K-12 Chemistry Project: Enthalpy of Solution of Group 2 Cation Salts
Experimental determination of enthalpy of solution
Enthalpy of solution or solvation energy is defined as the amount of heat released when 1 mole of a salt is dissolved in a solvent. The process of dissolution could either be endothermic or exothermic .
When the dissolution is accompanied by the evolution of heat energy the process or reaction is termed as an exothermic process while dissolution accompanied by the absorption of heat energy is an endothermic process.
For a salt solution if the process of dissolution is spontaneous despite being endothermic or exothermic, then it is an entropy driven process.
In an exothermic process, the final temperature(Tf) of the salt solution is greater than the initial temperature (Ti). There is an increase in temperature with respect to the initial room temperature and the container feels warm or hot to touch. In an endothermic process, there is a drop in the temperature on dissolution of the salt and the beaker is cold to touch.
Solvation can be thought of as a three step process – the breaking of bonds within the solute, the breaking of bonds within the solvent and finally the attraction of the solute ions with the solvent.
Method : A measured volume of water (100mL) was taken in a beaker where the temperature was recorded. A weighed amount of 0.05 moles of salt was dissolved in the beaker of water. With constant stirring the rise or fall in temperature was recorded.
After each salt was completely solubilized the final temperature was noted.
The three salts under study were Magnesium Chloride(MgCl2), Calcium Chloride (CaCl2)and Barium Chloride(BaCl2). All three salts have cations that belong to Group 2 (alkaline earth metals) of the periodic table. As the size of the cation increases the polarizing power of the cation decreases and hence the interaction between the water molecules and the cation of the ionic salt decreases. With a decrease in the polar interaction between the solvent and the cation the enthalpy of solvation decreases.
Although the experiment was simple it helped illustrate the periodic trends in solvation enthalpy. Our results complied with our expected trend despite the frugal resources of our chemistry laboratory.
The Enthalpy of Solution (Δsolution H) decreases as we go down the group. In the present study, we determined the enthalpy of solution for alkaline earth metal salts The enthalpy of solution depends on the attraction between the ions and water molecules. The attractions are stronger for smaller ions. Thus, greater the attraction, more is the energy released when the bonds are formed, and greater is the enthalpy of solution. We noted that all the reactions were exothermic as heat energy was liberated for each salt. This was in accordance with the expected trend.
Enthalpy of Solution for Calcium Chloride = ∆HCalcium Chloride = -949.278 J
Enthalpy of Solution for Magnesium Chloride = ∆HMagnesium Chloride = -1975.7298 J
Enthalpy of Solution for Barium Chloride = ∆HBarium Chloride = -502.42942 J
The experiment consists of evaluating the enthalpy of solvation of 4 salts -
Magnesium chloride hexahydrate
Strontium chloride hexahydrate
Barium chloride dihydrate
The enthalpy change for the process of dissolution was determined using the relationship q= m c delta T.
A plot of Energy in Joules vs size of the cation gives a straight line graph with a negative slope. indicating a steady decrease in the enthalpy of solution. This was anticipated as the enthalpy of solution is directly proportional to the polarizing power of the cation.
The experimental work for this project was conducted by Harshit Sinha and Akhil Viswatmula at R.N. Podar Sr Sec High School, Khar under the guidance of Ms Anjali B Gharpure