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What are electrochemical cells?
· Made from two different metals dipped in salt solutions of their own ions and
connected by a wire.
· The wire is the external circuit.
· Half-cells involving solutions of two aqueous ions of the same element use a
platinum electrode.
­ Conversion of ions happens on the surface of the electrode.
· Half-cells are connected by a salt bridge made of filter paper soaked in KNO3
­ K+ and NO3- ions flow through the salt bridge and balance out the charges in the two
· Redox reaction occurs in an electrochemical cell.
­ Electrons flow through the wire from the most reactive metal to the least reactive
· Voltmeter in the external circuit shows the voltage between the two half-cells.
­ This is called the cell potential or emf, Ecell.
­ Ecell = ERHS - ELHS
­ More negative standard electrode potential put on the left, so the cell potential is
always positive.
­ Swapping the sides the half-cells are on changes the sign of the cell potential.…read more

Slide 3

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What are electrode potentials?
· Standard electrode potential, E, of a half-cell is the voltage measured under
standard conditions when the half-cell is connected to a standard hydrogen
­ Standard conditions are solutions at 1M concentration, temperature of 298K and
pressure of 101kPa.
­ Ecell = ERHS - ELHS
­ Electrode potential can be positive or negative, depending on which way the
electrons flow.
· Standard hydrogen electrode is hydrogen gas bubbled through a 1M solution
of H+ ions with a platinized platinum electrode submerged in, at 101kPa
­ Platinized platinum electrode means the surface of foil is finely powdered with
­ The platinized surface absorbs hydrogen gas, causing an equilibrium to be set up.
­ H2(g) H+(aq) + 2e-
­ Electrode potential of a standard hydrogen electrode is 0.
­ Reference cell because it is used to measure the electrode potentials of other half-
­ Standard hydrogen electrode always put on the left.…read more

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What is the electrochemical series?
· The more reactive the metal, the more likely it is to lose
electrons to form a positive ion.
­ More reactive metals have more negative standard electrode
· The more reactive a non-metal, the more likely it is to gain
electrons to form a negative ion.
­ More reactive non-metals have more positive standard electrode
· Electrochemical series shows standard electrode potentials.
­ More positive electrode potentials mean the left-hand substance is
more easily reduced and the right-hand substance is more stable.
­ More negative electrode potentials mean the right-hand substance
is more easily oxidised and the left-hand substance is more stable.…read more

Slide 5

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What is the anticlockwise rule?
· Predicts whether a reaction is likely to happen.
1. Write half-equations out with the most negative standard
electrode potential on top.
2. Draw on anticlockwise arrows to show the direction of the
3. Swap the top half-equation, and change the sign of the
electrode potential.
4. Combine the half-equations and find the cell potential.
· Electrode potential chart is another way of showing the
anticlockwise rule; upside down y-axis showing electrode
potentials with the more negative electrode potential on
top.…read more

Slide 6

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What problems are there with
predicting electrode potential?…read more

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