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Solution chemistry enthalpy
- 1. Solutions Properties of Solutions Enthalpyof Solution
- 2. Solutions Solutions How does asolid dissolve into a liquid? What ‘drives’ the dissolution process? What are the energetics of dissolution?
- 3. Solutions How Does aSolution Form? 1. Solvent molecules attracted to surface ions. 2. Each ion is surrounded by solvent molecules. 3. Enthalpy (∆H) changes with each interaction broken or formed. Ionic solid dissolving in water
- 4. Solutions How Does aSolution Form? 1. Solvent molecules attracted to surface ions. 2. Each ion is surrounded by solvent molecules. 3. Enthalpy (∆H) changes with each interaction broken or formed.
- 5. Solutions How Does aSolution Form The ions are solvated (surrounded by solvent). If the solvent is water, the ions are hydrated. The intermolecular force here is ion- dipole.
- 6. Solutions What is enthalpy •a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume
- 7. Solutions Energy Changes inSolution 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.
- 8. Solutions Enthalpy Changes inSolution The enthalpy change of the overall process depends on ∆H for each of these steps. Start End EndStart
- 9. Solutions Enthalpy changes duringdissolution The enthalpy of solution, ∆Hsoln, can be either positive or negative. ∆Hsoln = ∆H1 + ∆H2 + ∆H3 ∆Hsoln (MgSO4)= -91.2 kJ/mol --> exothermic ∆Hsoln (NH4NO3)= 26.4 kJ/mol --> endothermic
- 10. Solutions Why do endothermicprocesses sometimes occur spontaneously? Some processes, like the dissolution of NH4NO3 in water, are spontaneous at room temperature even though heat is absorbed, not released.
- 11. Solutions Enthalpy Is OnlyPart of the Picture Entropy is a measure of: • Dispersal of energy in the system. • Number of microstates (arrangements) in the system. b. has greater entropy, ∴ is the favored state (more on this in chap 19)
- 12. Solutions Entropy changes duringdissolution Each step also involves a change in entropy. 1. Separation of solute particles. 2. Separation of solvent particles to make ‘holes’. 3. Formation of new interactions between solute and solvent.
- 13. Solutions SAMPLE EXERCISE: AssessingEntropy Change In the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is Does the entropy of the system increase or decrease?
- 14. Solutions Factors Affecting Solubility Thestronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.Example: ethanol in water Ethanol = CH3CH2OH Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces.
- 15. Solutions Factors Affecting Solubility Glucose(which has hydrogen bonding) is very soluble in water. Cyclohexane (which only has dispersion forces) is not water- soluble.
- 16. Solutions Factors Affecting Solubility •Vitamin A is soluble in nonpolar compounds (like fats). • Vitamin C is soluble in water.
- 17. Solutions Which vitamin is water-soluble and whichis fat-soluble?
- 18. Solutions Gases in Solution •In general, the solubility of gases in water increases with increasing mass. Why? • Larger molecules have stronger dispersion forces.
- 19. Solutions Gases in Solution •The solubility of liquids and solids does not change appreciably with pressure. • But, the solubility of a gas in a liquid is directly proportional to its pressure. Increasing pressure above solution forces more gas to dissolve.
- 20. Solutions Henry’s Law Sg =kPg where • Sg is the solubility of the gas; • k is the Henry’s law constant for that gas in that solvent; • Pg is the partial pressure of the gas above the liquid.
- 21. Solutions Ways of ExpressingWaysof Expressing Concentrations ofConcentrations of SolutionsSolutions
- 22. Solutions Mass Percentage Mass %of A = mass of A in solution total mass of solution × 100
- 23. Solutions Parts per Millionand Parts per Billion ppm = mass of A in solution total mass of solution × 106 Parts per Million (ppm) Parts per Billion (ppb) ppb = mass of A in solution total mass of solution × 109
- 24.
- 25. Solutions Boiling Point Elevationand Freezing Point Depression Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
- 26. Solutions Colligative Properties of Electrolytes Becausethese properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) show greater changes than those of nonelectrolytes. e.g. NaCl dissociates to form 2 ion particles; its limiting van’t Hoff factor is 2.
- 27. Solutions Colligative Properties of Electrolytes However,a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does. It doesn’t act like there are really 2 particles.
- 28. Solutions van’t Hoff Factor Onemole of NaCl in water does not really give rise to two moles of ions.
- 29. Solutions van’t Hoff Factor SomeNa+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.
- 30. Solutions The van’t HoffFactor • Reassociation is more likely at higher concentration. • Therefore, the number of particles present is concentration dependent.
- 31. Solutions The van’t HoffFactor We modify the previous equations by multiplying by the van’t Hoff factor, i ∆Tf = Kf m i i = 1 for non-elecrtolytes
- 32. Solutions Osmosis • Semipermeable membranesallow some particles to pass through while blocking others. • In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.
- 33. Solutions Osmosis In osmosis, thereis net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration). Water tries to equalize the concentration on both sides until pressure is too high.
- 34. Solutions Osmotic Pressure • Thepressure required to stop osmosis, known as osmotic pressure, π, is n V π = ( )RT = MRT where M is the molarity of the solution If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.
- 35. Solutions Osmosis in BloodCells • If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. • Water will flow out of the cell, and crenation results.
- 36. Solutions Osmosis in Cells •If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. • Water will flow into the cell, and hemolysis results.
- 37.
- 38. Solutions Tyndall Effect • Colloidalsuspensions can scatter rays of light. • This phenomenon is known as the Tyndall effect.
Editor's Notes
- #18 Figure: 13-32-03UNE13.06 Title: Exercise 13.6 Caption: Vitamins E and B6.