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SOLUTIONS

SOLUTIONS. How they Form. Mr. Shields Regents Chemistry U12 L04. The Solution Process. We’ve mentioned earlier that there are various kinds Of Soln’s but we’re only going to focus on a few … 1) Solids in liquid 2) Gas in liquid - ionic solids CO 2 , O 2 etc

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SOLUTIONS

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  1. SOLUTIONS How they Form Mr. Shields Regents Chemistry U12 L04

  2. The Solution Process • We’ve mentioned earlier that there are various kinds • Of Soln’s but we’re only going to focus on a few … • 1) Solids in liquid2)Gas in liquid • - ionic solids CO2, O2 etc • - molecular solids • 3)Liquid in Liquid • - Polar liquids • - Non-polar liquids Let’s start by looking at how an ionicsolid dissolves In a Polar liquid like water

  3. Dissolving Solids Let’s look at how an ionic compound like NaCl dissolves in water. Recall … - NaCl is ionic and exists as a crystal lattice - It contains both + & - ions - + • And secondly Water is a • polar molecular liquid • - It has both a + & - end

  4. Dissolving When a crystal of NaCl is dropped into water it Starts to dissolve. So what’s this mean? This means the arranged order Of the Crystal lattice Breaks down And it Becomes … A homogeneous mixture of Ions in sol’n

  5. Dissolving But how does the process of dissolving actually happen? Polar water aligns itself to the + and – ions at the surface of the NaCl crystal lattice. This is a form of Intermolecular Attraction between molecules. Note that here it is a force between different molecules.

  6. Dissolving The force of attraction between the solute and the solvent are Sufficient to overcome the force of attraction between the Positive and negative ions (Na+ and Cl-) in the crystal lattice. Once the water overcomes the force of the attraction of The ions in the crystal the separated ions are surrounded By the water molecules. This process is known as SOLVATION. If the Solvent is water it is Called HYDRATION.

  7. Solvated (Hydrated) Ion Cl- Cl- Na+ Na+ H20 Of Hydration - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Solvation (Hydration)

  8. Energy Balance of Solvation This whole process of dissolving involves 3 energy steps: • Breaking of attraction • between solvent particles • - Requires energy to be • absorbed • (it’s endothermic)

  9. Energy Balance of Solvation 2) Breaking of attraction between solute particles - requires energy (also endothermic) 3) Attraction between solute and solvent - Results in the release of energy (an exothermic process)

  10. Hsol = Hstep1 + Hstep2 + Hstep3 (+) (+) (-) The Heat of Solution The Total Energy (heat) absorbed or released during solvation is known as the HEAT OF SOLUTION (Hsol) If the heat of Sol’n (Hsol) is positive The solution process is endothermic +Hsol = endothermic process If the heat of Sol’n is negative The solution process is exothermic -Hsol = exothermic process

  11. Energy Balance of Solavation Whether the overall solvation process will be endothermic or exothermic depends on the Energies involved is steps 1, 2, and 3. When some compounds are dissolved in water The sol’n turns colder -This is the more common result Certain other compounds when dissolved in water will turn the solution warmer

  12. Heat of Solution Examples Solutes that are exothermic upon dissolving (i.e sol’n gets hotter): CaCl2 + H2O  exothermic solvation process (-82.8 KJ/mol) NaOH + H20  exothermic solvation process (-44.2 KJ/mol) Solutes that are endothermic upon dissolving (i.e sol’n gets colder): NH4NO3 +H2O  endothermic solvation process (+25.4 KJ/mol)

  13. Practical use Problem CaCl2 is used in a solvation process to help melt ice On roads and sidewalks (i.e. the solution gets hot). NH4NO3 is used in Cold Packs and Na2S2O3 is used in Heat packs. What is DH for each (is it positive or negative)? CaCl2 + H20  Negative Heat of Solution (-Hsol) NH4NO3 + H20  Positive Heat of Solution (+Hsol) Na2S2O3 + H20  Negative Heat of Solution (-Hsol)

  14. Table I • Open your reference tables to Table I • What is the Hsol of KNO3? +34.89 • What is the Hsol of LiBr? -48.83 • Which one when dissolved in water will decrease the temperature of the solution? KNO3 / Hsol is Positive / Hsol is Endothermic

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