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Solution Chemistry

Solution Chemistry. Solids, Liquids, and gases dissolve to form solutions. What is a solution? A mixture of 2 or more substances that are uniformly mixed with each other. What does it mean to dissolve?

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Solution Chemistry

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  1. Solution Chemistry

  2. Solids, Liquids, and gases dissolve to form solutions. What is a solution? A mixture of 2 or more substances that are uniformly mixed with each other. What does it mean to dissolve? When one substance mixes into another and completely disappears. For example, NaCl in H2O.

  3. The Solution Process In order for a solute to be dissolved in a solvent, the solute and solvent must be attracted to each other. The solute and the solvent molecules in a solution are then expanded (spread out), and able to mix with each other. In the example below, NaCl is broken apart because +/- water molecules are attracted to the +Na and the -Cl. The water breaks down the crystal, dissolving the salt.

  4. Three types of interactions in the solution process: • solvent-solvent interaction • solute-solute interaction • solvent-solute interaction Δ Hsoln= Δ H1+ Δ H2+ ΔH3

  5. Energy Changes in Solution To determine the enthalpy change, we divide the process into 3 steps. • Separation of solute particles. • Separation of solvent particles to make ‘holes’. • Formation of new interactions between solute and solvent.

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

  7. Enthalpy changes during dissolution ΔHsoln= ΔH1+ ΔH2+ ΔH3 The enthalpy of solution, ΔHsoln, can be either positive or negative. ΔHsoln(MgSO4)= -91.2 kJ/mol --> exothermic ΔHsoln(NH4NO3)= 26.4 kJ/mol --> endothermic

  8. The term solubility refers to the maximum amount of material that will dissolve in a given amount of solvent at a given temperature to produce a stable solution. Ack! What does this mean? It means how much solute (stuff) can dissolve in a solvent. It is dependant on temperature, how much solvent you have, how much solute you have, and whether or not you mix the solution.

  9. Nature of the solute and solvent A solute is soluble when it dissolves completely in a solvent. A solute is insoluble when it cannot dissolve in a solvent. Some solutes are partially soluble, meaning only a bit dissolves while the rest remains solid.

  10. When two liquids totally mix they are said to be miscible. An example of this would be alcohol and water. When to liquids do not mix they are said to be immiscible. An example would be oil and water:

  11. “like dissolves like” Two substances with similar intermolecular forces are likely to be soluble in each other. • non-polar molecules are soluble in non-polar solvents • CCl4 in C6H6 • polar molecules are soluble in polar solvents • C2H5OH in H2O • ionic compounds are more soluble in polar solvents • NaCl in H2O or NH3 (l) polar dissolves polar Nonpolar dissolves nonpolar like dissolves like

  12. SOLUBILITY The solubility of a solute in a solvent at a particular temperature is the number of grams of the solute necessary to saturate 100 gm of the solvent at that temperature. FACTORS AFFECTING SOLUBILITY There are six main factors that control solubility of a solute. (1) Temperature ** (2) Nature of solute or solvent (3) Pressure (4) Concentration ** (5) Time (6) Mixing **

  13. Effect of Temperature on Solubility • Solubility increases when temperature goes up (most of the time). • More solute dissolves in the solvent at higher temperatures. • Solubility decreases when temperature goes down (most of the time). • Less solute dissolves in the solvent at lower temperatures. • In endothermic reactions solubility increases with the increase in temperature and vice versa. • For example: solubility of potassium nitrate increases with the increase in temperature. • In exothermic reactions solubility decrease with the increase in temperature. • For example: solubility of calcium oxide decreases with the increase in temperature. • Gases are more soluble in cold solvent than in hot solvent.

  14. T  Solubility  Crystal is formed, Solubility  T  Solubility ↓ gas in liquid: T ↑ Temperature and Solutions Solubility: the maximum solute can dissolve in a given amount of solvent (in a given T).

  15. By looking at the plot of solubilities below, you can see that most solids increase in solubility with an increase in temperature. Solubility Curve The y-axis (dependant) is solubility: how much solute dissolves in the solvent. The x-axis (independent) is temperature. How much solute can dissolve is dependant on the temperature. Gases, however, decrease in solubility with an increase in temperature.

  16. Learning Check A. Why would a bottle of carbonated drink possibly burst (explode) when it is left out in the hot sun ? B.Why would fish die in water that gets too warm?

  17. Effect of Pressure on Solubility Henry's Law states that "The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of that gas in contact with the liquid..." What this means for divers is that gas molecules will dissolve into the blood in proportion to the partial pressure of that gas in the lungs (as "warm-blooded" creatures, our core body temperature remains relatively constant). Henry’s Law is used in breathalizers to determine how much alcohol a person has drunk.

