Thermodynamic of Borax

Thermodynamics of the Solubility of Borax

Purpose: To determine the thermodynamic quantities H and S for the solvation reaction of borax in water, by measuring the solubility product constant over the temperature range from 55 to15 C. Na2B4O7 10H2O(s) Introduction: As you have seen or will see in Thermochemistry (Chapter 18) the free energy change in a reaction, G, is related to the equilibrium constant for the reaction by the equation G = RTln(K) (Chang Eqn. 18.14) Looking at this equation we see that if the reaction is thermodynamically favored, G < 0, and we would expect K to be large, K 1 . If K is much greater than one, we will observe the reaction proceeding primarily to products. The free energy expresses the net effect of two thermodynamic quantities: the enthalpy, H, and the entropy, S. Under constant temperature conditions, this is expressed by the equation G = H T S (Chang Eqn. 18.10) The two physical contributions to the free energy are the heat absorbed or released during a reaction, H, and the molecular disorder created during a reaction, S. Because the above two equations are expressions of G , the right hand side of each equation must be equivalent. RTln(K) = H Rearranging this expression yields - H 1 S . R T R As long as we can determine the equilibrium constant at several temperatures, this equation is well-suited for determining H and S by linear regression. - H 1 S ln(K) R T R y m x b The above expression is valid only if H and S are not temperature dependent quantities. Over the small temperature range that we shall investigate, this assumption is valid for borax. ln(K) T S 2Na+(aq) + B4O5(OH)42 (aq) + 8H2O(l)

The method of determining the equilibrium constant in this experiment is worth discussing. The equilibrium for the reaction to be studied is actually a solubility product constant, Ksp. For the chemical reaction given above the equilibrium constant is given by Ksp = [Na+]2[B4O5(OH)42 ] Fortunately, the stoichiometry of the reaction...