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Unformatted text preview: Enthalpy of Formation of Magnesium Oxide PRELAB: Reading: Chapter 5, Brown, LeMay, and Bursten. 1. What is the change in enthalpy for the formation of methane, CH 4 , from solid carbon (as graphite) and hydrogen gas? Refer to the following reactions: Reaction H (kJ) C (s) + O 2 (g) CO 2 (g)-393.5 H 2 (g) + O 2 (g) H 2 O (l)-285.8 CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l)-890.3 2. If the ionization of 1.00 g Mg (s) in excess acid solution (Eq. 3) evolves 19.3 kJ of heat, what is the standard enthalpy of formation for Mg 2+ (aq) expressed in kJ/mol? Introduction : The goal of this exercise is to measure the enthalpies of formation of Mg 2+ (aq) and MgO (s) . The enthalpy of formation of Mg 2+ (aq) can be determined from the enthalpy of dissolution of 1 mol of Mg metal in a very large amount of very dilute acid (eq 1). Mg (s) + 2 H + (aq) Mg 2+ (aq) + H 2(g) H = H f (Mg 2+ ,aq) (1) The H for this reaction is the H f for Mg 2+ , because the H f for Mg (s) , H + (aq) , and H 2(g) are zero by definition. The only species in equation 1 with a nonzero H f is Mg 2+ (aq) , so that its enthalpy of formation is simply the enthalpy of this reaction. The enthalpy of formation of MgO is more difficult to measure directly. Mg (s) + 1/2 O 2(g) MgO (s) H = H f (MgO) (2) It is more convenient to use the first law of thermodynamics in the form of Hess's law to simplify the measurement of H f (MgO) . If a chemical reaction can be broken up into the sum of two or more other reactions, the sum of the enthalpies of these reactions will be the enthalpy of the desired reaction. Consider the following reaction equations: Mg (s) + 2 H + (aq) Mg 2+ (aq) + H 2(g) H 1 (3) Mg 2+ (aq) + H 2 O (1) MgO (s) + 2 H + (aq) H 2 (4) H 2(g) + 1/2 O 2(g) H 2 O (1) H 3 (5) _________ Mg (s) + 1/2 O 2(g) MgO (s) H = H 1 + H 2 + H 3 The sum of these three equations is the desired equation; thus H f (MgO) = H 1 + H 2 + H 3 . H 1 is simply the enthalpy of reaction of a mole of Mg metal in excess acid; H 2 is the negative of the enthalpy of reaction of MgO in excess acid; and H 3 is the molar enthalpy of formation of water (a known constant of 285.8 kJ/mol). The heat effect for a chemical reaction run at constant pressure (such as those run on the bench top in open vessels) is the enthalpy change, q rxn = H. The heat evolved for a chemical reaction can be determined by running the reaction in a calorimeter and measuring the temperature change. q cal = C sys T (6) where C sys is the heat capacity of the system. The heat capacity of the system is sum of the heat capacity of the solution in the calorimeter, C soln , and the heat capacity of the calorimeter, C cp ....
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