What factors affect the dissolution enthalpies of ionic compounds?
What is this experiment about?
Soluble ionic compounds produce ions when they dissociate in water, and this process may absorb or release heat. Solution calorimetry empowers students to measure the temperature change in the water and calculate the heat absorbed or released, which is known as a solution enthalpy. In this experiment, students learn the method of solution calorimetry and use it to measure solution enthalpies. Students use their measured solution enthalpies and the Born-Mayer equation as a model to calculate lattice enthalpies, which exhibit periodic trends.
What do students do?
Students use a coffee-cup calorimeter, and/or a more advanced solution calorimeter, to observe temperature changes when a salt dissolves in water. Students construct thermograms and calibrate their calorimeter to determine solution enthalpies for salts of monatomic ions. Students use the Born-Mayer equation as a model to construct lattice enthalpies that exhibit periodic trends. Students then explore, interpret, and consider extending periodic trends in lattice enthalpies of monatomic ions.
What equipment and supplies will you need?
You will need coffee cups, cardboard, thermometers, and an analytical balance. Anhydrous salts of alkali metal halides (such as NaCl) provide students with useful data with which to observe periodic trends. Some alkali metal salts may require careful dehydration and storage to avoid waters of hydration. Some representative student data are available upon request.
What makes this experiment a physical chemistry experiment?
Students connect their general chemistry concepts of enthalpy, lattice energy, Hess’ Law, and periodic trends, using the more rigorous physical chemistry model of the Born-Mayer equation. Students use the method of solution calorimetry with calibration and thermogram plots to observe the process of thermal equilibrium and to more accurately measure enthalpy changes in an aqueous solution at constant pressure. Students construct lattice energies from measured solution enthalpies using Hess’ Law and the physical chemistry concept of hydration enthalpy, clarifying the relationship between model and measurement in thermochemistry.
And what makes it a POGIL-PCL experiment?
Students make predictions and develop hypotheses about whether dissolution of specific salts will be exothermic or endothermic processes. Students engage in experimental design to establish a consistent and accurate protocol for measurement and analysis of time and temperature data, and in decision making to assign salts to student teams for reproducibility. Students process information by applying graphical analysis and the Born-Mayer equation with Hess’ Law to obtain lattice enthalpies. Finally, students share data to develop a fuller picture of periodic trends in lattice enthalpies. Students demonstrate understanding of periodic trends by extending these concepts to salts of polyatomic ions.
Reference
Matt Scanlon, Fairmont State University
Rob Whitnell, Guilford College
The Instructor’s Handbook with implementation details, sample data, and expected answers is available through the POGIL-PCL project.