CH4 (g) + 2 O2 (g) -> CO2 (g) + 2H20 (l)
The combustion of methane under standard conditions (25°C, 1atm) is spontaneous according to the change in Gibbs free energy, ie deltaG = -817.8 kJ/mol CH4. But if I spray with my deodorant, the methane (which pressurizes the liquid) that comes out with it doesn’t burn spontaneously, does it?
If there is an (electrical or thermal) spark, the methane will burn, but then the methane will not burn at 25 °C because the temperature is much higher at the spark.
For example, H2 gas can still exist in the air (as long as there is no spark nearby) while the combustion of H2 according to Gibbs free energy is still spontaneous (under standard conditions). Gasoline exposed to the air will not burn spontaneously anyway, only evaporate…
Answer
A reaction with a negative free energy change is indeed a spontaneous reaction. However, this only says something about the possibility of whether or not a reaction can proceed, but nothing about the speed of the reaction. Sometimes you do indeed have to give reactions “a push” before they can continue. The reactions must be catalyzed to proceed faster. What you have written down are thermodynamic data and only say something about the initial and final state of the reaction. The rate of the reaction (reaction kinetics) deals with how the starting products and end products are formed, and therefor intermediate products (intermediates) can be formed that are energetically unfavorable. This slows down the response.
Answered by
Prof. dr. Bart Devreese
Biochemistry, Biochemical Analysis proteins
http://www.ugent.be
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