General Guidelines on Solubility

Transcription

1 General Guidelines on Solubility Solubility is a measure of the maximum amount of substance (solute) that will dissolve in a given volume of solvent at a certain temperature. 1. The Dissolving Process General guideline: Like dissolves like If the type of bonding is similar between the solute and the solvent, there is a good chance that the solute will dissolve (the solvent bonds substitute for the bonds between the solute particles). Nonpolar substances dissolve best in nonpolar solvents (such as dry cleaning solvents, hexane) Polar substances dissolve best in polar solvents (like water, acetone). Substances that dissolve in water can either be: (a) an electrolyte - substance that dissolves to give an electrically conducting solution containing ions. e.g. NaCl(g) Na + (aq) + Cl (aq) (b) a non-electrolyte - substance that dissolves to give a non conducting solution containing only neutral molecules. They are produced from molecules (covalent bonds). No ions = no conductivity. e.g. C12H22O11 (s) C12H22O11 (aq) * Beware of acetate (CH3COO - ) and other organic ions. They will form ionic solutions. *

2 2. Degree of Saturation To determine solubility, one must first completely saturate a solution. Saturated Solutions At saturation, the maximum amount of solute will have dissolved and there will also be some undissolved solute present. In other words, the rate of dissolution = the rate of crystallization meaning equilibrium exists! Solubility: equilibrium concentration of a solute in solution at a given temperature (in other words, the concentration of a saturated solution). We show that a solution is saturated by writing an equilibrium equation. Ag2SO4(s) 2Ag + (aq) + SO4 2 (aq) Meaning, Solid Ag2SO4 dissociates into ions (dissolving reaction) Ag2SO4(s) 2Ag + (aq) + SO4 2 (aq) Ag + and SO4 2 ions come together to form Ag2SO4 (crystallization reaction) 2Ag + (aq) + SO4 2 (aq) Ag2SO4(s) Unsaturated Solutions Contain less than the maximum amount of substance which can dissolve Rate of dissolving > rate of crystallization No undissolved solute present Not at equilibrium Supersaturated Solutions Contains more than the maximum amount of solute Rate of dissolving < rate of crystallization Not at equilibrium

3 3. Solubility Curves Solubility curves can be used to tell how soluble a certain solute is at a certain temperature. On a solubility curve, the lines indicate the concentration of a saturated solution - the maximum amount of solute that will dissolve at that specific temperature. Values on the graph below a curve represent unsaturated solutions. Values above a curve represent supersaturated solutions. For most substances, solubility increases as temperature increases. Here's an example of reading the chart. Find the curve for KClO3. At 30 C approximately 10g of KClO3 will dissolve in 100g of water. If the temperature is increased to 80 C, approximately 40g of the substance will dissolve in 100g (or 100mL) of water. Another example: What term - saturated, unsaturated, or supersaturated - best describes a solution that contains 70g of NaNO3 per 100 ml H2O at 30 C? Find the curve for NaNO3. At 30 C a saturated solution would be able to dissolve approximately 95 g of NaNO3. Since there are only 70g in the solution, 25 more grams of NaNO3 could be added and it would all dissolve meaning the solution is unsaturated.

4 General Guidelines on Solubility is a measure of the amount of substance ( ) that will dissolve in a given volume of at a certain temperature. 1. The Dissolving Process General guideline: Like dissolves like If the type of bonding is between the solute and the solvent, there is a good chance that the solute will dissolve (the solvent bonds substitute for the bonds between the solute particles). Nonpolar substances dissolve best in solvents (such as dry cleaning solvents, hexane) Polar substances dissolve best in solvents (like water, acetone). Substances that dissolve in water can either be: (a) an electrolyte - substance that dissolves to give an electrically conducting solution containing. e.g. NaCl(g) Na + (aq) + Cl (aq) (b) a non-electrolyte - substance that dissolves to give a non conducting solution containing only molecules. They are produced from molecules (covalent bonds). No ions=no conductivity. e.g. C12H22O11 (s) C12H22O11 (aq) * Beware of acetate (CH3COO - ) and other organic ions. They will form ionic solutions. *

5 2. Degree of Saturation To determine solubility, one must first completely a solution. Saturated Solutions At saturation, the maximum amount of solute will have dissolved and there will also be some undissolved solute present. In other words, the rate of dissolution = the rate of crystallization meaning exists! Solubility: equilibrium concentration of a solute in solution at a given temperature (in other words, the concentration of a saturated solution). We show that a solution is saturated by writing an equation. Ag2SO4(s) 2Ag + (aq) + SO4 2 (aq) Meaning, Solid Ag2SO4 dissociates into ions ( reaction) Ag2SO4(s) 2Ag + (aq) + SO4 2 (aq) Ag + and SO4 2 ions come together to form Ag2SO4 ( reaction) 2Ag + (aq) + SO4 2 (aq) Ag2SO4(s) Unsaturated Solutions Contain than the maximum amount of substance which can dissolve Rate of dissolving > rate of crystallization No undissolved solute present Not at equilibrium Supersaturated Solutions Contains than the maximum amount of solute Rate of dissolving < rate of crystallization

6 3. Solubility Curves Solubility curves can be used to tell how soluble a certain solute is at a certain temperature. On a solubility curve, the lines indicate the concentration of a solution - the maximum amount of solute that will dissolve at that specific temperature. Values on the graph below a curve represent unsaturated solutions. Values above a curve represent supersaturated solutions. For substances,. Here's an example of reading the chart. Find the curve for KClO3. At 30 C approximately 10g of KClO3 will dissolve in 100g of water. If the temperature is increased to 80 C, approximately 40g of the substance will dissolve in 100g (or 100mL) of water. Another example: What term - saturated, unsaturated, or supersaturated - best describes a solution that contains 70g of NaNO3 per 100 ml H2O at 30 C? Find the curve for NaNO3. At 30 C a saturated solution would be able to dissolve approximately 95 g of NaNO3. Since there are only 70g in the solution, 25 more grams of NaNO3 could be added and it would all dissolve meaning the solution is unsaturated.