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Which type of salt conducts the most electricity?
Sean Williams
Reginald McClover
Woodroe Mitchell
Team Red
Period 1
Mr. Clark
Woodroe Mitchell, Reginald McClover, Sean Williams
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Statement of Purpose
The researchers conducted this project to find which type of salt produces more
electricity.
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Research
The sodium and chloride ions actually separate in water, turning solid NaCl into
Na+ and Cl- ions that can move freely through the solution. Electrons are one form
of charge carriers and the most common, being that they have a net negative
charge and are mobile inside of metals but free ions moving around in a
solution
also constitutes a current.
EDIT in response to comment: When you put two metal poles into a solution (a
negative anode and a positive cathode) and turn on a battery, you are making a
voltage difference between the two rods. As you may know from circuits, voltage
differences are what drive currents but how this is done are what separates
electrons
conducting current through a wire and the ionic salt water solution. I will
separate
this process into numbered steps, since I got very tangled up when I was
trying to
think of all the mechanisms at once.
Step 1. The battery is turned on and creates a voltage difference across the
electrodes. Nothing is conducting at this point and no current is flowing.
This is a tricky part that I would appreciate an answer to if someone more
knowledgeable is reading this but I believe this is correct.
Electrons will accumulate on the anode, giving that rod a negative net charge and
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the other rod a positive net charge.
Step 2. This voltage difference (and possible excess charge accumulation) sets up
an electric field in the solution. This attracts the positive Na+ ions to the negative anode,
as the positive cathode attracts the negative Cl- ions. Below is a picture I found in a PDF
titled 'Electrical Conduction in Solutions' that illustrates this nicely.
A picture taken from the PDF 'Electrical Conduction in Solutions', URL:
eee.uci.edu/programs/gchem/C05MANElectricalConduction.pdf
This is where the picture gets complicated. Simple attraction of ions is not enough
to sustain a current through a solution; if nothing else were occurring in the solution
besides the Na+ going to the anode and Cl- to the cathode, then once all the ions reach
their respective electrodes nothing else in the solution would move.
So there is something fishy going on in the 'simple' salt water battery. Naive
chemistry tells us this is the reaction that occurs when salt is dissolved in water:
NaCl(s) + H20(l) → Na+(aq) + Cl-(aq) + H20(l)
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A hint on what is happening comes from the unexpected sector of chlorine
production. Most of the chlorine used in the world is made using the following industrial
process of purifying salt water:
2 NaCl(s) + 2 H2O(l) → Cl2(g) + H2(g) + 2 NaOH(aq)
You may have noticed this yourself when one actually sets up a salt water battery:
gas bubbles accumulate on both electrodes and nothing precipitates out.
Step 3. You may know that electrolysis is the process by which a current is used
to drive an otherwise non-spontaneous chemical reaction. The non-spontaneous reactions
that the battery drives are so-called 'redox' reactions, where chemical species lose or gain
electrons.
In this case, the negative accumulation of electrons at the anode provides the
excess electrons needed to decomposes H20 into OH- and H+:
Anode (reduction): 2 H2O(l) + 2e- → H2(g) + 2 OH-(aq)
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These hydroxide ions are continuously made near the anode and the hydrogen gas
bubbles out, so two of three species from the brine equation are accounted for.
Step 4a. Around the anode, we now have a concentration of negative ions (OH-)
by a negative electrode. These negative ions are subject to the electric field in the solution
and are repelled away from the anode and attracted to the cathode, causing OH- to
migrate the cathode.
In pure water, this would be the complete picture. Water would pick up an
electron at the anode, decompose into hydroxide, which would migrate to the cathode,
pick up an electron and turn back into H20 and electrons would be ferried across the
solution. However, the reason that pure water alone is a poor conductor is that the
diffusion of OH- across the electrode gap is very slow and makes for weak conduction.
This is why we need to add a source of ions, such as NaCl, to get good conduction.
Step 4b. When NaCl is added to water, it is the Cl- ions are the ones that actually
reach the cathode and react to deposit their electrons:
Cathode (oxidation): 2 Cl-(aq) → Cl2(g) + 2e-
This accounts for the final species we were missing and also completed the cycle.
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H20 picks up electrons at the anode and the OH- atoms carry it to the cathode. At the
same time, the Cl- ions that dissociated in the water move towards the cathode and
deposit electrons to become a gas. Thus the net movement of electrons from anode to
cathode is complete and a current can flow.
I hope this is reasonably clean and clear after my edit. Also, I am a physics student, not a
chemistry student so I welcome anyone pointing out errors or missing subtleties in my
explanation.
o Connect the electrodes to the + and - terminals of a 9-volt battery. Place the
other ends of the electrodes in the 10% salt solution. See diagram below. Gas bubbles
will appear on the immersed electrodes.
o What to expect: • As the electricity from the battery passes through and
between the electrodes, the water splits into hydrogen and chlorine gas, which collect as
very tiny bubbles around the electrode tips. • Hydrogen collects around the cathode and
chlorine gas collects around the anode. • How can you get chlorine from H2O?
Sometimes in experiments, a secondary reaction takes place. This is what happens in this
experiment. • Oxygen is not given off in this experiment. That's because the oxygen
atoms from the water combine in the liquid with the salt to form hydroxyl ions. Salt's
chemical formula is NaCl - sodium chloride. The chorine gas is from the chloride in the
salt. The oxygen in the hydroxyl ions stay in the solution. So what is released in this
reaction is not oxygen but is chlorine gas that collects around the electrode tip. • In real
electrolysis systems, a different solution is used and higher levels of electricity help to
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split the water molecules into hydrogen and oxygen without this secondary reaction.
o OPTIONAL: If you have an ammeter that can be set to the microamp scale, you
can begin with pure distilled water and gradually add salt to the liquid. As you add more
salt to the solution, movement of the needle will indicate increased current flow. The
conductivity of a solution is proportional to the concentration of ions in the solution.
o Ask the students: "Which ions will move towards the cathode?" (Cations,
positively charged ions such as Na+ and H+, will move towards the negatively charged
cathode.) "Which ions will move towards the anode?" (Anions, negatively charged ions
such as Cl- and OH-, will move towards the positively charged anode.) Have the students
draw a diagram of the experiment set-up (i.e., similar to figure in the "Background"
section). On their diagram, indicate which ions are located near the cathode and the
anode.
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Hypothesis
The researchers believe that if the iodized salt is added to the water then it will help
produce the most electricity.
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Materials
Iodized Salt
Volt Meter
Epsom Salt
Table Salt
Beaker
9-volt Battery
Graduated Cylinder
Phone for pictures
3-ft Wire 3 Pieces
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Procedures:

1. Measure 100 ml of water pour into beaker.

2. Repeat with 2 more beakers.

3. Place 2 Tablespoons of (the type of salt) into each beaker.

4. Place wires connected to 9-volt battery into water with salt.

5. Use volt meter to see how much voltage was found.

6. Subtract 9 volts from the amount of voltage found to get correct voltage.

7. Repeat Steps 3-6 With each type of Salt.
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