Entropy

One A4 sheet on entropy regarding Edexcel A2.

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Helena Bonici Chemistry 26/09/2011
13EA2Kinetics
ENTROPY
In addition to enthalpy (heat content), there is another important thermodynamic aspect of all
chemical reactions - entropy.
Entropy = A measure of the amount of randomness or disorder in a system. (S ~ j/k-mole)
All substances, be they individual atoms of a single element or a molecule of a compound, possess
some degree of disorder because particles are always in constant motion. Thus, S is always a positive
number. This is because a substance cannot be less random than not random at all.
Typically concern lies with how entropy changes during a chemical reaction, or with S rather than S:
S = Ssystem + Ssurroundings
Import aspects with entropy:
Gas particles move about much more than do particles in the liquid phase, making them more
random or disordered. If a system changes from a gas state to a liquid state, it becomes less
random, or more ordered, because liquid particles move about less randomly than do gas
particles.
A negative value of S indicates a decrease in entropy-- the system becomes less random. A
positive value of S indicates an increase in entropy-- the system becomes more random.
The following changes suggest an increase in entropy:
Chan
ges in
state:
solid liquid
liquid gas
solid gas
solid or liquid aqueous state (the dissolving process)
An
increa
se in
the
numb
er of
moles
. If
the
produ
ct
side
of the
equati
on
has
more
moles

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Helena Bonici Chemistry 26/09/2011
13EA2Kinetics
than
the
reacta
nt
side,
the
syste
m has
beco
me
more
rando
m
more
partic
les
movi
ng
about
is a
more
rando
m
state
than
fewer
partic
les
movi
ng
about.
Increa
sing
the
tempe
rature
.…read more

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Helena Bonici Chemistry 26/09/2011
13EA2Kinetics
heat
energ
y,
increa
ses
molec
ular
motio
n
which
in
turn
increa
ses
the
degre
e of
rando
mness
+H -H
+Ssys CaCO3 CaO + CH4 H2O +
CO2 CO2
-Ssys - NH3 + HCl
NH4Cl
It
won't
occur
sponta
neously.
It is the
opposit
e of the
-H
and the
+Ssys.
Thus it
doesn't
exist as
a
reactio
n as
such.…read more

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Helena Bonici Chemistry 26/09/2011
13EA2Kinetics…read more

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