IF WHEN IF WHEN

is there a difference b/w "if" and "when"
is there a significant difference?
or it all depends on how the sentence is being structured?

sentence 1:-
in future, if we go shopping, we will blah blah blah.....

sentence 2:-
in future, when we go shopping, we will blah blah blah......


does it mean the same thing?
or sentence 1 means "IF WE EVER GO SHOPPING"
and 2 means "WHEN WE GO SHOPPING"

is there an uncertainty, ie not sure will go or not in sentence 1.....and sentence 2 is confirmed will go

so to say if some one says "next time, if we get marry, we will have 2 children"
is as good as saying "no committment here. no strings attached"?????? coz only IF we get marry ????

or my ang mo is just plain lousy.........