Subject: can you help explain to me Chemical reactions with calories

im doing a project on chemical reactions with calories for my chemistry class 
and i need an interview for part of my report.

first thing that i need help on is why reactants (particals) must come into 
contact (collide) in order for a chemical reactant to occur.

and the second thing is how can calories be determined in the foods you eat?  
and can you explain to me which provides more energy: carbohydrate, protien, 
or fat calories?

thanyou for you help

Re: can you help explain to me Chemical reactions with calories