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1 Solutions Unit #9 Chapter #11 A solution is a HOMOGENEOUS mixture of 2 or more substances in a single phase. One constituent is usually regarded as the SOLVENT and the others as SOLUTES. 1 Definitions Solutions can be classified as saturated or unsaturated or supersaturatedsaturated An unsaturated solution has less solute than it can hold at that temperature A saturated solution contains the maximum quantity of solute that dissolves at that temperature. Supersaturated solutions contain more than is possible and are unstable. Testing for each add more solute, if it dissolves (unsaturated), if it sinks (saturated) and if causes the solution to crystallize (supersaturated) 2 Energetics of the Solution Process If the enthalpy of formation of the solution is more negative than that of the solvent and solute, the enthalpy of solution is negative. The solution process is exothermic! 3 Page 1

2 Supersaturated Sodium Acetate 4 One application of a supersaturated solution is the sodium acetate heat pack. Sodium acetate has an ENDOthermic heat of solution. Supersaturated Sodium Acetate 5 Sodium acetate has an ENDOthermic heat of solution. NaCH 3 CO 2 (s) + heat ----> Na + (aq) + CH 3 CO 2- (aq) Therefore, formation of solid sodium acetate from its ions is EXOTHERMIC. Na + (aq) + CH 3 CO 2- (aq) ---> NaCH 3 CO 2 (s) + heat Concentration Units An IDEAL SOLUTION is one where the properties depend only on the concentration of solute. Need concentration units to tell us the number of solute particles per solvent particle. The unit molarity does not do this! 6 Page 2

3 7 Molarity Molarity = moles of solute/liter of solution M = mol/l Molarity is the most used measure of concentration of solutions. Sample: What is the molarity of a solution that contains 34.5 g of NaCl in 450. ml of water? Answer = 34.5 g 1 mol =.590 mol 58.5 g.590 mol = 1.31 M.450 L 8 Molarity and Dilutions Making dilutions involves starting with a more concentrated solution and adding water or another solvent. M 1 V 1 = M 2 V 2 where M is molarity and V is volume. Sample: What is the volume of a dilution made from 234 ml of a M HCl diluted to M? Answer: M 1 = 0.988M V 1 =234 ml M 2 = M V 2 =? V 2 = 4170 ml Other Concentration Units MOLE FRACTION, X For a mixture of A, B, and C X A = mol fraction A = mol A mol A + mol B + mol C MOLALITY, m mol solute m of solute = kilograms solvent WEIGHT % = grams solute / total grams solution x100 9 Page 3

4 Dissolve 62.1 g of ethylene glycol in 250. g of H 2 O. Calculate mol fraction, molality, and weight % of glycol. Get the formula from the picture Dissolve 62.1 g of ethylene glycol in 250. g of H 2 O. Calculate X, m, and % of glycol g H 2 O = 13.9 mol X glycol = 1.00 mol glycol 1.00 mol glycol mol H 2 O X glycol = Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H 2 O. Calculate X, m, and % of glycol. Calculate molality conc (molality) = Calculate weight % %glycol = 1.00 mol glycol kg H 2 O = 4.00 molal 62.1 g x 100% = 19.9% 62.1 g g Page 4

5 Normality Normality is a measure of how many ionizable hydrogen ions or hydroxide ions. It is very easy to calculate. It is molarity times # equivalents/mole. N = M x #equivalents/mol N = (mol/l) x (Eq / mol) = eq/l Try one A M solution of sulfuric acid (strong acids dissociate completely) has what Normality? H 2 SO 4 2 H + + SO 4 2- so N = M x 2 eq/mol= 1.10 N Like Dissolves Like This phrase is used to predict solubility. The word like refers to polarity. Therefore, a polar solvent will dissolve an ionic or polar solute. Only nonpolar solvents will dissolve nonpolar solutes. See Table 11.3 Page 5