Analysis Of The Enthalpy Of Formation Of Magnesium Oxide

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In part C, the enthalpy of formation of magnesium oxide was found by adding together three reactions, as shown in the table below. Experiments were performed to obtain the ΔHrxn value of the first two reactions in the table, which were reactions 4 and 5 respectively in the experimental section, while the ΔHrxn for the third reaction in the table was provided. Using the ΔT obtained experimentally, heat energy (q) was determined using the equation q = mCΔT + CΔT where q was the heat, m was the mass of the solution, ΔT was the change in temperature, C was the specific heat of water, and C was the calorimeter constant. The mass was calculated by multiplying the density of the solution, which was 1.01 g/mL, by the volume of solution. When these values were obtained, the number of moles of MgO and Mg were calculated by dividing their recorded masses by their molar masses. Then, ΔHrxn was calculated by dividing the heat energy released in each reaction by the number of moles of MgO in reaction 4 and Mg in reaction 5. The following values were obtained:

Reaction Equation ΔHrxn (kJ/mol)
1 MgO(s) + 2 HCl(aq) ⟶ MgCl2(aq) + H2O(l) -192.295
2 Mg (s) + 2 HCl(aq) ⟶ MgCl2(aq + H2(g) -509.444
3
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It can be calculated y subtracting the sum of enthalpies of formation of reactants from the sum of enthalpies of formation of the products. The enthalpy of formation is the ΔHrxn when 1 mole of a compound is formed from its elements in their standard states at 25℃ at 1 atm.2 The change in enthalpy per mole of a reaction was illustrated by part B of the experiment, because the change in enthalpy per mole of HCl or NH4OH was calculated. The standard molar enthalpy of formation was illustrated by part C of this experiment, because the various reactions added up to a reaction that led to the creation of 1 mole MgO from its elements at their standard states. Hence, the ΔHrxn in this reaction represented the enthalpy of

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