Redox and standard electrode potential

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Oxidising agent...
...is a substance that takes electrons from another substance and so it's reduced.
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Reducing agent...
...is a substance that gives electrons to another substance and so it is oxidised.
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Half equation of Cu2+ being reduced by Zinc metal, and what happens to the metals colour?
Cu2+(aq) + Zn(s) --> Zn2+(aq) + Cu(s) As Cu(s) is formed the zinc metal within the solution begins to change brown from is silvery surface to begin with.
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Salt bridge purpose?
A salt bridge is a piece of apparatus that connects the solutions in two half-cells so that the circuit can be complete and the current can flow without the solutions mixing.
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Example of metal/metal ions half cell
Zn(s) electrode with [Zn2+]=1moldm-3 within the solution another is Cu(s) electrode with Cu2+ions within the solution with the same concentration. Here you would see a colour change, as the blue solution loses it's colour as copper ions are reduced.
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Representing half-cells using cell a diagram between Copper and Zinc
Zn(s) I Zn2+(aq) II Cu2+(aq) I Cu(s)
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Standard electrode potential (E)
Measuring the ability of a half-cell to gain or lose it's electrons
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What does EMF show us?
It shows us the ease of oxidising or reducing the substance in each half-cell.
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Standard Hydrogen electrode...
...consists of a platinum electrode coated with fine platinum grains, called platinum black. This is dipped into 1 moldm-3 solution of H+(aq), used as a hydrochloric acid, and hydrogen gas is slowly passed over the electrode at a press 1atm, tem 298K
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Using standard electrode potential between zinc and hydrogen
Zinc E= -0.76V, so more negative than hydrogen. This means the electrons flow along the wire from the zinc electrode to the hydrogen electrode. This makes the hydrogen half-cell the positive electrode. (i.e Zn loses electrons more readily than H2)
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When do you know which electrode is classed as the positive electrode?
Electrons move along the wire towards the most positive electrode as they are more readily oxidised. This would make that electrode the positive electrode.
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How would the electrons move along the wire between Cu half-cell and H2 half-cell? You know Cu E = +o.34V
Electrons would move along the wire from the hydrogen towards the Cu half-cell because the H2 half-cell is a greater oxidising agent compared to Cu. Hydrogen electrode now becomes the negative electrode.
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EMF =
Ereduction - Eoxidation
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Calculating example: E = -0.76V (Zn), E= +0.34V (Cu) which is feasible
EMF = +0.34 - (-0.76) = +1.10V (feasible reactions always has to have a positive answer for EMF)
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Is 2H+(aq) + Zn(s) --> Zn2+(aq) + H2(g) a feasible reaction?
Zn E= -0.76V, H2 E= 0.00V EMF= 0.00 - (-0.76)= +0.76V, so feasible
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Hydrogen fuel cell
The fuel system passes the fuel over a platinum metal which acts as a catalyst, but also as an electrode for the electrochemical system. Electrons are removed from hydrogen atoms at one electrode, the other electrode receives the e- and O2 forms H20
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Benefits for a H2 fuel cell...
These are a convenient way of storing and releasing energy. Energy efficiency is much higher than standard fuel cells and emission from fuel cells are less damaging than the CO2 from traditional engines.
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Drawbacks for a H2 fuel cell...
Hydrogen fuel must be generated elsewhere, so likely to burn fossil fuels in the making. Gases needed are difficult to store compared to liquid fuels, and fuel cells operate at lower temperatures (80oC) so need very efficient catalyst (expensive)
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Other cards in this set

Card 2

Front

...is a substance that gives electrons to another substance and so it is oxidised.

Back

Reducing agent...

Card 3

Front

Cu2+(aq) + Zn(s) --> Zn2+(aq) + Cu(s) As Cu(s) is formed the zinc metal within the solution begins to change brown from is silvery surface to begin with.

Back

Preview of the back of card 3

Card 4

Front

A salt bridge is a piece of apparatus that connects the solutions in two half-cells so that the circuit can be complete and the current can flow without the solutions mixing.

Back

Preview of the back of card 4

Card 5

Front

Zn(s) electrode with [Zn2+]=1moldm-3 within the solution another is Cu(s) electrode with Cu2+ions within the solution with the same concentration. Here you would see a colour change, as the blue solution loses it's colour as copper ions are reduced.

Back

Preview of the back of card 5
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