Transcript Electrochemical Cells

Electrochemical
Cells
What is a Cell?
What is Electricity?
Oxidation and Reduction again
Drawing a Cell
Cell Shorthand
Author: J R Reid
What is a Cell?
In Physics at year 9-11 we
learnt that what we
normally call a battery is in
fact called a cell (by
scientists anyway).
A cell is a collection of
chemicals that can generate
electricity as long as we
attach wires and a device to
the positive and negative
terminals of the cell.
If we put a number of cells
together in series we
increase the total voltage
and we call this a battery
What is Electricity?
Electricity is the movement (or potential
movement) of electrons through a wire. We use this
movement because it can easily be turned into
other forms of energy such as light, heat, kinetic,
sound…
Electricity involves two main components – the
push given to the electrons to make them move
(voltage) and the actual resulting flow of electrons
(current)
Voltage (otherwise known as electric potential) – this is the
energy pushing the electrons. It will only turn into electron
movement if a circuit is created and the electrons are
allowed to flow. Voltage is measured in volts.
Current – this a measurement of the flow of electrons. It
counts the amount of electrons passing a certain spot per
second. It is measured in amps (or amperes).
Oxidation and Reduction
Again
We know that:
Oxidation is the loss of electrons
Reduction is the gain of electrons
Redox reactions involve the loss and gain of electrons (they swap
them – the oxidant is the taker and the reductant is the giver)
An oxidation reaction results in the electrons being pushed
away and the reduction reactions result in electrons being
attracted
We can use redox reactions to help generate a voltage (a push
of electrons) by separating the oxidation and reduction
reactions into different places, and then joining the two with
a wire
Oxidation
Reduction
(Electron
push)
(Electron attraction)
Drawing a Cell
Cells need a number of
components in order to work:
An oxidation reaction (always
drawn on the left - LEO)
A reduction reaction (drawn
on the right)
A two metal electrodes
A voltmeter connecting the
two reactions
A salt bridge connecting the
two solutions (making a
circuit)
Sometimes we don’t have a
metal as part of the oxidation
or reduction reaction. In this
case we need to add a
platinum electrode to
conduct the electricity
What is Happening?
In the diagram to the
right:
What is happening in the
reaction on the left hand
side?
What is happening in the
reaction on the right hand
side?
Which direction do the
electrons travel in? How do
you know?
What observations would
you see?
Shorthand
Instead of drawing a huge diagram each time, we
can use an abbreviated version to describe what is
happening in our electrochemical cell e.g.
Zn(s)/Zn2+(aq)//Cu2+(aq)/Cu(s)
Lets break it down into pieces:
Left side = Oxidation
Zn metal is turning into Zn2+ solution (aqueous)
A change of state is shown by a ‘/’
Middle (//) = separated by the salt bridge
Right side = Reduction
Cu2+ solution is turning into Cu metal
More Shorthand…
Sometimes we have situations where we don’t have any metal
electrodes to grab the free electrons from:
An example of this is…
I-(aq) → I2(s).
In this example neither the I- or the I2 can act as a conductor. A
platinum or graphite (carbon) electrode is normally used
If the pair of chemicals are different states i.e. a solid and a
gas or a gas and aqueous we separate the chemicals with the
‘/’ sign e.g.
Pt(s)/Cl-(aq)/Cl2(g)//Cu2+(aq)/Cu(s)
If the pair of chemicals are in the same liquid we separate the
chemicals using a comma e.g.
Zn(s)/Zn2+(aq)//Cu2+(aq),Cu+(aq)/Pt(s)
Exercises
Exam Practice - 2008
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search for 90696
Exam Practice - 2007
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Exam Practice - 2006
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Exam Practice - 2005
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search for 90696
Exam Practice - 2004
Have a go at questions:
•
Can’t see the exam paper below?
Go to the NCEA website and
search for 90696