Design of the experiment and relevant background for

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Design of the Experiment and Relevant Background For a series of borax-containing samples taken at various temperatures, the concentration of borate ion will be determined. The borate ion reacts with monoprotic acid (such as HCl) in a 1:2 fashion: B 4 O 5 (OH) 4 2 - ( aq ) + 2HCl( aq ) + 3H 2 O( l ) ----------> 4 B(OH) 3 ( aq ) + 2 Cl - ( aq ) The amount of HCl consumed (in L) multiplied by the molarity of the acid gives moles HCl. It is a trivial exercise to determine moles borate ion from there, and dividing by the volume of borate ion-containing sample in liters gives concentration of borate ion. This general procedure is repeated for borate ion- containing samples of constant volume obtained at various temperatures. Calculation of these temperature-dependent values for K sp of borate ion is but the first step in a greater sequence of obtaining complete thermodynamic parameters for the dissolution of borax in water. There are several other important thermodynamic parameters that can be found: free energy change, enthalpy, and entropy, upon further treatment of the equilibrium constant/temperature base data set. The next goal will be to estimate the free energy change for this solubility equilibrium. The following equation immediately shows the relationship between free energy change ( G ) and equilibrium constant ( K ): G = - RT ln K sp (A) A value for the equilibrium constant at a given temperature gives (after a sense) the value for free energy change directly. The free energy change at a given temperature is itself related to both the change in enthalpy, and the change in entropy, by the following equation: G = H - T S (B) Combining the two equations relates K , H, and S in a single statement: - RT ln K sp = = H - T S
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The preceding equation can be rearranged into a form that is linear . When re-arranged into a linear form, 1/T and ln K sp can be used as (x,y) points on a graph. The slope of such a graph is related to the change in enthalpy and the change in entropy is related to the y-intercept: A summary of the post-data collection activities reveals the true beauty of this experiment. Once the solubility product constants have been determined for 5 different temperatures, A table of free energy values ( G) with the temperatures those values correspond to should be compiled. This is most conveniently done using equation (A) , with values for K sp and the temperatures for which they are valid. A graph of ln K sp vs. 1/T should be made, the slope of which is related to change in enthalpy. The same graph has a relationship between change in entropy and its y-intercept The experiment requires that the solubility of borax be found at various temperature values. Samples of saturated borax solution are collected at no less than 5 different temperatures, four above room temperature, and one close to or at room temperature. These samples are then warmed (if necessary) to re-dissolve any precipitated borax, and titrated to the yellow bromocresol green endpoint with standardized aqueous hydrochloric acid.
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