Chem 162-2011 Lecture 3 2PLAN FOR TODAY: BALANCE OF LECTURE 3 •Electrolytes •Colloids LECTURE 4 - CHEMICAL KINETICS (13.3 – 13.6) •Reaction rates •Rate laws from experimental data - Initial Rates Method •Integrated rate laws - Half-lives
Chem 162-2011 Lecture 3 3COLLIGATIVE PROPERTIESVAPOR PRESSUREMost solution properties depend on the identity of the solute; e.g., solubility: KNO3is more soluble in H2O than AlPO4. Colligative properties don’t depend on the identity of the solute, only on the concentration (of particles) of the solute; e.g., 0.1m glucose and 0.1m urea (and 0.05m NaCl*) have the same number of particles, and therefore the same vapor pressure, boiling point, freezing point and osmotic pressure. *Note that 1 mole of glucose provides 1 mole of particles; 1 mole of urea provides 1 mole of particles; 1 mole of NaCl provides 2 moles of particles (0.5 mole NaCl provides 1 mole of particles). ET: Discuss vapor pressure Determine the vapor pressure of a 22oC solution made up of 1 mol H2O and 2 mol glucose (C6H12O6). Glucose does not dissociate and is not volatile. PoH2O@ 22oC = 21 mm. Raoult’s Law: Psoln/Xsolvent= Posolvent which means that the concentration of the substance in the gas phase to the concentration of the substance in the liquid phase is a constant, which means that if you decrease the concentration of the volatile substance in the liquid phase then the concentration must decrease in the gas phase. This is identical to Henry’s law, except for the state focus. Psoln= XsolventPosolventPsoln= (nH2O/(nH2O+ nC6H12O6)) x PoH2OPsoln= (1/(2+1)) x 21 = 7 mm Psoln= PXsolventPosolventPsoln= (inH2O/(inH2O+ inNaCl)) x PoH2OPsoln= ((1x1)/((1x1) + (2x1))) x 21 = 7 mm H2O C6H12O6C6H12O6H2O H2O H2O Na+Cl-H2O 2 molecules glucose = 2 particles1 molecule NaCl = 2 particles
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Chem 162-2011 Lecture 3 4SOLUTIONS OF ELECTROLYTESVAN’T HOFF FACTOR (“i”) The van’t Hoff factor (i) is a correction factor that must be incorporated into equations for colligative properties so that the equations may be applied to solutions of strong or weak electrolytes. 69 (mod) What is the van’t Hoff factor for the following: Na3PO4, CaBr2, KCl, CH3OH Al2(SO4)3→2Al3++ 3SO42-i = 5 Na3PO4→3 Na++ PO43-i = 4 CaBr2→Ca2++ 2Br-i = 3 KCl →K++ Cl-i = 2 CH3OH* →i = 1 * Assume that all salts dissociate completely, although the degree of dissociation depends on the concentration of the salt. Organic molecules, in general, other than organic acids do not dissociate. Organic acids dissociate slightly.