How Solutions FormAnimation of NaCl Dissolving • The intramolecular forces holding solute particles must be overcome, separating an individual particle from the group of particles. • The intermolecular forces holding solvent particles must be overcome, thus making an opening within the group of solvent particles for a solute particle to fit. • Attractiveforces between solute and solvent particles must be formed in order to make the solution or solvation process complete.
Solutions: Homogeneous Mixtures • Homogeneous-same properties spread throughout • Sample off the top and sample off the bottom will have similar properties (no settling) • Solutions-NO Tyndall effect • Demonstration of the Tyndall Effect: • http://www.silver-lightning.com/tyndall/
Solution ConcentrationHow much solute is dissolved in a specific amount of solvent or solution.
We will focus on Molarity… • A 1 molar (1M) solution has 1 mole of solute for every liter of solution. • How many grams is 1 mol? See the Periodic Table!!
Mol Review Quickie • Mole – amount of substance that contains Avagadro’s number of particles • Mass of one mole-use the molar mass from the Periodic Table. • Sample molarityProblem: • What is the Molar concentration of a solution that has 52.0 g of carbon dioxide dissolved in 2500 mL of water?
Solution Moles of CO2 2 X 16.00 = 32.00 1 X 12.01 = 12.01 44.01 g/mol 52.0 g X mol/ 44.01g = 1.18 mols • By definition, Molarity is the number of mols per Liter of solution. • 1.18 mols/2500 mL = 1.18 mols/2.5 L = 0.473 M BIG M means Molarity (and the University of Michigan!)
Some Terms • Concentrated vs. Dilute
Dilutions • M1V1=M2V2
Let’s Do Some Dilution Problems • Given any three, solve for the unknown • Two terms: stock and diluted solution; stock is always stronger. • M1V1=M2V2 Annabel has 50.0 mL of stock solution with a concentration of 6.0 M. What volume of 1.5 M solution can she make?
Solution to Solution Dilution Problem • M1V1=M2V2 • It doesn’t matter to which “substance,” you assign the subscripts, only that you are consistent. • In this case, let’s make M1V1 the stock: • M1= 6.0 M • V1 = 50.0mL • V2 = ? • M1 = 1.5 M M1V1=M2V2 ; M1V1/ M2=V2 6.0 M X 50.0 mL / 1.5 M = 200 mL
Solvation Basics • “Like dissolves like” • Polar solutes dissolve in polar solvents. • Nonpolar solutes dissolve in nonpolar solvents. • Bonding, polarity, and intermolecular forces (IMFs) need to be considered. • IONIC vs. MOLECULAR (sucrose)
What makes a molecule polar? • An electronegativity difference between 2 bonded atoms of 0.5-1.7 • Asymmetrical distribution of electrons • Remember “like dissolves like” so since water is polar, it will dissolve polar solutes! • Oils & grease are nonpolar and will dissolve nonpolar solutes!
Heat of Solution: the overall energy change that occurs during solution formation • Energy is required to overcome the attractive forces within the solute and within the solvent. • ENDOTHERMIC • When solute and solvent particles mix, the particles attract each other and energy is released. • EXOTHERMIC
Factors Affecting Solvation • Agitation • Stirring/shaking allows new collisions between solute & solvent particles. • Surface Area • Smaller pieces means greater surface area. • Greater surface area allows for more COLLISIONS & faster dissolving. • Temperature • For solid solutes, increasing temperature increasessolvation. • For gas solutes, increasing temperature decreasessolvation. (higher KE allows gas particles to escape from solution!)
What’s the main trend here? • As T increases, so does solubility – a direct relationship. • What’s the deal with Cerium (III) Sulfate? • An exception to the rule!! It’s unstable in the presence of water and is exothermic when it dissolves. These types of substances dissolve better in cold water than warm! • When else would we see a substance that dissolves better in cold water? • GASES!!!
Solubility Terms • Unsaturated Solutions • Contains less dissolved solute for a given temperature & pressure than a saturated solution. • More solute can be dissolved. • Saturated Solutions • Contains the maximum amount of dissolved solute for a given amount of solvent at a specific T & P. • Equilibrium between solvation & crystallization. • Supersaturated Solutions (**UNSTABLE**) • Contains MORE dissolved solute than a saturated solution at the same T. • To make a SS Solution, make a saturated solution, then cool it s l o w l y.
Colligative Properties of Solutions • Physical properties of solutions that are affected by the number of particles but not by the identity of dissolved solute particles. • Electrolytes in aqueous solution • Ionic compounds dissociate in water to form a solution that conducts electric current • Ex. 1 mole of NaCl yields 2 mol of solute particles in solution … 1 mol Na+& 1 mol Cl-. • Nonelectrolytes do not ionize / dissociate in solution • Ex. Sucrose
**Which would have a greater effect on colligative properties, NaCl or sucrose? • NaCl – it dissociates forming twice as many particles!!
4 Colligative Properties1. Vapor Pressure Lowering REVIEW • Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped as a gas. • When Pvap=Patm, boiling occurs. • Adding a nonvolatile solute (one that has little tendency to become a gas) to a solvent lowers the solvent’s vapor pressure. • Solute particles prevent solvent particles from escaping at the surface as much. • Which lowers vapor pressure most: 1 mol of NaCl, 1 mol of Na2SO4, or 1 mol of AlCl3? • AlCl3, then Na2SO4, then NaCl – greater number of ions effects colligative properties the most!!
4 Colligative Properties 2. Boiling Point Elevation • Adding a nonvolatile solute lowers Pvap so it takes more heat & KE to raise thePvap so it equals Patm & boils! • Remember: molality = moles of solute kilograms of solvent Solute particles “get in the way!” Pure Water 1.0 M NaCl solution - NaCl produces Na+ ions (blue) and Cl- ions (green) when dissolved in water.
4 Colligative Properties 3. Freezing Point Depression - LINK • At a solvent’s freezing point temperature, particles don’t have enough KE (kinetic energy)to overcome the interparticleattractive forces so they become a solid. • In a solution, solute particles interfere with the attractive forces among solvent particles & lower the freezing point.
Dashed lines show the differences in freezing and boiling points for water when solute is added.
4 Colligative Properties4. Osmotic Pressure • Osmosis is the diffusion of a solvent through a semipermeable membrane. Ex. cell membranes • Solute particles can’t pass through the membrane but water molecules can. • Water always moves from areas of low solute concentrations to higher solute concentrations . • Osmotic Pressure: pressure caused by water molecules moving into the more concentrated solution. • More solute particles on one side of the membrane increases osmotic pressure.