Photosynthesis is certainly critical, because neither plants nor animals could exist on Earth without it (ultimately, all animal derive their food from plants), but photosynthesis is a network of numerous reactions - which ONE is most important? Same applies to all of the metabolic processes that allow human beings and other animals to function -- ATP or Acetyl coA perhaps?
I'll go with the largest and most important SYNTHETIC chemical reaction, which I believe is unquestionably the Haber-Bosch reaction for making ammonia, a critical synthetic fertilizer without which the Earth could not support more than a modest fraction of the people living on it today:
N2 + 3H2 --> 2NH3
Based on their positions in the Periodic Table, the chemical reaction between single atoms that releases the most energy should be that between francium and fluorine.
One possible answer is - photosynthesis.
The type reaction that changes the reactants into products is called a chemical reaction. Most chemical reactions can run forward or backward.
Reaction equilibrium
This is a chemical reaction.
It depends: just try to relate, try to write a chemical equation for and see if there are byproducts. Most likely, if the same chemical still has its same properties After the reaction, a chemical reaction has not occurred.
combustion
The type reaction that changes the reactants into products is called a chemical reaction. Most chemical reactions can run forward or backward.
do not break bonds
How big is the biggest chemical reaction depends on what you mean by "big." Is it a reaction with the most spectacular result, the most ingredients, the most steps, or the most of something else? One suggestion was making Luminol, which is an 8 step reaction, and each step is a chemical reaction its self. Look at NurdRage video on how to make it on YouTube
Reaction equilibrium
This is a chemical reaction.
what would most likely result in the greatest decrease in the rate of a chemical reaction
It depends: just try to relate, try to write a chemical equation for and see if there are byproducts. Most likely, if the same chemical still has its same properties After the reaction, a chemical reaction has not occurred.
Photosynthesis
Most definitely.
combustion
For most chemical reactions, energy is required to supply an "activation energy" required before reaction.
The strongest chemical in a science lab is Hydrogen Fluoride. The second strongest chemical is Hydrogen Chloride. Hydrogen Fluoride isn't the world's strongest chemical. The King of Liquids is the mixture of 1 Hydrogen Fluoride and 3 Hydrogen Chloride.