Question: Which group first actively experimented with chemicals by applying theory and not just experimental observations?
About the article "Chemical reaction", section "History".

Candidate passages:
The discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and Heinz London in 1935.
This theory explained resistanceless transport and the Meissner effect, and allowed the first theoretical predictions for superconductivity to be made.
However, this theory only explained experimental observations—it did not allow the microscopic origins of the superconducting properties to be identified.
This was done successfully by the BCS theory in 1957, from which the penetration depth and the Meissner effect result.
However, some physicists argue that BCS theory does notexplain the Meissner effect.  
The Big Bang model, or theory, is now the prevailing cosmological theory of the early development of the universe and was first proposed by Belgian priest Georges Lemaitre, astronomer and professor of physics at the Catholic University of Louvain, with a PhD from MIT.
Lemaître was a pioneer in applying Albert Einstein's theory of general relativity to cosmology.
Bill Bryson wrote that the idea was decades ahead of its time, and that Lemaitre was the first to bring together Einstein's theory of relativity with Edwin Hubble's cosmological observations, combining them in his own "fire-works theory".
Lemaitre theorized in the 1920s that the universe began as a geometrical point which he called a "primeval atom", which exploded out and has been moving apart ever since.
The idea became established theory only decades later with the discovery of cosmic background radiation by American scientists. 
During the course of history, one theory has succeeded another, and some have suggested further work while others have seemed content just to explain the phenomena.
The reasons why one theory has replaced another are not always obvious or simple.
The philosophy of science includes the question: "What criteria are satisfied by a 'good' theory".
This question has a long history, and many scientists as well as philosophers have considered it.
The objective is to be able to choose one theory as preferable to another without introducing cognitive bias.
Several often proposed criteria were summarized by Colyvan.
A good theory:  Is elegant (Formal elegance; no "ad hoc" modifications)  Contains few arbitrary or adjustable elements (Simplicity/Parsimony)  Agrees with and explains all existing observations (Unificatory/Explanatory power)  Makes detailed predictions about future observations that can disprove or falsify the model if they are not borne out.
Is fruitful: the emphasis by Colyvan is not only upon prediction and falsification, but also upon a theory's seminality in suggesting future work.
Stephen Hawking supports items 1-4, but does not mention fruitfulness.
On the other hand, Kuhn emphasizes the importance of seminality.
The goal here is to make the choice between theories less arbitrary.
Nonetheless, these criteria contain subjective elements, and are heuristics rather than part of scientific method.
Also, criteria such as these do not necessarily decide between alternative theories.
Quoting Bird:It also is debatable whether existing scientific theories satisfy all these criteria, and they may represent goals not yet achieved, a set of "New Year's resolutions", if you like.
For example, Item 3: explanatory power over all existing observations, is satisfied by no one theory at the moment.
The desiderata of a "good" theory have been debated for centuries, going back perhaps even earlier than Occam's razor, which often is taken as an attribute of a good theory.
Occam's razor might fall under the heading of "elegance", the first item on the list, but too zealous an application was cautioned by Einstein: "Everything should be made as simple as possible, but no simpler."
It is arguable that "parsimony" and "elegance" "typically pull in different directions."
The falsifiability item on the list is related to the criterion proposed by Popper as demarcating a scientific theory from a theory like astrology: both "explain" observations, but the scientific theory takes the risk of making predictions that decide whether it is right or wrong:Thomas Kuhn argued that changes in scientists' views of reality not only contain subjective elements, but result from group dynamics, "revolutions" in scientific practice and changes in "paradigms".
As an example, Kuhn suggested that the Sun-centric Copernican "revolution" replaced the Earth-centric views of Ptolemy not because of empirical failures, but because of a new "paradigm" that exerted control over what scientists felt to be the more fruitful way to pursue their goals (Colyvan's requirement of "fruitfulness"). 
The special theory of relativity, the successor to classical mechanics, was first proposed in 1905 by Albert Einstein.
Part of the theory is based on two observations ‒ that the Galilean transformation ("addition of velocities") is valid, and that light does not obey the Galilean transformation.
Einstein constructed a system in which both observations were either correct or approximately correct, by altering the equation for the Galilean transformation such that it became less accurate as speed approached the speed of light.
This was reasonable because the previous (confirming) tests of the Galilean transformation did not reach speeds where this effect would have been apparent.
This new theory explained more experimental observations than previous theories, and its mathematics also made further testable predictions that could be confirmed.
The theory of relativity also explains why classical (Newtonian) mechanics makes accurate predictions: Newton's laws are a very good approximation in almost all circumstances.
If relativity could not explain why Newton's laws were successful, then its explanatory power would have been less than the previous theory, and it would not have been an eligible successor theory.
Einstein subsequently replaced special relativity with general relativity, and the theory has been advanced further since then, through the efforts of scientists such as Stephen Hawking and Roger Penrose.
However, classical mechanics still remains useful today (e.g.
for calculating spacecraft trajectories), because its equations are much simpler and its predictive power is sufficiently accurate in most circumstances; the difference is that the accuracy of relativity is even greater.
If a future theory with even greater accuracy were to succeed relativity, a similar situation might occur. 
The rate of living theory held broad scientific acceptance for the better portion of the twentieth century.
It is generally believed that its widespread acceptance was due to the intuitive nature of the observations.
Created shortly after the industrial revolution, it could be readily observed that machines that operated more frequently also broke down quicker.
Its exit from the scientific main stage came after a number of different studies that contradicted the metabolic/lifespan inverse connection.
The primary observation that served to disprove the rate of living theory was that birds did not follow this connection.
Work examining the lifetime energy expenditure per gram of tissue, which would be predicted to be constant by Rubner’s theory, was shown to vary twenty-fold between birds and mammals.
Many other experimental manipulations in model systems were shown to be able to increase metabolic expenditure without decrease in lifespan.
One such experiment had rats stand in a cold water bath for four hours a day, for five days a week.
In order to keep sufficiently warm, these rats had to increase metabolic expenditure, as was observed by a 44% increase in food intake over control.
Contrary to what would be predicted by the rate of living theory, the experimental group showed no change in lifespan versus control.
Numerous findings have since shown that the rate of living theory’s prediction is incorrect.
Concurrent with these findings have been other theories of aging that more adequately explain the observed phenomenon.
The disposable soma theory of aging has gained much prominence for being able to explain these observations accurately, and is now considered a more proper theory in explaining this relationship.