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Solutions and Mixtures

Solutions and Mixtures. A solution is a homogenous mixture of 2 or more substances. The solute is(are) the substance(s) present in the smaller amount(s). The solvent is the substance present in the larger amount. Aqueous solutions : water is the dissolving medium, or solvent .

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Solutions and Mixtures

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  1. Solutions and Mixtures

  2. A solution is a homogenousmixture of 2 or more substances. The solute is(are) the substance(s) present in the smaller amount(s). The solvent is the substance present in the larger amount.

  3. Aqueous solutions: water is the dissolving medium, or solvent. • One of most important properties of water is its ability to dissolve many different substances.

  4. Water is Polar • Covalent bonds in water • Polarity gives water its great ability to dissolve many substances.

  5. Hydration (Dissolving) of Ionic Compounds • “Positive ends” of water molecules are attracted to the negatively charged anions and the “negative ends” are attracted to the positively charged cations. • This process is called hydration. • The hydration of its ions tends to cause a salt to “fall apart” in the water or dissolve.

  6. Very Important!! • When ionic substances (salts) dissolve in water, they break up into the individualcations and anions. • Example: ammonium nitrate (NH4NO3) dissolved in water

  7. Polar covalent compounds will also dissolve in water. • Polar and ionic substances are more soluble in water than nonpolar substances (animal fat).

  8. Many substances will not dissolve in water. • Nonpolar substances (like animal fat) do not interact with water (polar substance). • “Like dissolves like” is a general rule for predicting solubility.

  9. 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.

  10. A saturated solution contains the maximum amount of a 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 supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate.

  11. Concentration Most commonly used in chemistry is molarity. • Molarity (M) is defined as moles of solute per volume of solution in liters. • A solution that is 1.0 molar (1.0 M) contains 1.0 mole of solute per liter of solution. • Molarity can be used as a conversion factor.

  12. Dilution • Preparing solutions from stock solutions. • Example: HCl is purchased as a concentrated solution (12 M) and is diluted as needed. M1V1 = M2V2 M1V1 = mol solute before dilution = mol solute after dilution = M2V2

  13. The Energies of Solution Formation

  14. Three types of interactions in the solution process: • solute-soluteinteraction (separating the solute) • solvent-solventinteraction (overcoming intermolecular forces) • solvent-solute interaction

  15. Steps 1 and 2 require energy, since forces must be overcome to expand the solute and solvent. • Step 3 usually releases energy. • Steps 1 and 2 are endothermic and step 3 is often exothermic. • Enthalpy change: enthalpy (heat) of solution (∆Hsoln) is the sum of the ∆H values for the steps: ∆Hsoln = ∆H1 + ∆H2 + ∆H3

  16. (a) ΔHsoln has a negative sign (exothermic) if step 3 releases more energy than that required by 1 and 2 (b) ΔHsoln has a positive sign (endothermic) if steps 1 and 2 require more energy than is released in 3

  17. Factors Affecting Solubility Temperature Pressure

  18. solubility increases with increasing temperature solubility decreases with increasing temperature due to entropy (not covered yet) Temperature and Solubility Solid solubility and temperature – the solubility of most solids increases with temperature.

  19. Temperature and Solubility Gas solubility and temperature solubility usually decreases with increasing temperature (more energy to escape from solvent)

  20. Separation of Liquid Solutions • Chromatography (paper, thin-layer, and column) • separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components of the solution (the mobile phase) and with the surface components of the stationary phase.

  21. Separation of Liquid Solution • Distillation – the separation of a liquid mixture by vaporization and then again condensing the vapor into a liquid. • It involves a change of state from liquid to vapor and again to liquid.

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