SOLUBILITY AS AN EQUILIBRIUM PHENOMENA

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Georgia Smith
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1 SOLUBILITY AS AN EQUILIBRIUM PHENOMENA

2 Equilibrium in Solution solute (undissolved) solute (dissolved)

3 Solubility A saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a specific temperature An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature A supersatured solution contains more solute than is present in a saturated solution at a specific temperature

4 Crystallization Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate

5 Ways of Expressing Concentration The concentration is the amount of solute present in a given quantity of solution or solvent. Parts by mass Percent by mass (parts per hundred) Parts per million (ppm) Parts per billion (ppb) Mole fraction Molarity Molality

6 Ways of Expressing Concentration Mole Fraction (X) moles of A X A = sum of moles of all components Molarity (M) moles of solute M = liters of solution Molality (m) moles of solute m = mass of solvent (kg) Converting between molarity (M) and molality (m) requires density.

7 Ways of Expressing Concentration Percent by Mass % by mass = mass of solute x 100% mass of solute + mass of solvent Parts per milion = mass of solute x 100% mass of solution ppm = mass of solute x 10 6 mass of solution

8 Ways of Expressing Concentration For dilute solutions (in the ppm and ppb level), density is almost equal to 1 g/ml Parts per milion ppm = mass of solute x 10 6 mass of solution ppm = g solute 1 g solution 1000 ml solution 1000 mg x x solute g solution ml x solution 1 L solution 1 g solute ppm = mg solute L solution

9 Exercise 1) A solution is made by dissolving 13.5 g of glucose, C 6 H 12 O 6, in kg of water. What is the mass percentage of solute in this solution? 2) A 2.5 g sample of ground water was found to contain 5.4 μg of Zn 2+. What is the concentration of Zn 2+ in parts per million? 3) The solubility of MnSO 4 H 2 O in water at 20 C is 70 g per 100 ml of water. Is a 1.22 M solution of MnSO 4 H 2 O in water at 20 C saturated, supersaturated or unsaturated?

10 Exercise 4) What is the mass percentage of NaCl in a solution containing 1.50 g of NaCl in 50.0 g of water? 5) What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is g/ml? 6) What is the mole fraction of HCl in a solution of hydrochloric acid that has 36% HCl by mass? What is its molality? What is its molarity? The density of the acid solution is g/ml

11 Exercise 7) Calculate the molality, molarity and mole fraction of FeCl3 in a 28.8 mass % aqueous solution (with density of g/ml) 8) An automobile antifreeze mixture is made by mixing equal volumes of ethylene glycol (d=1.114 g/ml; MW = g/mol) and water (d=1.00 g/ ml) at 20 C. The density of the resulting mixture is g/ml. a) % volume c) molarity e) mole fraction b) % mass d) molality

12 Factors Affecting Solubility Intermolecular Forces Temperature Pressure

13 Intermolecular Forces Substances with similar intermolecular forces tend to be soluble in one another. The stronger the attractions between solute and solvent, the greater the solubility. Liquid-Liquid Interactions: Miscible when two liquids mix in all proportions, e.g. H 2 O, C 2 H 5 OH Immiscible when two liquids do not dissolve significantly in one another, e.g. H 2 O, C 6 H 14

14 Temperature on Ionic Salts The change in enthalpy of salt solutions (ΔH sol n ) can either be positive or negative solute + solvent + heat solution solute + solvent solution + heat Temperature is a measure of internal energy in a system. It is a state Heat is the transfer of energy due to differences in energy of interacting systems. It is a path taken (which for a path that is constant in pressure is equal to the ENTHALPY, ΔH)

15 Temperature on Ionic Salts Solubility is the amount of solute needed to form a saturated solution in a given quantity of solvent Solids: T Solubility

16 Thermodynamics of different processes For the any process, Gibbs free energy(g) = the energy that s free to do work (other than expansion work!) The higher the G, the more work a system can do

17 Thermodynamics of different processes When G final is higher than G initial, the energy to do work of a system increase, then you need an input of energy (from the surrounding!) this process does not happen spontaneously

18 Thermodynamics of different processes When G final is less than G initial, the energy to do work of a system decreases the system loses energy, it has less energy to do further work this process happens spontaneously

19 Thermodynamics of different processes When G final is equal to G initial, there is no change in energy in the system, and we call it a state of equilibrium.

20 Thermodynamics of solution process For solutions to occur, G sol n <0 How is G related to enthalpy ( H), entropy ( S) and Temperature? (+/-) (+) (+) Temp Negative x positive x positive = negative

21 Temperature on Gases Gases: T Solubility Why don t carbonated beverages taste as good when they re no longer cold?

22 Thermodynamics of solution process For solutions to occur, G sol n <0 (-) (+) (-) Negative x positive x negative = positive

23 Pressure The solubility of a gas in a liquid is proportional to its partial pressure over the solution (Henry s law). S g = k H P g Carbonated beverages are bottled under P CO2 > 1 atm. As the bottle is opened, P CO2 decreases and the solubility of CO 2 decreases. Therefore, bubbles of CO 2 escape from solution.

24 Crystallization playing with Solubility Lower temp low solubility Higher temp high solubility

25 Exercise For a saturated aqueous solution of each of the following at 20 C and 1 atm, will be the solubility increase, decrease or the same? He (g), decrease T RbI (s), increase P Caffeine is about 10 times soluble in hot water as in cold water. A chemist puts a hot-water extract of caffeine into an ice bath and some caffeine crystallizes. Is the remaining solution saturated, unsaturated, or supersaturated?

SOLUTIONS. Engr. Yvonne Ligaya F. Musico

SOLUTIONS. Engr. Yvonne Ligaya F. Musico SOLUTIONS SOLUTION A homogeneous mixture of two or more substances, the relative proportion of which may vary within certain limits. COMPONENTS OF SOLUTION SOLUTE component which is in small quantity SOLVENT