Write a separate balanced chemical equation for the combustion of each of the fo

Question

Write a separate balanced chemical equation for the combustion of each of the following: C(s), H_2(g), and C_2 H_5OH(l). Consider the only products to be CO_2(g) and/or H_2O(l). (b) In principle, ethanol can be prepared by the following reaction. 2C(s) + 2H_2(g) + H_2O(l) - C_2H_5OH(l) Calculate the standard enthalpy change (Delta H degree) for the preparation of ethanol as shown in the reaction above. (c) Calculate the standard entropy change (Delta S degree) for the reaction given in part (b). (d) Calculate the value of the equilibrium constant at 25 degree C for the reaction represented by the equation in part (b).

Explanation / Answer

(a)

Balanced chemical equation for the combustion of Carbaon (s) or C(s) is

C(s) + O2(g) CO2 (g)               H0 = -393.5(kj/mol)                       Equation (a)

Balanced chemical equation for the combustion of Hydrogen (g) or H2(g) is

2H2 + O2 2H2O (l)                       H0 = -285.8(kj/mol)                     Equation (b)

Balanced chemical equation for the combustion of Ethanol(l) Or C2H5OH (l) is

C2H5OH (l) + 3O2 (g) 2CO2 (g) + 3H2O (l)   H0 = -1366.7(kj/mol)           Equation (c)

(b)

The standard enthalpy change for the preparation of ethanol is 293.9kj/mol

2C (s)+ 2H2 (g)+ H2O (l) C2H5OH (l)                             Equation (d)

The above equation can be obtained by the following equation

2[Equation (a)] + Equation (b) - Equation (c)

Or

Multiplied equation (a) by 2 followed by addition of equation (b) and equation (c) is reversed so that the required equation (d) can be obtained

2[C(s) + O2(g) CO2 (g) ]                  2[-393.5 kj/mol]

Or

2C(s) + 2O2 (g) 2CO2 (g)         -787kj/mol

2H2(g) + O2(g)   2H2O (l)          -285.8 kj/mol

2CO2(g) + 3H2O (l) C2H5OH (l) + 3O2(g)    -(-1366.7kj/mol)

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2C (s)+ 2H2 (g)+ H2O (l) C2H5OH (l) 293.9kj/mol

(c) The standard entropy change (S0reaction) for the the reaction in part (b)

2C(s) + 2H2(g) + H2O(l) ---> C2H5OH(l)

S0reaction = S0products - S0reactants

Soreaction = 1 mol SoC2H5OH - (2 mol SoC + 2 mol SoH2 + 1 mol SoH2O)

Soreaction = 1mol x 160.7J/molK - 2mol x 5.74J/molK - 2mol x 130.6J/molK - 1mol x 69.91J/mol K

Soreaction = - 181.9 J/K or Soreaction = - 0.1819 kJ/K

(d) Calculation for the value of the equilibrium constant at 25oC for the reaction represented by the equation in part (b)     ie 2C (s)+ 2H2 (g)+ H2O (l) C2H5OH (l) is

Goreaction =  Horeaction - T Soreaction

Goreaction = 293.9 kJ - 298 K x (- 0.1819 kJ/K) = 348.1 kJ

Goreaction = - 2.303 RT log Keq

                         - Goreaction                                       - 348.1 kJ
Keq = antilog(--------------) = antilog( -------------------------------------------------------)
                        2.303 RT                     2.303 x 0.008314 kJ mol -1 K -1 x 298 K

Keq = 9.83 x 10 -62

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