Why methane burn at high temperature?

[chewchun]

Methane burns at 1200k without any spark or ignition.
But why is this so?Why so high temperature?
Isn't it that when there is the right proportion of methane and oxygen, combustion will occur with a small energy needed?
Or is it because of high activation energy?

 

[chill_factor]

methane reacting with oxygen is thermodynamically favorable at essentially all temperatures.

That's because according to the constant temperature (which is a very nice approximation) free energy equation, dG = dH - TdS, dG will ALWAYS be negative: the binding energy of methane + oxygen is higher (less negative) than the binding energy of the products water and carbon dioxide, so dH is negative, and dS is always positive because the equation

CH4 + 2 O2 -> CO2 + 2 H2O has molecules with more degrees of freedom on the right hand side than on the left hand side.

To make it quantitative, you look up numbers on a thermodynamic data chart.

However, thermodynamically favorable =/= actually happens because to actually happen, the activation energy must be overcome. There's 2 ways to do it. One is to apply a small amount of energy in a tiny place so that the energy density is very high, such as with a spark plug, and a reaction occurs at that point, and the energy released by the reaction causes a chain reaction. Or you can just raise the temperature of everything.

 

[chewchun]

methane reacting with oxygen is thermodynamically favorable at essentially all temperatures.

That's because according to the constant temperature (which is a very nice approximation) free energy equation, dG = dH - TdS, dG will ALWAYS be negative: the binding energy of methane + oxygen is higher (less negative) than the binding energy of the products water and carbon dioxide, so dH is negative, and dS is always positive because the equation

CH4 + 2 O2 -> CO2 + 2 H2O has molecules with more degrees of freedom on the right hand side than on the left hand side.

To make it quantitative, you look up numbers on a thermodynamic data chart.

However, thermodynamically favorable =/= actually happens because to actually happen, the activation energy must be overcome. There's 2 ways to do it. One is to apply a small amount of energy in a tiny place so that the energy density is very high, such as with a spark plug, and a reaction occurs at that point, and the energy released by the reaction causes a chain reaction. Or you can just raise the temperature of everything.

Is there any reason for the high activation energy of methane combustion?
To me,such a simple molecule seems to be pretty complex upon what you have said...

 

[chill_factor]

yes there is a reason but it is complicated. i think i remember being taught this in my physical chemistry class but i did not learn it well and forgot.

 

[LudusRex]

The high temperature required for methane to burn is due to a combination of factors. First, methane has a very stable chemical structure, meaning that it takes a lot of energy to break the bonds between its carbon and hydrogen atoms. This is known as the activation energy, and it is the minimum amount of energy required for a chemical reaction to occur. In the case of methane, this activation energy is quite high, which is why a high temperature is needed to provide enough energy to start the reaction.

Furthermore, the reaction between methane and oxygen is highly exothermic, meaning that it releases a lot of heat energy. This heat energy, in turn, helps sustain the reaction and keep it going at high temperatures. Without this heat energy, the reaction would not be able to continue, and the temperature would not be able to reach the required 1200k.

It is also important to note that the presence of oxygen is crucial for the combustion of methane. Without oxygen, the reaction cannot occur at all, no matter how high the temperature is. This is because oxygen is a necessary reactant in the chemical reaction that converts methane into carbon dioxide and water.

In summary, the high temperature required for methane to burn is a result of its stable chemical structure, the activation energy needed to break its bonds, and the exothermic nature of the reaction. Additionally, the presence of oxygen is essential for the reaction to occur. Understanding these factors is important for safely and efficiently using methane as a source of energy.