2. Consider electrolysis of water into constituent gases H2 and O2 through the r
ID: 702867 • Letter: 2
Question
2. Consider electrolysis of water into constituent gases H2 and O2 through the reaction (a) Calculate the enthalpy ?? of the reaction for a mole of water from known values of ?? (Schroeder, p. 404) for the reactant (H20) and the products (H2, O2). Start with the equation for ??, and a list of ?? values to be uses. What does ?? stand for? (b) Calculate the change in internal energy ?U due to the reaction and the work PAV done to make room for the products in gaseous form, relating these to ?? of the reaction. (c) Calculate the change in entropy ?S due to the reaction from a table of entropy values (see Schroeder, p. 404) for the reactant and the products; remember to show the equation used (d) Finally calculate ?G of the reaction using the relation between ?? and ?G. Compare it with the ?G calculated from the tabulated ?,G values for the reactant and the products What does AG represent in this example? S (J/K) 212 205 214 70 ??? - 1273 0 - 393.5 285.8 - 918 0 394.4 285.8 Glucose Schroder p. 404 CO2 H20Explanation / Answer
Ans 2
Change in enthalpy of reaction = sum of Enthalpy of formation of products - sum of Enthalpy of formation of reactants
H = 0.5*Hf(O2) + Hf(H2) - Hf(H2O)
= 0.5*0 +0 - (-285.8)
= 285.8 kJ/mol
Hf = heat of formation
Entropy change of reaction =sum of Entropy of formation of products - sum of Entropy of formation of reactants
S = 0.5*Sf(O2) + Sf(H2) - Sf(H2O)
= 0.5*205 + 130.6 - (70)
= 163.1 J/mol·K
Gibbs free energy change of reaction
G1 = H - TS
= 285.8*1000 - 298 *163.1
= 237196.2 J/mol
= 237.196 kJ/mol
From the table
G2 = 0.5*0 + 0 + 285.8
= 285.8 kJ/mol
On compare,
G2 > G1 by 50 kJ energy
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