This is a common point of argument on internet forums. I think it is fairly well established theoretically that there is a very small amount of mass converted to energy in an exothermic chemical reaction. Has this ever been observed experimentally?
If not, how far off is our current experimental state of the art from being able to detect this change? Is it something that could conceivably be accomplished in the next few decades, or are we unlikely ever to be able to observe it?
There is an earlier question on this site that addresses essentially the same issue: Conversion of mass to energy in chemical/nuclear reactions. As written in the answers there, the amount of energy that is lost or gained in a chemical reaction is roughly 10 (or more) orders of magnitude smaller than the mass of the participating molecules.
I took a look around the web (i.e. top few results of a Google search) and some of the results I found suggest that this difference is too small to be measured by modern devices. But according to this article (paywalled, unfortunately), there are devices that can measure mass to a precision of about $10^{-10}$ under certain circumstances, generally for macroscopic masses. So it appears that we're pretty close, and it wouldn't surprise me if a chemical mass defect becomes measurable sometime in the next decade or two, if it isn't already. Whenever that happens, I'm sure someone will do the experiment, but I doubt that it will be particularly big news unless the mass defect repeatedly fails to be detected.
I haven't done the literature research, instead, I want to propose a path that could lead to accomplishing what you're talking about.
First, I want to draw your attention to K-shell emission of X-rays. This is a process where a hole in the innermost electron shell of a heavy atom is created by some external influence. This could be photoelectric absorption or another electron, as is the case in certain x-ray machines. With present technology, a hole is created and then some ordinary outer shell electron within the material moves in to replace it, emitting an x-ray in the process.
Why would this be of use? K-shell emissions show up on nuclear spectroscopy graphs, albeit comparatively low energies. This question is about chemical processes creating a measurable mass deficit. In the sense that movement of electrons are involved and no nuclear processes are involved, this can be called a "chemical" process. Additionally, a relatively large amount of energy is involved, making it a promising candidate for detection of the mass deficit by direct mass measurement.
I will defer to Wikipedia for a specific example and numbers:
The two X-ray contrast media iodine and barium have ideal K shell binding energies for absorption of X-rays, $33.2\,\rm keV$ and $37.4\,\rm keV$, respectively, which is close to the mean energy of most diagnostic X-ray beams.
I will use iodine and write the following. Note iodine-127 has a mass of $126.904473(4)\,\rm amu$.
$$ \begin{align} M(\mathrm{^{127}I}) &= 118\,210.766\, \frac{\mathrm{MeV}}{c^2} \\ M(e) &= 0.511\, \frac{\mathrm{MeV}}{c^2} \\ Q &= 0.033\,\rm MeV \end{align} $$
Do you see what I did there? I selectively picked a chemical transition that has an energy just barely within the accuracy of the best mass measurement of the atom. This is why the problem is difficult. Most chemical transition are in the $\rm eV$ range. That is, the $0.000\,001\,\rm MeV$ range. And when you measure them on any inertial or weight scale, you are weighing them against the $100\,000$s of $\mathrm{MeV}/c^2$ mass scale.
Here is my proposal.
Create two species of $+1$ ionized $\mathrm{^{127}I}$, one where it is stripped of an electron in the outermost electron shell, and one where it is stripped of an electron in the innermost electron shell.
Have both of these very quickly enter a magnetic field at a high speed. Should you measure the predicted path curvature difference between the two, you have measured a mass difference from a chemical process.
Consider the reason for a second. Ordinary $\mathrm{I^+}$, the iodine-127 that is missing an electron in the outermost shell, has a very small binding energy for accepting an electron. For all practical purposes, the mass of the ion plus the mass of a free electron is almost exactly equal to the mass of a neutral Iodine atom. That is not the case by exactly $33\, \mathrm{keV}/c^2$ in the case of an electron displaced from the K-shell. The chemical transition you will be measuring the mass difference of is the transition of an outer shell electron to the inner shell.
Chemical reactions satisfy mass conservation and energy conservation independently. Mass conservation implies the mass of the system remaining unchanged (zero change) as the reaction proceeds. Small changes - however small - are deemed to violate the law of conservation of mass. This has nothing to do with our ability to measure very small values of mass change.
Nucleii of atoms do not take part in chemical recations.
In mass-energy conversion processes, nucleii do take part in the process and as a result undergo change. Such processes do not fall in the category of chemical reactions.
