"Prove" that LPG burns with a blue flame ....

[KedarMhaswade]

TL;DR Summary

What all information do we need to "prove" on paper the color of the flame we see when we burn the household LPG?

The household LPG burns with a blue flame. There's nothing to prove!

But what if we attempt to do that? How do we go about it?

I started with the assumption that it is a complete combustion of the LPG. A Google search tells me that the calorific value (the amount of heat a substance gives off upon complete combustion is $46.1 MJ/Kg$. The efficiency of the LPG is about 70%. I take this to mean that when LPG burns, its atoms and molecules are thermally agitated and their temperature rises. Some of that heat is conducted to the things that get heated (70%) and some of it is converted into electromagnetic radiation (30%) in the form of the visible light.

This will give me the energy contained in the radiation. What next? I can't seem to be able to use the amount of energy in a photon, $E=h\nu$ because it is unclear how many photons there are. Am I on the right track? How do I make progress toward establishing a $\lambda$ of about $500 nm$ of the radiation emitted by the LPG flame?

 

[hutchphd]

Summary:: What all information do we need to "prove" on paper the color of the flame we see when we burn the household LPG?

This will give me the energy contained in the radiation. What next? I can't seem to be able to use the amount of energy in a photon, E=hν because it is unclear how many photons there are. Am I on the right track? How do I make progress toward a λ of about 500nm?

What instrument is used to measure the wavelength of light?

 

[KedarMhaswade]

What instrument is used to measure the wavelength of light?

Spectrometer? But that is not household.

 

[hutchphd]

Sorry I don't understand the scope of the question. I would use my eye, then a spectrometer...The question says the gas is household LPG.

 

[Lnewqban]

http://archive.boston.com/news/scie...9/why_are_some_flames_blue_and_others_yellow/

 

[KedarMhaswade]

Sorry I don't understand the scope of the question. I would use my eye, then a spectrometer...The question says the gas is household LPG.

I can understand the strangeness of the question. I thought I accepted the strangeness of it by stating that there's nothing to prove: the eyes see a blue flame, therefore it must be blue light.

But how do we verify it using equations and known values?

 

[vanhees71]

I'm not sure, whether you ask about the spectrum of a flame? Then maybe that helps:

https://en.wikipedia.org/wiki/File:Spectrum_of_blue_flame.png

 

[KedarMhaswade]

I'm not sure, whether you ask about the spectrum of a flame? Then maybe that helps:

https://en.wikipedia.org/wiki/File:Spectrum_of_blue_flame.png

Thanks @vanhees71, but I guess I am trying to find out if what we see by eyes or measure by spectrometer could be in agreement with what we can calculate by using, e.g., conservation of energy.

 

[jbriggs444]

If one wanted to skip the (important) details of chemistry, one could pretend that a natural gas flame behaves like a black body. Then one could use Wien's law to associate the flame temperature with the flame spectrum.

 

[vanhees71]

As the spectrum shown in this Wikipedia figure illustrates, such a flame is fortunately not delivering a black-body spectrum but the spectral lines of the involved atoms and/or molecules. This gave rise to the utmost important discoveries of Kirchhoff and Bunsen and can well be cosidered as one of the empirical prerequisites for the development of modern quantum mechanics.

 

[anorlunda]

Might I recommend this wonderful book.

The Chemical History of a Candle by Michael Faraday
https://www.gutenberg.org/ebooks/14474

 

[hutchphd]

Too bad the authors are unknowns...

 

[bigfooted]

This topic in combustion is called chemiluminescence https://en.wikipedia.org/wiki/Chemiluminescence, which is describing the light emission from chemical reactions.
In chemistry class, we are taught that fuel + oxygen $\rightarrow$ products.
Let's assume that LPG is propane. We get as a chemical reaction:

$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$

This is a so-called global mechanism. In reality, many other chemical reactions are taking place. For instance, the nitrogen in the air also reacts at high temperature with oxygen and forms NO. When there is not enough oxygen available, CO is formed instead of $CO_2$. Also, for the reactions to take place, the molecules basically 'break apart' and then form new bonds. This means that we will also have H and O atoms in the flame. They combine into OH, which is called hydroxyl.
Let's visualize molecules as a bunch of protons and neutrons collected in the core of the molecule, with electrons in an orbit around them. These electrons circle around in fixed orbits called ground states, like the moon circling around the earth. Sometimes, these electrons are temporarily bumped into a higher orbit called an excited state when we somehow manage to give the molecule some energy to do so. This is usually an unstable situation and the electron quickly returns to it's ground state. When it returns to the ground state, there is some leftover energy that is being released in the form of a photon. Hydroxyl does this, and also $CH$ and $C_2$. Also water vapor emits light in combustion products.

Now, this a rough explanation of where the light comes from. As for the color of the light, we can get that by the Planck relation: https://en.wikipedia.org/wiki/Planck_relation

$E_1 - E_2 = hc\nu$,

So if we know exactly the difference in energy between the ground state and the excited state, we can determine the wavelength of the light $\nu$. Computing this energy is complicated, and depends on the type of molecule and it's rotation and vibration and we have to resort to quantum mechanics for computations. This topic is treated in spectroscopy https://en.wikipedia.org/wiki/Spectroscopy
This analysis does not lead to a single unique light frequency, but to a large number of discrete wavelengths that @vanhees71 is referring to, this was indeed a big discovery.

There are also other sources of light emission, like thermal radiation. hot metals radiate light, but also small carbon particles (soot) emit light, which is why candles are orange (and why your finger turns black when you move it through a candle flame).

 

[Lnewqban]

...
There are also other sources of light emission, like thermal radiation. hot metals radiate light, but also small carbon particles (soot) emit light, which is why candles are orange (and why your finger turns black when you move it through a candle flame).

Which opposite may be an acceptable demonstration by evidence for this problem.

 

[bigfooted]

I want to add that the blue light in hydrocarbon flames is due mainly to the OH and CH molecules. Hydrogen flames only have OH, which is why they are much fainter and often almost invisible. The water vapor emits a light orange-red glow.

 

[KedarMhaswade]

Now, this a rough explanation of where the light comes from. As for the color of the light, we can get that by the Planck relation: https://en.wikipedia.org/wiki/Planck_relation

$E_1 - E_2 = hc\nu$,

Should that be $\frac{hc}{\lambda}$?

So if we know exactly the difference in energy between the ground state and the excited state, we can determine the wavelength of the light $\nu$.

You mean $\lambda$.

(Some minor edits for completeness. Thank you for your answer!)