Given the reaction of gaseous H2 + liquid Br2 yields gaseous HBr. Does
the entropy increase or decrease?

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Request for Question Clarification by livioflores-ga on 25 Jul 2005 20:39 PDT

Hi!!

H2 + Br2 --> 2HBr

Not sure if it this the proper answer, but all indicates that the entropy increase:
·There is no increase or decrease in the number of moles, so no
assumptions of entropy change can be made here.
· We have one mol of liquid plus one mol of gas at the start, then we
get 2 moles of gas, this suggest an increase in the entropy (entropy
of gases is greater than entropy of liquid).
· Increase in molecular complexity generally leads to increase in S,
at start we have two simple element molecules then we get 2 more
complex molecules.
· It is an exothermic reaction, normally in this reactions the entropy increases.

For further references see the following PowerPoint presentation:
"Entropy and Free Energy":
http://www.mc.maricopa.edu/~gilletti/CHM152/Thermo/Ch19_Thermo_1%20kotz.ppt


Let me know if this is the info you are requesting and I will post it
in the answer box.

Regards,
livioflores-ga

Hi!!

Thank you for giving me the opportunity to answer your question.

H2 + Br2 --> 2HBr

All indicates that the entropy increase:
·There is no increase or decrease in the number of moles, so no
assumptions of entropy change can be made here.
· We have one mol of liquid plus one mol of gas at the start, then we
get 2 moles of gas, this suggest an increase in the entropy (entropy
of gases is greater than entropy of liquid).
· Increase in molecular complexity generally leads to increase in S,
at start we have two simple element molecules then we get 2 more
complex molecules.
· It is an exothermic reaction, normally in this reactions the entropy increases.

For further references see the following PowerPoint presentation:
"Entropy and Free Energy":
http://www.mc.maricopa.edu/~gilletti/CHM152/Thermo/Ch19_Thermo_1%20kotz.ppt


I am really glad to know that this answer helped you in the
understanding of this topic.


Regards,
livioflores-ga

very clear and precise anser!

Considering that the number of moles of gases increases (from 1 mol of
hydrogen to 2 moles of HBr), there is an increase in the disorder of
the system.  Thus, the entropy of the system increases.

I am right if I say that entropy is always non negative?

The entropy of a system may not always be positive, but the entropy of
the universe (system + surroundings) is always greater than (or equal)
to zero.

So I would look at the number of moles to see if it would increase or
decrease. But wouldnt it be decrease since you are going from H2 + Br2
--> HBr

The entropy of a gas is always larger than that of a liquid, so the
entropy change of your reaction is positive.  (IN fact, at STP, the
change in entropy is about 144 Joules/Kelvin for one mole of H2(gas)
reacting with 1 mole of Br2(liquid) to produce 2 moles of HBr(gas)).

To livioflores, yes, the entropy of a system must always be positive. 
The general definition of entropy in statistical mechanics is:

 S = -k*{SUM over i of [P_i*ln(P_i)]},

where k is Boltzmann's constant (a positive number), P_i is the
probability that system will be in a particular "microstate", and the
sum extends over all the possible microstates of the system.
Probabilities must be between zero and 1 (inclusive), so each term in
the sum must be <=0, and the sum itself must therefore be <=0.  The
minus sign in front turns the whole r.h.s. positive (or zero).

To rmpalpha, the entropy of a system must always be positive.  The
entropy CHANGE of a system can be negative, but the entropy CHANGE of
the universe must always be positive as a result of a spontaneous
process.

Yes livioflores-ga...that is very helpful! Thank you very much..its
exactly what I was trying to figure out.