@Binyamin Tsadik,
The problem with trying to scientifically apply chemistry to consciousness is that you are trying to use some of the concepts of science to explain a realm where science is still unproven...if you want this to be of use, then go get your degree in biochemistry and start running tests.
Yes, you can mix a ratio of chemicals in a petri dish and be quite certain that they will always react in the same manner. But the human brain is still far from being understood by science. We can say that certain chemicals and hormones will lead to more of a chance that someone will report being "happy". But we can not define what this emotion means. We can not determine that mixing a certain ratio of chemicals in the brain, like in the petri dish, will give us any consistent result in the line of the petri dish result. You can simply observe how different people react to different drugs--almost any drug you ingest has certain psychological side effects that can occur. Most people don't experience them, but some do, and some in different ways. Yet the same chemical and amount of chemical has crossed the blood-brain barrier. Look at how people react differently to psychedelic drugs.
If you want some scientific "proof" of free will, I would mention physics prof. Richard Muller's thoughts on the matter. The weak nuclear force results in the regularly predictable decay of radioactive particles, whereby they release energy in the form of radiation, and return to the "stable" state. Science can tell you the rate of decay, but there is no way to tell which particle is going to decay at the time. So, you can have a lump of radioactive uranium, and you know the half-life, but one atom within that mass might cease to be radioactive in the next minute, while another might take hundreds of years. Some of the best physicists say that not only do we not know a mechanism to determine which particle will decay when, but that it is probably impossible to know. Do radioactive atoms have free will?
I believe in an aspect of free will, in that humans can reason to base their decisions on what is logical. They don't always do so, but they can, and this makes us different from animals. Of course you can argue that what is "logical" is really just a pathway of reasoning that allows us to justify an already determined emotional or chemical response. I don't believe this is the case, but I can't prove this, and you can't disprove it.
I would think of humans having free will in the sense of a mouse walking through a gigantic maze. You can put the cheese down to influence the mouse's actions, and the maze in itself will confine his own will to being only so powerful over his existence. But there is still free will that exists there, and many paths that can be taken due to free decision making, even if all of the available paths have been predetermined.