1 Why does Mentos make Cola “explode“? o Introduction o The Phenomenon o Different Explanations...
-
Upload
bernadette-griffith -
Category
Documents
-
view
222 -
download
2
Transcript of 1 Why does Mentos make Cola “explode“? o Introduction o The Phenomenon o Different Explanations...
1
Why does Mentos make Cola “explode“?
o Introductiono The Phenomenono Different Explanationso The Composition of Colao CO2 in Cola
o Henry‘s Lawo Two States of Equilibriumo CO2 in Solution
o The Bubbles – Young-Laplaceo The Bubbles – Nucleationo The Mentos Effecto Coca Cola vs. Diet Coke vs. Cheap
Cola
Philipp Schüdel- Why does Mentos make Cola “explode”?
3
White Mentos, thrown into a bottle of
Cola, results in a violent eruption of the
soda-drink
It is found, that this works best with
Diet Coke but the principle works with
any drink containing CO2
Why does that happen?
Why does it work best with Diet Coke?
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Phenomenon
4
A chemical reaction? According to a
certain “professor“ from Brasil, some of
the aromas in Mentos and the
Acesulfam K found in Diet Coke react
to give an explosive substance called
Ta9V4.
Catalysis of the desorption of CO2 by
heterogenous nucleation
Philipp Schüdel- Why does Mentos make Cola “explode”?
Different Explanations
5
Main ingredients of normal Coca Cola: water, sugar,
CO2 (called carbonic acid), phosphorus acid, food
dyes, other
Main ingredients of Diet Coke: water, CO2 (called
carbonic acid), phosphorus acid, citric acid, acesulfame
K, aspartame, food dyes, other
=> Most probable reason for violent eruption: CO2
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Composition of Cola
6
CO2 is dissolved in the Cola Mixture by applying a pressure of
approximately 4 atmospheres
Questions: - What does dissolved CO2 look like?
- What happens when the pressure is reset to 1
atm? (opening of the bottle)
In approximation, Cola can be described as a solution of CO2 in
water. For such a system, Henry‘s law can be applied.
Philipp Schüdel- Why does Mentos make Cola “explode”?
CO2 in Cola
7
The chemical potential of CO2 in water is
It has to be in equilibrium with its gas phase
The equation can be transformed in a way that the term on the left is a constant.
That means that the term on the right side has to be a constant, too
For an ideal solution, we obtain Henry‘s law
Philipp Schüdel- Why does Mentos make Cola “explode”?
Henry‘s Law
8
Another interesting application of Henry‘s law: The
so called “Martini Effect“:
Divers can suffer from a sort of drunkness in deep
waters. This is due to the higher solubility of N2 in
blood at higher pressure (Henry)
An empirical law connects the depth of the diver to
the amount of Martinis he would have to drink in
order to get the same effect. Every ten meters
corresponds to one Martini where the feeling of
being drunk starts at 30 meters.
Because a Martini is not a very common unit, a
recalculation has been done to form the “Beer
Effect“ according to which every 17 meters is like
drinking half a litre of beer.
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Phenomenon
9
Henry‘s law can also be written this way: c(CO2)= KH*p(CO2)
with KH= 0.034 atm*l*mol-1 at 25°C
Sealed bottle of Cola:
p(CO2)= 4 atm => c(CO2)= 0.136 mol*l-1
Opened bottle:
p(CO2)= 2.96*10-4 atm => c(CO2)= 0,010 mmol*l-1
How does Mentos help in getting from one state of equilibrium to the
other?
What does CO2 dissolved in water look like?
Philipp Schüdel- Why does Mentos make Cola “explode”?
Two States of Equilibrium
10
Contrary to common belief, CO2 in water is not
carbonic acid H2CO3 or hydrogencarbonate HCO3-.
Only about 1% of the dissolved CO2 reacts to give
these species. In Cola, this reaction is even further
reduced due to the low pH (phosphoric acid).
According to some scientists, CO2 in solution
corresponds to small bubbles of a size of around
100 nm being dispersed in water. That would mean
that it‘s not an actual solution but a metastable two
phase system.
The best explanation is given by the CO2 forming a
hydrationkomplex with 12 water molecules
Philipp Schüdel- Why does Mentos make Cola “explode”?
CO2 in solution
11
How does the system get to equilibrium?
Bubbles formed in the solution are only stable above a certain diameter due to the surface tension and the
pressure difference between the inside and the outside of a bubble.
The free energy of the surface is G= 4r2where is the surface tension
A change of the radius of the bubble results in a change of the free energy: G= 8rdr
This is compensated by the pressure-volume-work
p*dV= p*4r2dr
In equilibrium, those two terms are the same and we obtain the Young-Laplace-Equation:
p= 2/r
One can see that the difference in pressure and therefore the instability of the bubble is smaller for small
surface tensions and big bubble diameters.
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Young-Laplace
12Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
Free energy
-2
-1,5
-1
-0,5
0
0,5
1
1,5
2
0 0,2 0,4 0,6 0,8 1 1,2 1,4
radius
free
en
erg
y
volume => -r3
surface => r2
free energy
maximum
All units in this diagram are arbituary
13
In a homogeneous solution, the bubbles can only
surpass the critical diameter through pressure
fluctuations. This is called homogeneous nucleation.
Another example of homogenous nucleation are hand
warming pads, where an overcritical liquid is crystallized
by inducing nucleation through a charge emitted by a
small metal plate.
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
The process of nucleation can also be catalyzed,
which is called heterogeneous nucleation. A good
example is rice which is put into Weizen. The CO2
in the beer can form bubbles on the rice-surface
which grow and make the rice “dance“.
14Philipp Schüdel- Why does Mentos make Cola “explode”?
The Bubbles - Nucleation
The formation of raindrops is also a
nucleation process. Rain can only occur
if small particles like dust, pollen etc. are
present which is why it never rains in the
Antarctic.
Furthermore, in regions with high
particle concentration, like big cities,
there are too many nucleation sites.
Because of that, the raindrops can
never grow enough in order to be able
to fall.
15
How do Mentos affect the CO2 in solution?
Atomic Force Microscopy (AFM) pictures
have shown that Mentos have a very rough
surface.
Philipp Schüdel- Why does Mentos make Cola “explode”?
The Mentos Effect
On the cavities of this surface, CO2-
molecules can accumulate to form bubbles
which surpass the critical size.
16
Why does the Mentos-Cola experiment work best with
Diet Coke? => Higher CO2 concentration?
According to the Young-Laplace-equation ( p= 2/r ) a
higher surface tension means that for a given bubble
size, the pressure difference between the inside and the
outside of the bubble is bigger than for a lower surface
tension. That means that the bubbles in a system with
high surface tension must be bigger in oder to be
stable.
Surface tension measurements of normal Coca Cola,
Diet Coke and cheap Cola showed that there is almost
no difference.
The reason for a higher CO2 concentration could also
be a higher pressure applied in the production process,
but an actual measurement of the amount of dissolved
CO2 showed no difference between the sodas.
Philipp Schüdel- Why does Mentos make Cola “explode”?
Coca Cola vs. Diet Coke vs. Cheap Cola
17
In conclusion, the real reason
why the experiment works best
with Diet Coke has yet to be
found. The problem seems to be
a bit more complicated than
expected and there is no valid
explanation found so far.
Philipp Schüdel- Why does Mentos make Cola “explode”?
Coca Cola vs. Diet Coke vs. Cheap Cola