Solutions Definitions A solution is a homogeneous mixture A solute is dissolved in a solvent. solute is the substance being dissolved solvent is the liquid in which the solute is dissolved an aqueous solution has water as solvent A saturated solution is one where the concentration is at a maximum - no more solute is able to dissolve. A saturated solution represents an equilibrium: the rate of dissolving is equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate so that there appears to be nothing happening. Aqueous Solution Solutions LiCl (s) + H2O (l) LiCl Solute Solvent solution Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent. (s) (L) Dissolution of Solid Solute What are the driving forces which cause solutes to dissolve to form solutions? 1. Covalent solutes dissolve by H-bonding to water or by LDF 2. Ionic solutes dissolve by dissociation into their ions. How Does a Solution Form? 1. Solvent molecules attracted to surface ions. 2. Each ion is surrounded by solvent molecules. 3. Enthalpy (DH) changes with each interaction broken or formed. 1
How Does a Solution Form? 1. Solvent molecules attracted to surface ions. 2. Each ion is surrounded by solvent molecules. 3. Enthalpy (DH) changes with each interaction broken or formed. Ionic solid dissolving in water How Does a Solution Form The ions are solvated (surrounded by solvent). If the solvent is water, the ions are hydrated. The intermolecular force here is iondipole. Solutions 3 Stages of Solution Process How does a solid dissolve into a liquid? What drives the dissolution process? Separation of Solute must overcome IMF or ion-ion attractions in solute requires energy, ENDOTHERMIC ( + DH) Separation of Solvent must overcome IMF of solvent particles requires energy, ENDOTHERMIC (+ DH) Interaction of Solute & Solvent attractive bonds form between solute particles and solvent particles Solvation or Hydration (where water = solvent) releases energy, EXOTHERMIC (- DH) Dissolution at the molecular level? Consider the dissolution of NaOH in H2O Definitions Solubility- a substance that dissolves in water. - a substance that does not dissolve in water. Miscible (miscibility)- two liquids that mix together Ex. Alcohol and water Immiscible- two liquids that do not mix evenly, forms layers. Ex. Oil and water 2
Review Table F: Solubility Table F: Self Check soluble = insoluble = precipitate NH 4 CH 3 COO PbI 2 Na 2 CO 3 PbCrO 4 NaOH MgS LiClO 3 KNO 3 Ag 2 SO 4 AlPO 4 Ca(OH) 2 NaHCO 3 Table F: Self Check review LiCl Li 2 S K 2 CO 3 NiCrO 4 KOH CaS CaCO 3 NaCO 3 PbSO 4 Mg 3 (PO 4 ) 2 Ba(OH) 2 HCO 3 1. Nature of Solute / Solvent. - Like dissolves like 2. Temperature - i) Solids/Liquids- Solubility increases with Temperature Increase K.E. increases motion and collision between solute / solvent. ii) gas - Solubility decreases with Temperature Increase K.E. result in gas escaping to atmosphere. 3. Pressure Factor - i) Solids/Liquids - Very little effect Solids and Liquids are already close together, extra pressure will not increase solubility. ii) gas - Solubility increases with Pressure. Increase pressure squeezes gas solute into solvent. Example: ethanol in water Ethanol = CH 3 CH 2 OH The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve. Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water. Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces. 3
Which vitamin is watersoluble and which is fatsoluble? Solubilities of several ionic solid as a function of temperature. MOST salts have greater solubility in hot water. A few salts have negative heat of solution, (exothermic process) and they become less soluble with increasing temperature. Which three substances decrease in solubility when temperature increases? HCl NH 3 SO 2 Why? Predict another substance that would also decrease in solubility. Temperature Higher temperature drives gases out of solution. Carbonated soft drinks are more bubbly if stored in the refrigerator. Warm lakes have less O 2 dissolved in them than cool lakes. Energy Changes in Solution To determine the enthalpy change, we divide the process into 3 steps. 1. Separation of solute particles. 2. Separation of solvent particles to make holes. 3. Formation of new interactions between solute and solvent. Degree of saturation Saturated solution Solvent holds as much solute as is possible at that temperature. Undissolved solid remains in flask. Dissolved solute is in dynamic equilibrium with solid solute particles. Degree of saturation Unsaturated Solution Less than the maximum amount of solute for that temperature is dissolved in the solvent. No solid remains in flask. 4
