Here is my explanation about the reaction between hydrogen or methane and oxygen.

Hydrogen molecules and oxygen molecules can collide sucessfully and react if they have enough energy. In room temperature, molecules are moving slowly, but they do have a chance to collide, and they do have a chance to have high energy level to collide successfully. Therefore, even in room temperature, hydrogen can slowly react with oxygen to form carbondioxide and water.

And as temperature increases, the chance to collide and to have enough energy gets bigger, so rate of reaction increases.

PS: the kenetic energy of particles satisfies something like normal distribution, so molecules with high energy must exist even in low temperature.

My chemistry teacher don't agree with me. why?

Which brought me to a second question: what is the definition of ignition point(the minimal temperature required to burn) if reaction can happen just at any temperature?

  • $\begingroup$ @Mithoron My question is NOT about organic molecules. The link you gives doesn't explain inorganic burning! $\endgroup$
    – Ma Joad
    Feb 8 '18 at 2:54
  • $\begingroup$ But methane is an organic molecule. $\endgroup$ Feb 8 '18 at 8:31
  • $\begingroup$ @IvanNeretin But obviously organic or inorganic is not the main concern. $\endgroup$
    – Ma Joad
    Feb 8 '18 at 8:46
  • $\begingroup$ Whatever the answer remove carbon dioxide from your sentence. $\endgroup$
    – Alchimista
    Feb 8 '18 at 10:29
  • $\begingroup$ What you say is theoretically correct. But if you did the calculations, you would find that the reaction rate is so slow it it would be impossible to even measure it. $\endgroup$
    – matt_black
    Feb 8 '18 at 11:16

Your reasoning is correct. When we say a reaction occurs only with sufficient energy (activation energy), it is truly a tail of a single (pair of) molecule. But the ensemble of molecules still provides molecules of high energy, able to react, and molecules of low energy which will collide and not react. That effect is expressed macroscopically by the reaction kinetics, with a higher rate of collisions resulting in a higher reaction rate. The kinetic effect of temperature is thus continuous and there is no single temperature at which a system has the activation energy to react (as some people believe). It is indeed true that a reaction may become spontaneous in a given direction only above a certain temperature, but that is a matter of thermodynamics and not kinetics.

Despite all of this, it sort of happens that indeed some reactions do seem to begin to take place at a certain temperature while not changing its thermodynamic spontaneity. That is because the kinetic effect of temperature is not linear, it is actually much more intense that a linear model might predict, sometimes doubling the reaction rate at every 10⁰C increase - this is captured by the Arrhenius equation. Sometimes you do have some molecules reacting, but it is so slow at a the temperature you're looking it's not even noticeable. Bring the temperature up and you will see a rapid increase in reaction rate in a short temperature interval. So it looks like it was an on/off transformation.

More often than not in chemistry, the theory predict really small quantities, but the theory is also naive to assume the molecular distribution is continuous and not made of discrete entities as we know. So it sometimes happens that the idea that there no molecules reacting is in this situations actually closer to reality than the theory suggests.

A remark: molecular energy distribution is not normal generally, but follows something like a Maxwell-Boltzmann distribution.


Not the answer you're looking for? Browse other questions tagged or ask your own question.