 Thin clouds hang over Olympus Mons, the largest known volcano in the solar system. 25 km high, it is more than three times higher than Mt. Everest, and at 550 km across is as wide as Saskatchewan. This photograph of the colossal mountain, with clouds of frozen CO2, shows that Mars truly does have an atmosphere. (Photo courtesy: Malin Space Science Systems/NASA) |
You should now understand equilibrium, and how to predict the effects caused by changes in temperature or pressure using le Châtelier's principle. You can use it to make some sense of the reactions proposed to synthesize fuel from the Martian atmosphere. Recall that the Sabatier methanation reaction is the starting point for this process:
CO2 (g) + 4 H2 (g) 
CH4 (g) + 2 H2O (g)
The CO2 (g) required for the reaction is abundant in the Martian atmosphere. Even though the atmospheric pressure on Mars is very low – only 0.7 kPa, less than one hundredth of Earth's – almost 95% of the atmosphere is carbon dioxide. Despite its thin atmosphere, this is almost 20 times as much CO2 (g) as on Earth. There will be no shortage of this resource!
H2 is a different story altogether. Until we find water on Mars, we
will have to import the hydrogen from Earth. Hydrogen is very light, but the fact
that it boils at -253 °C means there will be large evaporation losses unless it is
incredibly well insulated.
| In the Sabatier reactor, Martian CO2 will be reacted with H2 brought from Earth. Using le Châtelier's principle, predict the pressure conditions that will give maximum yield of products. |
The Sabatier methanation reaction takes place rapidly at a temperature of about 400 ºC in the presence of a nickel catalyst. Which of the following statements is correct?
In order to be able to predict the effect of temperature on this reaction, you need to
know its 
H. This can be calculated from a table of thermodynamic
values using Hess' law.
What is the calculated H for the reaction?