01MANborax - Enthalpy and Entropy of a Borax Solution...

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Enthalpy and Entropy of a Borax Solution Revised 4/02/09 1 ENTHALPY AND ENTROPY OF A BORAX SOLUTION OBJECTIVES For you, the student, to write…. INTRODUCTION The relationship between free energy ( Δ G), enthalpy ( Δ H), entropy ( Δ S) and the equilibrium constant (K) for a chemical reaction at a specific temperature (T), is shown in equation (1) below. The Gas Constant, R, is equal to 8.314 J/mol · K. (1) Δ = – R T ln K = Δ T Δ A process at constant temperature and pressure will be spontaneous in the direction in which free energy decreases (– Δ G°). When equation (1) is rearranged to eliminate Δ G°, ln K is expressed in terms of Δ H, Δ S and T only: (2) ln K = ( −Δ H ° )(1) + ( Δ S ° ) ( R )(T) ( R ) Note that equation (2) resembles the point-slope form of a line (y = mx + b) . A plot of ln K versus (1/T) should result in a straight line with a slope equal to (– Δ H°/R) and a y-axis intercept equal to ( Δ S°/R). Enthalpy, Δ H, is determined from the slope but entropy, Δ S, is usually calculated from (2) instead of the graph. The reason is that the intercept of the y-axis occurs when the ratio (1/T) = 0 (that is, when T = ). Since the temperature of the reaction will be ~310 to 340 K, the graph would have to be unusually large to show T = .
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Enthalpy and Entropy of a Borax Solution Revised 4/02/09 2 The solubility of a salt is dependent on the temperature of the solution. When equilibrium is established in a saturated solution at a specific temperature, the rate of formation of ions in solution is equal to the rate of deposition of solid. The equilibrium constant for the dissolution of a solid in a solvent is called the "solubility product constant" (K sp ). It is equal to the product of the concentration of ions in solution. Since the concentration of ions can change with temperature, K sp is temperature dependent. Sodium tetraborate decahydrate ("borax") dissociates in water to form sodium and borate ions and water molecules: (3) Na 2 B 4 O 7 . 10H 2 O(s) ' 2Na + (aq) + B 4 O 5 (OH) 4 2- (aq) + 8H 2 O(l) And the solubility product (equilibrium) constant, K sp , is: (4) K sp = [Na + ] 2 [B 4 O 5 (OH) 4 2- ] Note that two sodium ions are produced for each borate ion (B 4 O 5 (OH) 4 2- ) in the reaction. Therefore, the concentration of Na
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This note was uploaded on 11/23/2009 for the course BIO 005 taught by Professor Zil. during the Spring '08 term at UC Riverside.

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01MANborax - Enthalpy and Entropy of a Borax Solution...

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