Question

1.            Consider the following redox reaction to produce methanol:

                                    2CH₄(g) + O₂(g) --> 2CH₃OH(g).

a. Calculate the change in enthalpy for the reaction at standard conditions using bond dissociation energies. See the b. appendix for the table of bond energies.

b. Name two things that you could do to favor the formation of methanol by this reaction.

c. Using the value of ∆H calculated in 1a, is this an exothermic or endothermic reaction? Explain your answer.

d. Methane gas is a byproduct of oil extraction and of oil refining. Release of the methane gas into the atmosphere, which contains 20% oxygen, does not result in the formation of methanol. That’s a surprising observation: Despite the fact that the reaction of methane and oxygen to produce methanol is product-favored at normal temperatures, methanol is not formed spontaneously when methane is released to the atmosphere. Suggest why no methanol is formed.

Answer 1

1. a. change in enthalpy for the reaction at standard conditions using bond dissociation energies:

H^o = H_Products - H_reactants

= 2[H_CH3OH]-[2(H_CH4)+H_O2]

= 2[3(H_C-H)+H_O-H+H_C-O]-[2(4(H_C-H))+H_O=O]

= 2[3(410)+460+350]-[2(4(410))+498] = 2[1230+810]-[3280+498] = 4080-3778 = 302 kJ/mol

b. To favor the formation of methanol, we can increase the amount of methane and oxygen. If we increase the amount of these two compounds, the reaction will move in forward direction.

c. The enthalpy change for the reaction is positive, so the reaction is endothermic. If the enthalpy change would have been negative, then the reaction is exothermic. But for this reaction, it is endothermic.

d. Methanol is not formed spontaneously when methane is released in atmosphere, because the Gibbs free energy change for the reaction would be positive for room temperature. The temperature should be high for the reaction to be spontaneous. As Gibbs free energy change is dependent on temperature and entropy. The entropy change for the system is positive. The temperature should be quite high to make the term T​S high. The Gibbs free energy change will become negative.