Why does a chemical reaction takes place at all?

As a "Beginner" in the study of Chemistry one always wonders why a chemical reaction takes place at all, why A gives B and not the reverse.
Let us consider the flow of water. It always flows from a higher level down to the lower level Why?. Because water at a lower level is associated with less potential energy and therefore greater stability.

A High-energy state is not a stable state. Processes move towards stability, therefore towards lower energy state.

Similarly a chemical reaction will proceed in the direction of stability. If the products are associated with less energy, then the conversion of the reactants to products is a spontaneous process.

The energy content of a compound is expressed in terms of the Gibbs free energy, which is defined as the energy available to do useful work. The total energy of a chemical compound cannot be estimated but the energy available to do work in a chemical change ( ΔG ) can be easily measured. In a chemical reaction if the products are of a lower free energy than the reactants that is if the change is negative that reaction is thermodynamically a feasible process. (Energy of the products – energy of the reactants).

A criterion for the spontaneity of a reaction is that ΔG should be negative.

The free energy change is usually expressed under standard conditions as ΔG⁰
For instance the actual energy content of a piece of paper cannot be estimated, but when the paper burns (a chemical change), the amount of energy released in conversion to ash etc, can be easily measured.
The free energy change of a chemical reaction is a thermodynamic property.

Let us go back to the example of flow of water.
Suppose there is a tank on the top of a hill or the top of your home. That water is definitely at higher level yet it is remains on the top. Even though the flow of water from a higher level is spontaneous yet it is not happening because some thing is preventing it. There is a height barrier coming in the way. How can we make that water come down, well one way is to punch a hole in the wall of the tank or break the wall, by doing this the height barrier is being lowered.
Another way is get into the tank use a bucket and scoop the water and pour over the wall or use a pump the water will get down.
By doing this we are overcoming the height barrier.

A chemical reaction is similar to this

• Conversion of paper to ash and other products (burning) is thermodynamically a spontaneous process.
• Yet ordinarily paper does not burn (fortunately!!). This is because something is preventing it.
• In a chemical reaction of this type there is an energy barrier preventing it. How can this be overcome?
• Well like in the example of flow of water there are two ways one is to lower the energy barrier, another to overcome the energy barrier.
• A catalyst lowers the energy barrier and makes the reaction go faster.
• Supplying energy to the system by heating, increasing pressure among others is a second way of overcoming the energy barrier.
• The free energy change is a thermodynamic property and the energy barrier is a kinetic property.
• The free energy change between the reactants and products is a constant, no matter how the reaction is carried out.
• The energy barrier can be different for the same reaction, for instance in the presence of a catalyst it is lowered, hence it is dependent on the reaction conditions.
• A sontaneous reaction has negative free energy change. Positive free energy change the reaction is non spontaneous.
• Large activation barrier it is a slow reaction. Low energy barrier fast reaction.
• Molecular reactions are slow. Ionic reactions are fast.