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Experiment_17_-_Chem_1B

# Experiment_17_-_Chem_1B - Chem istry 1B Experim ent 17 89...

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Chemistry 1B Experiment 17 89

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90 17 Thermodynamics of Borax Solubility Introduction In this experiment, you will determine the values of H° and S° for the reaction which occurs when borax (sodium tetraborate octahydrate) dissolves in water. In previous experiments, you have determined H° values directly, by measuring temperature changes when the reaction occurred. However, in many cases, this technique is not practicable. For example, the reaction may not go to completion, or it may give off such a small amount of heat that the temperature change is too small to measure. In addition, there is no direct method for measuring S° for a reaction. It is therefore useful to be able to determine H° and S° indirectly, by using their relationship to the equilibrium constant of a reaction. The equilibrium constant of any reaction can be related to the free energy change of the reaction: G° = –RT ln K The free energy change is also related to the enthalpy and entropy changes during the reaction: G° = H° – T Combining these two equations gives the general relationship between K, H°, and S°: –RT ln K = H° – T Dividing both sides by –RT gives a particularly useful form of this relationship: ln K = – ! H ° R 1 T " # \$ % + ! S ° R This represents a linear equation of the form y = mx + b. In this case, y = ln K and x = 1/T; a plot of ln K against 1/T will therefore be linear. In addition, the slope of this line (m) will equal –( H°/R), and its y-intercept (b) will equal ( S°/R). It is therefore
Chemistry 1B Experiment 17 91 possible to determine H° and S° by simply measuring the equilibrium constant at two different temperatures, graphing ln K against 1/T, and measuring the slope and intercept of the resulting line. In practice, K is measured at several temperatures, so that the effect of any experimental errors in one measurement will be minimized. The reaction you will study is the dissolution of borax (sodium tetraborate octahydrate) in water. “Borax” is a naturally occurring compound; it is in fact the most important source of the element boron, and it has been used for many years as a water softening agent. Borax is a rather complicated ionic salt which has the chemical formula Na 2 B 4 O 5 (OH) 4 8 H 2 O. When it dissolves, it dissociates as follows: Na 2 B 4 O 5 (OH) 4 8 H 2 O ( s ) 2 Na + ( aq ) + B 4 O 5 (OH) 4 2– ( aq ) + 8 H 2 O ( l ) (1)

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Experiment_17_-_Chem_1B - Chem istry 1B Experim ent 17 89...

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