  18. Pressure and Solutions Henry’s law P  Solubility  (gas in liquid)

  19. low P high P low c high c Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). c is the concentration (M) of the dissolved gas c = kP P is the pressure of the gas over the solution k is a constant (mol/L•atm) that depends only on temperature 12.5

  20. Effect of Concentration: Degrees of Saturation • When referring to solutions, there are three degrees of saturation — unsaturated, saturated, and supersaturated. • If a solution is unsaturated, the solvent is capable of dissolving more solute. • When the solution is saturated, the solvent has dissolved the maximum amount of solute that it can at the given temperature. • At this point we say that the solution is in a state of dynamic equilibrium—the processes of dissolving and precipitation are happening at the same rate. • A supersaturated solution is one in which the solvent contains more solute than it can theoretically hold at a given temperature. • Supersaturated solutions are often formed by heating a solution and dissolving more solute, then cooling the solution down slowly. These solutions are unstable and crystallize readily.

  21. Supersaturated solution Seeding A surface on which to being crystallizing.

  22. When a solution reaches saturation, some of the solid precipitates out and forms a solid on the bottom of the beaker. Sometimes a reaction happens when a solution is made, but one of the products is insoluble in the solvent and precipitates out. A precipitate is a solid that forms from a solution, typically because the solid is either insoluble in the solvent, or is no longer soluble.

  23. Concentration Terms Solutions are often referred to as being concentrated or dilute. These two terms are very general. While concentrated indicates that there is a lot of solute dissolved in the solvent (perhaps the solution is near to being saturated) and dilute indicates that a small amount of solute is dissolved in the solvent, we often need to be exact with quantities in chemistry.

  24. Effect of Time: Diffusion & Dissolving As time goes on, due to the random (Brownian) movement of the molecules, the solute will completely dissolve in the solvent.

  25. Effect of Mixing You have done several experiments in which you were instructed to add a solute to a solvent and stir. What was the reason behind the stirring? When you stir a solute in a solution, you increase the movement of the molecules. When the molecules of the solute move more and faster, they come in higher contact with the molecules of the solution. The solution is able to attack the solute and pull it apart, dissolving the solution. Mixing causes the solute to dissolve in the solvent faster than if not mixed.

  26. Molarity (M) The molarity of a solution is a measure of the number of moles of solute per liter of solution. This is the most common concentration unit used in chemistry. Let’s now run through how you calculate the molarity of a solution. Example Calculate the molarity of a solution prepared by dissolving 20.0 g of solid NaOH in enough water to make 100 mL of solution. Explanation Convert grams to moles: Then convert mL to liters: Then divide:

  27. Molality (m) The molality of a solution is a measure of the number of moles of solute per kilogram of solvent. The molality is dependent on the mass of the solvent in the solution. Try an example: A solution is prepared by mixing 80.0 g of sodium hydroxide (NaOH) with 500.0 g of water. Calculate the molality of this solution. Explanation Convert grams of solute to moles: Convert grams of solvent to kg: Divide: .

  28. moles of solute moles of solute m = m = moles of solute M = mass of solvent (kg) mass of solvent (kg) liters of solution 5.86 moles C2H5OH = 0.657 kg solvent What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL? • Assume 1 L of solution: • 5.86 moles ethanol = 270 g ethanol • 927 g of solution (1000 mL x 0.927 g/mL) mass of solvent = mass of solution – mass of solute = 927 g – 270 g = 657 g = 0.657 kg = 8.92 m

  29. Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

  30. Mass Percent (Weight Percent) The mass percent of a solution is another way of expressing its concentration. Mass percent is found by dividing the mass of the solute by the mass of the solution and multiplying by 100. A solution of NaOH that is 28% NaOH by mass contains 28 g of NaOH for each 100 g of solution. Here’s the equation: Now try a problem involving the equation: A solution is prepared by mixing 5.00 g ethanol (C2H5OH) with 100.0 g water. Calculate the mass percent of ethanol in this solution. Explanation Plugging the values we were given into the mass percent equation, we get:

  31. Convert % mass to Molarity • What is the Molarity of a 95% acetic acid solution? (density = 1.049 g/mL) If you assume 1 L, that amount of solution = 1049 g 95% of the solution is acetic acid 1049 g solution x 0.95 = 997 g solute 997 g X 1 mol/60.05 g = 16.6 mol solute Since we assumed 1 L, that’s 16.6 mol / 1 L or 16.6 M

