# Can O2 combine with H2 to form water without activation energy?

1. May 31, 2014

### kevin_tee

Let say that I have hydrogen gas and oxygen gas mix together, is there any chance that some H2 and O2 will react to form water without doing anything to it? I know that there needs to be activation energy to start the reaction, but are there any chance of reaction happening without activation energy? Thank you

2. May 31, 2014

### hilbert2

Even at room temperature, some small fraction of the gas molecules has high enough kinetic energy to react with each other (look up Boltzmann distribution). However, the fraction is very small and the reaction would probably take literally billions of years at room temp.

3. May 31, 2014

### kevin_tee

Thank you, so in lower temperature the less probability of H2 and O2 combining is lower because the gas molecule with high kinetic is less than high temperature, did I understand it correctly?

4. May 31, 2014

### hilbert2

Yes, the reaction rate is proportional to $e^{-\frac{E_{a}}{kT}}$, where $E_{a}$ is the activation energy and $k$ is the Boltzmann constant. Because of the behavior of the exponential function, the reaction rate very rapidly becomes slower when temperature is decreased.

5. May 31, 2014

### kevin_tee

Thank you, now I understand

6. Jun 7, 2014

### HeavyMetal

No, you still need activation energy.

$2~H_{2}(g)+O_{2}(g)~\xrightarrow{\Delta}~2~H_{2}O(g)~\ \ \ \ \ \ \ \ \ \ \Delta H^{\circ}=-483.6~kJ~mol^{-1}$