1 Enthalpy of Formation of Magnesium Oxide Adapted with permission from the United States Air Force Academy. Revised: 2008 BACKGROUND In this laboratory, we will introduce one of the most often used techniques in thermochemistry, calorimetry. Although we often think of calorimetry in terms of finding the number of calories in a certain amount of food, calorimetry is valuable to the chemist in measuring basic thermodynamic data. Along with learning calorimetry techniques, you will use the data you collect, other reaction enthalpies, and Hess’s Law, to determine o f H Δ for MgO(s). Read and/or review Chapter 6 in your textbook. Enthalpy and Hess’s Law The enthalpy change, o rxn H Δ , of a chemical reaction is called the enthalpy of reaction or the heat of reaction and represents the amount of heat gained or lost by the reaction system as the reaction proceeds from reactants to products. The standard (molar) enthalpy of formation, o f H Δ , is defined as the o rxn H Δ when one mole of a compound is formed from its elements in their reference form and in their standard states. Enthalpy is a state function; the enthalpy change of a reaction is independent of its path and depends only on the initial and final states of the reactants and products. This principle, applied to enthalpy, is known as Hess’s Law. Hess’s Law states that the enthalpy change of a reaction is the same whether it occurs in one step or in many steps. For example, the enthalpy change for the reaction between carbon and oxygen to form carbon monoxide: C( s ) + ½ O 2 ( g ) → CO( g ) o rxn H Δ = ? cannot be directly measured since carbon dioxide is also a product of this reaction (there is no way to run this reaction to ONLY give CO). However, to obtain the desired heat of reaction, we can react carbon and carbon monoxide in large excesses of oxygen to form carbon dioxide and measure the enthalpies of these reactions: C( s ) + O 2 ( g ) → CO 2 ( g ) o H Δ = -393.5 kJ CO( g ) + ½ O 2 ( g ) → CO 2 ( g ) o H Δ = -283.0 kJ According to Hess’s Law, we can combine the above two reactions in a manner that will give the desired reaction. Note that if we reverse the second reaction and add it to the first reaction, we will obtain the desired reaction (see below) when the two reactions are then added together. Since we reversed the second reaction, the sign on o H Δ must also be changed.