  32. Percent concentration: Weight solute × 100 Weight / volume (W / V)% = Volume of solution (mL) Weight solute × 100 Weight / Weight (W / W)% = Weight of solution Volume solute (mL) × 100 Volume / volume (V / V)% = Volume of solution (mL)

  33. Solubility The maximum amount of solute that can dissolve in a specific amount of solvent usually 100 g. g of solute 100 g water

  34. Learning Check At 40C, the solubility of KBr is 80 g/100 g H2O. Indicate if the following solutions are (S) saturated or (U) unsaturated A. ___60 g KBr in 100 g of water at 40C B. ___200 g KBr in 200 g of water at 40C C. ___25 KBr in 50 g of water at 40C

  35. Concentration Parts per Million (ppm): g solute ppm = × 106 g solvent Parts per billion (ppb): g solute ppb = × 109 g solvent

  36. Dilution is the process of taking a more concentrated solution and adding water to make it less concentrated. The more concentrated solution before the dilution is performed is known as the stock solution. You can relate the concentration of the stock solution to the concentration of the diluted solution using the equation below: C1V1 = C2V2 where C is concentration (molarity) and V is the volume, in liters, of the solution. Try the following example using this equation. What volume of 6.0 M sulfuric acid (H2SO4) must be used to prepare 2.0 L of a 0.10 M H2SO4 solution? Explanation Just plug the numbers into the formula! Be careful to read closely. C1V1 = C2V2 (6.0 M) (V1) = (0.10 M) (2.0 L) V1 = 0.033 L or 33 mL should be measured out and then diluted by adding enough water to make 2.00 L total volume. This should be a review!!!

  37. Moles of solute before dilution (i) Moles of solute after dilution (f) = Dilution Add Solvent = MfVf MiVi Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution.

  38. How would you prepare 60.0 mL of 0.2 M HNO3 from a stock solution of 4.00 M HNO3? MfVf 0.200 x 0.06 Vi = = 4.00 Mi MiVi = MfVf Mi = 4.00 Vi = ? L Mf = 0.200 Vf = 0.06 L = 0.003 L = 3 mL 3 mL of acid + 57 mL of water = 60 mL of solution

  39. What is the mole Fraction of Gas A in mixture I? What is the mole fraction of Gas B in mixture II? Partial Pressure in terms of mole fraction: XAPtotal = PA (XA = mole fraction of A)             Example:  If there are 3 moles of gas A, 4 moles of gas B and 5 moles of gas C in a mixture of gases and the pressure of A is found to be 2.5 atm, what is the total pressure of the sample of gases?

  40. Fractional crystallization is the separation of a mixture of substances into pure components on the basis of their differing solubilities. Suppose you have 90 g KNO3 contaminated with 10 g NaCl. • Fractional crystallization: • Dissolve sample in 100 mL of water at 600C • Cool solution to 00C • All NaCl will stay in solution (s = 34.2g/100g) • 78 g of PURE KNO3 will precipitate (s = 12 g/100g). 90 g – 12 g = 78 g

  41. Fractional Distillation Apparatus

  42. Gravimetric Analysis • Dissolve unknown substance in water • React unknown with known substance to form a precipitate • Filter and dry precipitate • Weigh precipitate • Use chemical formula and mass of precipitate to determine amount of unknown ion

  43. Titrations In a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL the indicator changes color

  44. What volume of a 1.420 M NaOH solution is Required to titrate 25.00 mL of a 4.50 M H2SO4 solution? WRITE THE CHEMICAL EQUATION! H2SO4 + 2NaOH 2H2O + Na2SO4 M M rx volume acid moles acid moles base volume base base acid coef. 4.50 mol H2SO4 2 mol NaOH 1000 ml soln x x x 1000 mL soln 1 mol H2SO4 1.420 mol NaOH 25.00 mL = 158 mL

  45. electrolyte + - Electrolytes bulb Electrolyte:conduct an electriccurrent. Na+ Cl- Ionization NaCl → Na+ + Cl- strong electrolytes: molecules dissociate completely to ions (NaCl). weak electrolytes: molecules dissociate partially to ions (CH3COOH). nonelectrolytes: molecules do not dissociate to ions (DI water).

  46. nonelectrolyte weak electrolyte strong electrolyte An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity.

  47. Osmotic Pressure osmotic pressure Semipermeable membrane Higher concentration → Higher osmotic pressure

  48. Osmotic Pressure (p) Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (p) is the pressure required to stop osmosis. more concentrated dilute

  49. Osmotic Pressure (p) High P Low P p = MRT Osmolarity (osmol) = M × i M is the molarity of the solution M: molarity i: number of particles R is the gas constant T is the temperature (in K) Osmolarity ↑ → Osmotic pressure ↑

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