Calculating a Heat of Formation Using Hess’s Law As discussed in lecture, the simplest and most general method for determining the energy change during a chemical reaction involves the use of the heat of formation (ΔHf) for the reactants and products. Conceptually, the heat of formation is the heat released or absorbed when one mole of the substance is created from its elements at standard temperature and pressure. In some cases, this number can be measured directly (i.e. by reacting the elements at 1.0 atm and 25oC and observing the heat given off). In most cases, however, this is not practical. For instance, the direct calculation of the ΔHf for magnesium oxide would involve burning magnesium metal in a pure oxygen atmosphere; while such a reaction is certainly possible, it would difficult to measure changes in temperature since the reaction is highly exothermic and the product is a solid powder. Instead, in this lab you will be calculating the ΔHf for magnesium oxide by determining the heat of reaction for other reactions whose heats of reaction are either known or easily determined by experiment, and then combining them according to Hess’s Law to generate the heat of formation for MgO. The two equations you will be working with in the lab are: Reaction 1: Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) Reaction 2: MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l) Calculating a Heat of Formation Using Hess’s Law As discussed in lecture, the simplest and most general method for determining the energy change during a chemical reaction involves the use of the heat of formation (ΔHf) for the reactants and products. Conceptually, the heat of formation is the heat released or absorbed when one mole of the substance is created from its elements at standard temperature and pressure. In some cases, this number can be measured directly (i.e. by reacting the elements at 1.0 atm and 25oC and observing the heat given off). In most cases, however, this is not practical. For instance, the direct calculation of the ΔHf for magnesium oxide would involve burning magnesium metal in a pure oxygen atmosphere; while such a reaction is certainly possible, it would difficult to measure changes in temperature since the reaction is highly exothermic and the product is a solid powder. Instead, in this lab you will be calculating the ΔHf for magnesium oxide by determining the heat of reaction for other reactions whose heats of reaction are either known or easily determined by experiment, and then combining them according to Hess’s Law to generate the heat of formation for MgO. Grams of MgO in reaction 2 : 0.35 g Grams of HCl solution in reaction 2: 30.99 g Initial temperature in reaction 2: 24.C Final temperature in reaction 2: 29.C heat of formation of MgO: Mg (s) + O2 (g) à MgO (s) = -569 kJ/mol (calculation from experiment) Question: If you had failed to grind up a large chunk of magnesium oxide in the second part of the experiment, would your resulting ΔHf for magnesium oxide be too positive or too negative? Thoroughly justify your response.