Solutions

1. Solutions • Solutions are homogeneous mixtures of two or more pure substances. • In a solution, the solute is dispersed uniformly throughout the solvent.

2. Solutions The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

3. How Does a Solution Form? As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

4. How Does a Solution Form If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

5. Energy Changes in Solution • Simply put, three processes affect the energetics of solution: • separation of solute particles, • separation of solvent particles, • new interactions between solute and solvent.

6. Energy Changes in Solution The enthalpy change of the overall process depends on H for each of these steps.

7. Why Do Endothermic Processes Occur? Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.

8. Why Do Endothermic Processes Occur? Yet we know the in some processes, like the dissolution of NH4NO3 in water, heat is absorbed, not released.

9. Enthalpy Is Only Part of the Picture The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.

10. Enthalpy Is Only Part of the Picture So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.

11. Types of Solutions • Saturated • In a saturated solution, the solvent holds as much solute as is possible at that temperature. • Dissolved solute is in dynamic equilibrium with solid solute particles.

12. Types of Solutions • Unsaturated • If a solution is unsaturated, less solute than can dissolve in the solvent at that temperature is dissolved in the solvent.

13. Types of Solutions • Supersaturated • In supersaturated solutions, the solvent holds more solute than is normally possible at that temperature. • These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

14. Factors Affecting Solubility • Chemists use the axiom “like dissolves like." • Polar substances tend to dissolve in polar solvents. • Nonpolar substances tend to dissolve in nonpolar solvents.

15. Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

16. Factors Affecting Solubility Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.

17. Factors Affecting Solubility • Vitamin A is soluble in nonpolar compounds (like fats). • Vitamin C is soluble in water.

18. Gases in Solution • In general, the solubility of gases in water increases with increasing mass. • Larger molecules have stronger dispersion forces.

19. Gases in Solution • The solubility of liquids and solids does not change appreciably with pressure. • The solubility of a gas in a liquid is directly proportional to its pressure.

20. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

21. Temperature • The opposite is true of gases. • Carbonated soft drinks are more “bubbly” if stored in the refrigerator. • Warm lakes have less O2 dissolved in them than cool lakes.

22. Colligative Properties • Changes in colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. • Among colligative properties are • Vapor pressure lowering • Boiling point elevation • Melting point depression • Osmotic pressure

23. Vapor Pressure Because of solute-solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.

24. Vapor Pressure Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

25. Boiling Point Elevation and Freezing Point Depression Nonvolatile solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

26. Capillary Action–the movement of liquid within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. The liquid is able to move up the filter paper because its attraction to itself is stronger than the force of gravity. Solubility – the degree to which a material (solute) dissolves into a solvent. Solutes dissolve into solvents that have similar properties. (Like dissolves like) This allows different solutes to be separated by different combinations of solvents. Separation of components depends on both their solubility in the mobile phase and their differential affinity to the mobile phase and the stationary phase. Important Concepts in Paper Chromatography

27. Paper/TLC Chromatography Animation

28. Simple Example of Paper Chromatography using “Sharpie” Pens

29. Dye Separation in a Black “Sharpie” 1. Dyes separated – purple and black 2. Not soluble in low concentrations of isopropanol 3. Partially soluble in concentrations of isopropanol >20% 0% 20% 50% 70% 100% Concentration of Isopropanol

30. Thin Layer Chromatography • Works mainly on principle of adsorption. • Mobile phase is the solvent • Stationary phase is the solid on the plate.

31. TLC vs. Column Chromatography • Thin-layer chromatography and column chromatography and are different types of liquid chromatography. • The mobile (moving) phase is a liquid. The stationary phase is usually silica or alumina. This phase is very polar. • The principle of operation is the same!

32. Thin Layer Chromatography The surface of a plate consists of a very thin layer of silica on a plastic or aluminum backing. The silica is very polar. This is the stationary phase. Material is spotted at the origin (bottom) of the TLC plate. The plate is placed into a glass jar with a small amount of a solvent in a glass jar. This solvent acts as the moving phase. The plate is removed from the bottle when the solvent is close to the top of the plate. The spots are visualized (explanation to follow). Non-polar compounds will be less strongly attracted to the plate and will spend more time in the moving phase. This compound will move faster and will appear closer to the top of the plate. Polar compounds will be more strongly attracted to the plate and will spend less time in the moving phase and appear lower on the plate.

33. s o l v e n t f r o n t o r i g i n Thin-Layer Chromatography: A Two-Component Mixture s o l v e n t f r o n t Less polar! c o m p o n e n t B s o l v e n t f r o n t c o m p o n e n t B More polar! c o m p o n e n t A c o m p o n e n t A mixture o r i g i n o r i g i n Increasing Development Time

34. Thin-Layer Chromatography: Determination of Rf Values Rf of component A = dA dS Rf of component B = dB dS The Rf value is a decimal fraction, generally only reported to two decimal places