Advanced Projects in Chemistry - Enthalpy of Solution

Thermochemistry investigates the relationship between chemical reactions and energy changes involving heat. The study of thermochemistry is important in order to understand energy efficient processes and reactions. Development of alternative fuel sources, such as fuel cells, hybrid gas-electric cars or gasoline supplemented with ethanol using energy efficient reactants is the need of today’s industry. From the viewpoint of a K-11-12 grade student, the fundamentals of thermochemistry are important. Theoretical study of thermochemistry demonstrates direct relationship between the forces holding molecules or ionic compounds with the heat evolved or absorbed in a chemical reaction. It is therefore imperative for a chemist to understand energy changes accompanying chemical reactions- be it the solubility of lead salts in drinking water or the metabolism of glucose in the blood stream. The amount of heat generated or absorbed in a chemical reaction can be studied using a calorimeter.

The thermodynamic "system" of our chemical reaction will be an inorganic salt which is placed in a well-insulated vessel surrounded by water, our thermodynamic surrounding. A thermometer placed in the system is used to measure the heat transferred to or from the system to the surroundings. Ideally, only water is the "surroundings" in thermodynamic sense, as the vessel does not allow heat to pass. Enthalpy or the heat content of a system, is the amount of energy in the system that can be converted to work. It is the sum of the internal energy and the pressure volume energy. To quantify the heat flowing into and out of the system in a constant pressure system, chemists introduced the term Enthalpy represented by the symbol ‘H’. When a reaction takes place at constant pressure, which in our case is the atmospheric pressure, then there can be either an increase or decrease in the volume of the reactants in product formation. If expansion occurs then some heat is utilized in doing work against the constant pressure. The heat evolved would be less than the heat evolved at constant volume. If the reaction proceeds with a decrease in volume, then, work is done on the system and the heat evolved will be more than the heat evolved at constant volume. Therefore heat changes that take place at constant pressure are not solely due to change in internal energy but also due to the expansion or contraction of the system against atmospheric pressure.

The enthalpy of a solution is defined as the enthalpy change when one mole of the solute is dissolved in a definite quantity of solvent at a given temperature to produce a solution of desired concentration. It is represented as ∆sol H or ∆sol Ho if the reactants and products are in their standard state at a specified temperature. The concentration of the solution is generally stated as the ratio of the amount of solute to that of solvent. The process of dissolution can be endothermic or exothermic depending on the nature of the salt and its interaction with the solvent. The process of dissolution of a salt in solvent involves -

- The breaking of the ionic lattice between the ions that constitute the salt. The heat change accompanying the separation of one mole of anionic compound into its gaseous atoms is called its lattice enthalpy.
- The gaseous constituents then interact with water as the solvent and the process is called enthalpy of solvation.

If a large volume of solvent is taken such that further addition of solvent doesn’t produce any heat change, it is called enthalpy of solution at infinite dilution. Enthalpy of solution can be positive or negative. Enthalpy of solution determines the extent of solvation. When the enthalpy of solution is a negative value with a high magnitude it indicates that the substance is highly soluble in the given solvent. Higher the lattice enthalpy less is the solubility of the solvent in the salt. Experimental work in the laboratory related to studying heat of solution involves simple experiments and apparatus. Using a coffee cup calorimeter, the enthalpy of solution can be determined by simple temperature changes accompanying the dissolution of different salts.

Enthalpy of solution can be studied in two ways-
a. A study of the different sulfate salts of transition metals can be studied.
b. The study of heat changes accompanying dissolution as a function of concentration can be plotted as a graph of ∆ T vs concentration of salt in moles/L.

Actual hands on laboratory work, collection of data related to enthalpy of solution impresses the understanding of everyday life applications such as cold and hot compresses, drug interactions to lower body temperature as also act therapeutically at optimum body temperatures. With the growing need of Green Chemistry, study of enthalpy is an important aspect in developing eco friendly processes to reduce thermal pollution.

Acknowledgments: The experimental work related to enthalpy of solution was diligently carried out by Binaya Swain and Someshwar Dash of R. N. Podar Senior Secondary CBSE High School, Santacruz (W).