Question

In combustion, hydrogen gas ignite and produce 'pop' sound. Why ? Please explain using quantum.

Answer 1

well pop you hear is from explosion because hydrogen burns but in right mixture with oxigen (air) it forms explosive mixture which upon ignition explodes and forms water and because you need two gasseous compound to form one liquid compound you get sort of vacuum (explosive vacuum) and that is that pop sound of air recieding in test tube or container.

H2 (g) + 1/2O2 (g) -> H2O (l)

Answer 2

In short, the heat forms a positive-pressure p-wave (i.e. a blastwave) which as the product settles down from the excited state creates a decompression. This looks something like this on a microphone (microphones are designed to pickup P-waves and convert into electrical signals).

Answer 3

@agentx5 remind me to discuss this later but i think your point is not valid and ill explain when i come back, have to hellp my bilnd friend change pc battery...

Answer 4

in short while im still here... when two gaseous reactants are "egnited" one liquid product is formed, as energy of explosion is not enough to "dissasemble" water there is no pressure wave but under pressure wave or hipobaric wave and this there is no typical boom but pop....

Answer 5

Kryten, the combustion of H2(g) and O2(g) will produce H2O(g), not H2O(l). For one thing, consider the energetics: the reaction enthalpy is more than the enthalpy of vaporization of water. For another, consider the kinetics: the water molecules are formed by collisions between H2 and O2 molecules in the gas phase, and these collisions can't be any more densely packed than the H2 and O2 molecules in the first place. So the water molecules must necessarily form at a density of a gas, and can only later concentrate to form a liquid.

That said, It seems to me you're also right that the pressure necessrily drops, because you go from having 3 moles of gas (2 H2(g) + 1 O2(g)) to only 2 (2 H2O(g)).

I would imagine the sound wave is in part an initially positive overpressure wave, from the heating of the surrounding air by the energy release, which is typical for an explosion, and perhaps backed up by an unusually strong underpressure wave, because the final volume of gas is lower than the initial. Whether under the right conditions the underpressure wave could be in advance or overtake the overpressure wave is a very interesting question.

Answer 6

@Kryten I would refute by citing that liquid water turning into steam (gas) is about a 1:1700 volume ratio. (although exact values depend on temperature starting & ending, pressure changes, and saturation or unsaturated)

There is a tremendous expansion ratio here, far more than the mole ratio, and it's exothermic on top of that so we're going to be seeing an increase in temperature giving us a little addition increase in volume and/or pressure. Agreed?

Answer 7

I disagree highly with the concept of "explosive vacuum", at the very least you'd have to call it an implosion.

Answer 8

agentx i agree on explosive vacuum thing cause i was in hurry and implosion didnt come to me at the time... now let me make myself coffe and read trough what you and Carl have written...