There isn't really an answer to this question. Chemical reactions are not, in and of themselves, "lethal" unless they're occurring in or to your body. It's possible, with the proper precautions, to work with chemicals that are extremely toxic, corrosive, flammable or just out and out nasty.
The most toxic chemical is another thing that's really hard to quantify, since there are lots of chemicals for which extremely small amounts can be fatal, but larger amounts aren't always necessarily fatal (just because Biology can be weird like that). Some good candidates would be ricin and botulin.
Duodecaplyatomite.
Okay, that's not real; it's a literary reference (assuming you consider pulp space Opera "literary").
Pound for pound, it's hard to do much better than the combustion of oxygen and hydrogen.
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
Theobromine is present in many, if not all, forms of chocolate. It is incredibly lethal to Dogs and is one of the primary reasons in which chocolate is lethal to most, if not all, breeds of canine.
Reaction equilibrium
what would most likely result in the greatest decrease in the rate of a chemical reaction
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.
Photosynthesis
Most definitely.
combustion
For most chemical reactions, energy is required to supply an "activation energy" required before reaction.