Enzymes are noy changed in chemical reactons. If enzymes were changed
or were used up by reaction, how would this affect the rate of
catalyzed reactions

Your question is answered on  a web page on Louis J. Gross's website. 
Gross is a professor of evolutionary biology at the University of
Tenessee.

http://www.tiem.utk.edu/~gross/bioed/webmodules/enzymes.htm

I recommend you read this web page yourself, as it gives a
step-by-step explanation of enzyme catalyzed reactions.  The relevant
key facts from this page are:

- enzymes are proteins that speed up the rate of a chemical reaction
without being used up
- the rate of a chemical reaction is affected by the total number of
enzymes as well as the concentration of substrates
- in the presence of enzymes the reaction rate is higher, especially
when there are enough enzymes to handle the amount of substrates
present

This is illustrated in this graph:
http://www.tiem.utk.edu/~gross/bioed/webmodules/enzyme1.gif

If enzymes were changed (i.e., became less active or even inactive) or
used up in the reaction, the reaction rate would slow down and
approach the rate for a reaction with no enzymes present.

If this does not answer your question, please request an answer
qualification.  If it helped you, please feel free to add a rating to
the answer.

"- enzymes are proteins that speed up the rate of a chemical reaction
without being used up
- the rate of a chemical reaction is affected by the total number of
enzymes as well as the concentration of substrates
- in the presence of enzymes the reaction rate is higher, especially
when there are enough enzymes to handle the amount of substrates
present"

Hmm.. Sounds about right, BUT from what I know, enzymes can also start
a chemical reaction that wouldnt occur without the enzyme being
present..

An example:
Milk fat doesnt decompose by itself, but if you add Lipase enzymes (
from Pencilium Roquefortii for example ) the fat will start
decomposing.. In other words: The fat develops a rancid taste.. This
is sometimes utilised in the production of cheese. Particularly Feta
and Bluecheese.

More info here: 
http://www.foodsci.uoguelph.ca/dairyedu/grading.html#rancid


/naeslund

I do not agree with naeslund's comment.  Enzymes can only speed up a
reaction that is already taking place, they cannot trigger a reaction
that does not occur without the precence of the enzyme.  However, the
reaction rate of the non-catalyzed reaction may be so slow that it is
not practically detectable.