University Of Central Lancashire
Department of Forensic and Investigative Science
FZ1025 Experimental Techniques in Chemistry
17. Determining the thermodynamics of calcium hydroxide solubility in water
When an ionic solid is added to water the solid dissolves by dissociating into its component ions. As more and more solid is added, it will continue to dissolve until the solution becomes saturated with ions. At this point the solid is in equilibrium with its component ions in solution. This is called the solubility equilibrium and will persist as long as the temperature is constant. For example, the solubility equilibrium of calcium hydroxide in water is:-
Ca(OH)2 (s) Ca2+ (aq) + 2OH- (aq)
It is straightforward to determine the molar solubility of a solid hydroxide by titration. First of all a saturated solution is prepared at a fixed temperature. Then the solid is removed and finally the concentration of hydroxide ions is determined by titration with standardised hydrochloric acid. The solubility will vary with temperature,
It is then possible to determine the equilibrium constant (sometimes known as the solubility product) at a particular temperature for the above equation.
Ksp = [Ca2+][OH-]2
In this experiment you will measure the molar solubility of calcium hydroxide in water at two different temperatures and then use the values to determine the solubility product at each temperature. From the solubility product it is them possible to determine the Gibbs free energy of solution and the standard enthalpy and entropy changes.
1) If a reaction is spontaneous would the Gibbs free energy be a positive or negative number? (1 mark)
2) How is the solubility product related to the Gibbs free energy? Write the equation defining all the terms. (3 marks)
3) What is entropy and how is it related to the Gibbs Free energy of a reaction?
4) If was found by titration that the...
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