Solubility As A Function Of Temperature: Laboratory Report

Laboratory Report Solubility as a function of temperature

Summary The experiment aims to represent the cooling curves for 2 systems: potassium nitratewater and naphthalene- toluene and to note the temperature of the beginning of crystallization as a function of concentration. By plotting the logarithm of the solute concentration against the reciprocal of the absolute temperature the dissolution heats for these saturated solutions could be estimated. Theoretical aspects The dissolution is generally accompanied by a thermal effect (examples: the dissolution of NH4Cl in water takes place with absorption of heat, the dissolution of NaOH in water-with releasing of heat). Under constant pressure these thermal effects represent the enthalpies of dissolution ∆dH, also called dissolution heats. Dissolution heat depends on the concentration of solution, reaching the maximum value at saturation, where thermal effect is ∆Hsat. The integral heat of dissolution ∆Hsat of a solid could be directly determined, by measuring its solubility at several temperatures. the dependence of saturation molar fraction of the solid, xsat, on absolute temperature T is:

d (ln x sat ) H sat  dT R T 2

(eq.1)

By integration, this yields: ln x sat (T )  

H sat  ct R T



lnx sat (T )  

H sat  ct 2.3  R  T

Plotting ln xsat=f(1/T) , the slope of the obtained straight line is -∆H sat/2.3R, which allows the calculation of ∆Hsat. In ideal solutions the molar heat value of don’t depend on the solvent used and is approximately equal to the melting heat of solid.

Laboratory set-up -water electrical bath -thermometer -4 glass tubes with different compositions of potassium nitrate-water and naphthalene- toluene -magnetic stirrer Working procedure The sample is a special vial tightly closed to prevent the vaporization. The thermometer is introduced into the interior tube of each vial for a better measurement of the temperature. The samples are heated in the water electrical bath (shake it from time to time) till the whole solid is dissolved. Then each sample is cooled at the room temperature under permanent stirring (with magnetic stirrer).Every 15 seconds the temperature will be read on the thermometer. Then one plots these values taking the point when the slope of the cooling curve T=f(t) is changed. This is the temperature when the solution concentration becomes equal to the concentration of saturation (the solid solubility at the respective temperature). Each vial is labeled and contains a solution whose composition is written in Table 1.