literally the product of the solubilities of the ions in units of molarity (mol/l) NOTE: There is no denominator in the solubility product equilibrium constant. The key word to remember is PRODUCT which can remind you that you should have a multiplication (or product) of the concentrations of the ions. When K sp is very small, there is a higher concentration of reactants, (solid) than ions. Therefore, reactants are favoured, and the solid is considered insoluble. Would NaCl (s) have a large or small K sp value? How do you know? Ksp values are found in your text book on page 725. These values are constant at 25 0 C Which is more soluble Calcium carbonate or Silver carbonate? Which has a higher concentration of carbonate ions? 4

5 Sample Calculations Ksp can be calculated from the solubility of a salt. Conversely, the solubility of a salt can be calculated from Ksp. Finding K sp when given the molar solubility Calculate the K sp of silver sulfide(ag 2 S). The molar solubility is 1.3 x mol/l. 1. What is the K sp expression for silver sulfide? 2 What symbol or value does molar solubility represent? 3 Create an ICE table for the solubility of Ag 2 S. 4 Substitute values into your K sp expression. 5 Interpret the K sp value you obtained. What does this value tell you? Calculating Molar Solublility [Finding (x)] Molar Solubility refers to the amount that dissolves per amount of solvent. Find the molar solubility of PbCl 2(s). The K sp of PbCl 2 is 1.2x Write the dissociation equation for PbCl 2(s) PbCl 2(s) 2. Create an I.C.E table to find the amount that dissociates (x) PbCl 2(s) Pb +2 (aq) + 2Cl (aq) I C E 3. Solve for x. X refers to the molar solubility, the amount that dissolves per litre of solution. 4. Change the molar solubility you just calculated to units of g/l. 5