Enthalpy of Reaction

Thermodynamics is the study of heat and its relation to energy and work. One of the most important thermodynamic properties is enthalpy, which indicates the heat energy transferred by a process at constant pressure. Enthalpy is used to describe chemical reactions, where the enthalpy change, ΔH, tells us how much heat is absorbed or released during a chemical reaction.

Enthalpy is a state function, meaning that the change is independent of the path and only takes into account the initial and final state. Thus, the ΔH of a reaction is calculated by subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products.

If ΔH is positive, the reaction is endothermic and absorbs heat from the surroundings. If ΔH is negative, the reaction is exothermic and releases heat to the surroundings.

We can measure the enthalpy change of a reaction using an isolated system like a calorimeter. A simplified calorimeter, like the one we have here, consists of an insulated sample cell with a stirrer and a thermometer. As the reaction proceeds inside of the insulated cell, the temperature changes and we are able to measure ΔT.

We can assume that no energy is transferred into or out of the sample cell because it is insulated. So, we can then assume that the temperature change is due to the heat change in the reaction.

So, how do we use ΔT to calculate ΔH? At constant pressure, ΔH is equal to the heat flow, Q. Q is equal to the mass of the substance times its specific heat capacity and the change in temperature. The specific heat capacity, c_s, is a measure of how much heat is required to increase the temperature of 1 g of a material by 1 °C.

A material with a higher specific heat capacity requires more energy to raise its temperature and vice versa. Specific heat capacity values for common substances can be found in the literature. Thus, we can use the specific heat capacity, along with the mass of the substance and the temperature change, to calculate ΔH for the reaction.

In your experiment, we will measure the change in enthalpy for the formation of magnesium oxide from magnesium and oxygen. Using our simple calorimeter requires us to measure the heat of reaction indirectly with a series of separate reaction steps. So, we will run each reaction separately and measure ΔT and, by extension, ΔH for each reaction. Then, according to Hess's Law, the enthalpy of the overall reaction is equal to the sum of each of the enthalpies of reaction for each reaction step.

In this lab, you will construct a simple calorimeter using polystyrene cups and then experimentally determine the enthalpy of formation of magnesium oxide.