Properties of Solutions_part_1-2

Properties of Solutions_part_1-2 - PHYSICAL EQUILIBRIA...

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PHYSICAL EQUILIBRIA SOLUTION COMPOSITION A solution is a homogeneous mixture of a solvent and solute(s). The solvent is the dissolving substance and the solute is the dissolved substance. Examples: clean air, salt and water, 14K gold. Can you identify the solvent and the solute(s) in each? Solution composition can be described in different terms: Molarity Mass Percent Mole Fraction Molality THE TERMODYNAMICS OF SOLUTION FORMATION We can look at solubility as consisting of three hypothetical steps (fig 17.1): Step 1: Breaking up the solute-solute interactions or expanding it Step 2: Breaking up the solvent-solvent interactions or expanding it Step 3: Forming interactions between solute and solvent particles to form the solution Ehthalpy of the solution (fig 17.2) We expect the first step to be ----------------------- . This is because -------------------- -----------------------------------------------------. This energy is represented as H 1 . We expect the second step to be ------------------------. This is because ------------------ -----------------------------------------------------. This energy is represented as H 2 . We expect the third step to be -------------------------. This is because -------------------- -------------------------------------------------------. This energy is represented as H 3 . 1
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The enthalpy of the solution is therefore, H soln = H 1 + H 2 + H 3 The standard enthalpy of solution is H ° soln = H ° 1 + H ° 2 + H ° 3 The enthalpy of hydration, H ° hyd , is defined as the sum of H ° 2 and H ° 3 Therefore, the standard enthalpy of the solution is: H ° soln = H ° 1 + H ° hyd Example: NaCl in water NaCl(s) Na + (g) + Cl - (g) H ° 1 = 786 kJ/mol H 2 O(l) + Na + (g) + Cl - (g) Na + (aq) + Cl - (aq) H ° hyd = -783 kJ/mol H ° soln = H ° 1 + H ° hyd
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