May
24,
2009
Los
Angeles,
CA
IYPT
2008
#5:
Voltaic
Cell:
Make
a
voltaic
cell
using
paper
tissues
as
a
salt
bridge.
Study
and
explain
how
the
electromotive
force
of
the
cell
depends
on
time.
Chemistry
teachers
love
this
problem
and
they
do
this
experiment
in
labs
when
they
teach
Redox
and
Electrochemistry.
They
usually
show
cell’s
work
but
do
not
study
dependence
on
time.
This
problem
is
about
the
time
behavior
of
the
cell
References:
1.
Any
textbook
for
Honors
Chemistry
or
the
College
Board
standardized
tests
(SAT
and
AP)
prep
books
explain
how
voltaic
cells
work.
SAT
books
tend
to
explain
the
phenomenon.
AP
books
give
more
tools
for
calculating
cell
voltage
depending
on
chemical
composition
of
the
various
components.
These
tools
may
include
temperature
but
will
certainly
involve
concentration.
Very
few
of
the
texts
at
this
level
(if
any)
will
discuss
time
dependencies.
2.
In
order
to
research
time
dependencies,
you
have
to
search
for
ionic
conductivity
and
diffusion
in
books
on
electrochemistry.
One
we've
found
that
looks
promising
is
Electrochemistry
By
Carl
H.
Hamann,
Andrew
Hamnett,
and
Wolf
Vielstich
(Wiley,
New
York)
2nd
Edition
2007.
There
are
many
more.
Guide:
1.
Chemistry
of
Voltaic
Cell
for
Physics
teachers
who
do
not
teach
Chemistry.
a.
What
is
redox?
Redox
is
a
concatenation
of
the
words
"reduction"
and
"oxidation"
reaction.
If
you
drop
zinc
metal
into
a
glass
with
an
aqueous
solution
of
Cu2+
ion,
the
redox
reaction
will
start
immediately.
Cu2+
ions
will
gain
electrons
from
Zn
atoms.
Cu2+
ion
reduce
(means
become
neutral
Cu
atoms)
and
Zn
atoms
become
Zn2+
ions.
You
will
see
Cu
layer
on
the
surface
of
zinc.
This
is
called
electroplating.
b.
How
can
redox
initiate
electric
current?
In
our
example,
redox
moves
electrons
from
Zn
atom
to
Cu2+
ion.
We
can
take
motion
of
electrons
from
atoms
to
ions
outside
of
the
glass
and
make
them
move
in
a
wire
from
a
glass
where
Zn
atoms
will
loose
electrons
(oxidation)
into
another
glass
where
Cu2+
ions
will
gain
them
(reduction).
Picture
1
shows
this
setup
and
it
almost
represents
a
Voltaic
cell.
There
is
one
problem:
electrons
move
from
the
glass
with
zinc
to
the
glass
with
copper
and
this
will
build
up
negative
charge
on
Cu‐electrode
will
repel
Cu2+
ions
and
positive
charge
on
Zn‐electrode.
c.
What
can
prevent
electric
charges
build­ups?
The
answer
is
a
“salt
bridge”.
A
salt
bridge
is
a
glass
tube
or
diffusive
paper
(such
as
filter
paper
or
tissue
paper).
The
bridge
contains
an
aqueous
solution
of
an
electrolite
whose
components
do
not
participate
in
the
redox
reaction.
KNO3
is
a
good
example
of
an
electrolite
that
can
be
used
in
the
bridge.
Positive
ions
K+
will
move
into
Cu
glass
in
order
to
neutralize
positive
bult‐up
and
negative
ionic
compaund
of
(NO3)‐
will
move
into
the
Zn
glass
and
will
neutralize
negative
built‐up.
So,
the
redox
reaction
can
continue
now.
Figure
2
represents
complete
voltaic
cell
with
the
bridge.
2.
The
IYPT/USIYPT
Problem
The
problem
is
about
the
time
dependence
of
the
EMF
of
your
cell
and
they
will
depend
on
various
parameters
which
all
should
be
studied.
Here's
a
partical
list
a.
Chemical
solutions.
These
include
the
chemical/material
that
you
select
for
the
metal
electrodes,
water
solutions,
and
bridge
solutions.
Temperature
and
concentration
of
water
solutions
are
closely
associated
parameters.
b.
Paper
tissue.
The
problem
states
that
you
must
use
paper
tissues
to
build
your
bridge,
but
nothing
is
said
beyond
that.
In
order
to
understand
how
different
kinds
of
tissues
affect
time
dependence
of
your
setup,
study
their
diffusion
and
absorbtion
properties.
They
both
affect
ionic
conductivity
of
the
bridge.
c.
Will
the
cell
“die”
soon?
You
might
be
surprised
that
your
cell
will
not
attain
equilibrium
(when
your
cell
will
be
dead)
in
an
hour.
It
lasts
long
time.
Good
luck!
T.Bibilashvili