Degree of saturation Degree of saturation Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can often be stimulated by adding a seed crystal or scratching the side of the flask. Unsaturated, Saturated or Supersaturated? How much solute can be dissolved in a solution? More on this in Chap 17 (solubility products, p 739) A solution which is on the line is saturated A solution below the line is unsaturated A solution above the line is supersaturated Saturated, unsaturated supersaturated How many grams of NaNO 3 do you need to make a saturated solution at 34⁰C in 100ml of water? How many grams of HCl can be dissolved in 200ml of water at 75⁰C? Describe the solution: 85g of KNO 3 in 100ml of water at 50⁰C. 105g of KNO 3 in 100ml of water at 50⁰C. 70g of KNO 3 in 100ml of water at 50⁰C. Saturated, unsaturated supersaturated How many grams of KCl do you need to make a saturated solution at 45⁰C in 100ml of water? How many grams of NaCl can be dissolved in 400ml of water at 90⁰C? Describe the solution: 140g of KI in 100ml of water at 10⁰C. 45g of KCl in 100ml of water at 60⁰C. 100g of KNO 3 in 100ml of water at 70⁰C. Chemists use the axiom like dissolves like : Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar solvents. 5
Glucose (which has hydrogen bonding) is very soluble in water. Cyclohexane (which only has dispersion forces) is not water-soluble. Math of solutions Definitions Concentration indicates how much dissolved solute is in solution (or solvent). Concentration calculations can be found at Reference Table T 2 ways of expressing concentration Parts per million(ppm) - parts solute / million parts solvent Molarity(M): moles solute / Liter solution Parts per million is usually used when there is a very small amount of solute dissolved in solvent. Molarity expresses moles per liter of solvent. Most of the time you will solve for moles before solving molarity. Molarity Concentration: Molarity Example If 0.435 g of KMnO 4 is dissolved in enough water to give 0.250 L of solution, what is the molarity of KMnO 4? As is almost always the case, the first step is to convert the mass of material to moles. If 0.435 g of KMnO 4 is dissolved in enough water to give 0.250L of solution, what is the molarity of KMnO 4? number of moles = 0.435 g KMnO 4 = 0.00275 mol KMnO 4 158.0 g KMnO 4 Now that the number of moles of substance is known, this can be combined with the volume of solution which must be in liters to give the molarity. Molarity KMnO 4 = 0.00275 mol KMnO 4 = 0.0110M 0.250 L solution 6
Parts per Million Preparing Solutions ppm = mass of A in solution total mass of solution 1,000,000 Weigh out a solid solute and dissolve in a given quantity of solvent. Dilute a concentrated solution to give one that is less concentrated. Dilution When a solution is diluted, solvent is added to lower its concentration. Suppose you have 0.500 M sucrose stock solution. How do you prepare 250 ml of 0.348 M sucrose solution? Concentration 0.500 M Sucrose Titration The amount of solute remains constant before and after the dilution: moles BEFORE = moles AFTER M 1 V 1 = M 2 V 2 250 ml of 0.348 M sucrose A bottle of 0.500 M standard sucrose stock solution is in the lab. Give precise instructions to your assistant on how to use the stock solution to prepare 250.0 ml of a 0.348 M sucrose solution. Acid Base Reactions Titrations Setup for titrating an acid with a base H 2 C 2 O 4 + 2 NaOH Na 2 C 2 O 4 + 2 H 2 O(l) acid base salt water Carry out this reaction using a TITRATION. Oxalic acid, H 2 C 2 O 4 7
Titration 1. Add solution from the buret. 2. Reagent (base) reacts with compound (acid) in solution in the flask. Indicator shows when exact stoichiometric reaction has occurred. (Acid = Base) This is called NEUTRALIZATION. Saturated, unsaturated supersaturated Review How many grams of HCl do you need to make a saturated solution at 90⁰C in 100ml of water? How many grams of NH 4 Cl can be dissolved in 200ml of water at 45⁰C? Describe the solution: 136g of KI in 100ml of water at 10⁰C. 80g of HCl in 100ml of water at 60⁰C. 5g of SO 2 in 100ml of water at 20⁰C. Quiz Questions How many grams do you need to make a saturated solution of KCl at 30 C in 100 grams of water? A 70 gram solution of NaCl at 90 C in 100mL of water is a solution. 8