18CH 18.1
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18CH 18.1

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18CH 18.1 18CH 18.1 Presentation Transcript

  • 18.1 Properties of Solutions
    • “ Like dissolves like.”
    • Materials with similar polarity are soluble in each other. Dissimilar ones are not.
    • Miscible
    • Liquids that are soluble in each other in all proportions such as ethanol and water.
    • Immiscible
    • Liquids that are not soluble in each other such as hexane and water.
  • Factors Affecting rate of solution
    • Nature of solvent ( polar or non or ionic)
    • Temperature- for most, not all solids in a liquid, solubility increases with temp.
    • Agitation- more shaking or stirring, faster a solute dissolves ( solids in liquids)
    • Particle size- smaller the particle size, more surface area, so faster dissolving takes place
    • All these can be explained by understanding how solvation occurs
  •  
  • Solution of solids
  • Solubility
    • Amount of solute that dissolves in a given amount of solvent at a particular temperature
    • Usually expressed in:
    • g of solute/100g of solvent
    • Solubility chart shows solubility of different solutes at different temperatures. (pg 504)
  • Few terms first..
    • Saturated- solution is holding max amount of solute for that solvent; if you try to add more solute, it does not dissolve
    • Unsaturated – solution is holding less than the max amount of solute
    • Supersaturated- solution is holding more solute than it should for that temp. ( we’ll talk about this later)
    • At saturation, the solute is in dynamic equilibrium. The concentration is constant.
    • Solute species are
    • constantly in
    • motion, moving
    • in and out of
    • solution.
  • The lines represent saturated solutions- solutions which are holding the maximum amount of solute possible for these conditions. Handout problems
  • Handout hints
    • Make sure you read the axis of the graphs- this one is for 100g of water
    • If the problem asks for solubility in 200g of water, then you need to double your reading/or if it asks for solubility in 50g of water then you cut your reading in ½ ; You must always adjust your readings to the amount of solvent in the problem
    • If your reading is below the line for your solute, the solution is unsaturated; if it is above the line it, is either supersaturated, or you can calculate how much extra solute will precipitate out ( amount above the line)
  • Supersaturated Solutions
    • At higher temperatures, more solute can be dissolved than at a lower temp.
    • So, if you heat up water and make a saturated solution, then slowly cool the solution down in an uncontaminated env. then more solute will be dissolved than should be for the lower temp- this is supersaturated
    • As soon as you add an additional crystal of solute, or scratch the glass of the container, the excess solute will precipitate out
  • Gases and solubility
    • Look at the chart on pg.505
    Notice- for gases, solubility decreases with temperature
    • This is why you seen tiny bubbles when heating up water ( long before it boils)- these are N 2 and O 2 coming out of solution
    • This is why thermal pollution can affect life in waterway- warmer the water, the less Oxygen (DO) can be dissolved in it, less oxygen available for living things
  • Pressure and solubility of gases
    • Increasing the pressure of a gas above a liquid increases the concentration of the gas.
    • This shifts the equilibrium, driving more gas into the liquid.
  • Pressure and solubility of gases
    • At constant temperature, the solubility of a gas is directly proportional to the pressure of the gas above the solution.
    • Henry’s Law
    • S 1 = S 2
    • P 1 P 2
    • This law is accurate to
    • within 1-3% for slightly soluble gases and pressures up to one atmosphere.
  • Henry’s Law problem
    • If the solubility of a gas in water is .77g/L at 3.5 atm, what is the solubility at 1 atm? Assume temp stays same
    • .77g/L = S 2
    • 3.5 atm 1.0 atm
    • .77g/L x 1 atm = S 2
    • 3.5 atm
    • .22g/L=S 2