3.1. Redox and Standard Electrode Potential

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Oxidation
1. The loss of electrons. 2. The removal of hydrogen. 3. The addition of oxygen.
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Reduction
1. The gain of electrons. 2. The addition of hydrogen. 3. The loss of oxygen.
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Redox reaction
Oxidation and reduction reactions.
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Oxidising agent
The species that is reduced is accepting electrons from another species, causing it to lose electrons and be oxidised. Therefore, it is a oxidising agent.
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Reducing agent
The species that is oxidised is donating electrons to another species, causing it to be reduced. It is therefore the reducing agent.
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Oxygen almost always has an oxidation state of...
-2, unless in peroxides, in which it is -1.
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Hydrogen almost always has an oxidation state of...
+1, unless in hydrides, in which it is -1.
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When forming half equations, the electrons are always added to the side...
with the highest oxidation state.
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To balance out electrons in half equations...
multiply be the difference in oxidation states (but also be aware of balancing).
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How are redox equations formed?
1. Form half equations. 2. Balance out the number of electrons, then cancel. 3. Combine.
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Acidified manganate ion?
MnO4^-, with oxidation state +7 (this is usually reduced to form Mn or Mn2+)
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Acidified dichromate ion?
Cr2O7^2-, with oxidation state +6 (this is usually reduced to form Cr^3+)
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We know from previous topics that acidified dichromate and manganate ions are used as oxidising agents. How can this be used?
It shows that they must be reduced.
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What happens if a piece of metal is left standing in a solution of water?
There will be a tendency for some of the metal atoms to dissociate, forming positive ions and leaving electrons on the metal. The ions will be attracted to the electrons left, and so some will reform metal atoms. This sets up a dynamic equilibrium.
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Dynamic equilibrium
When the rate of the forward reaction equals that of the reverse reaction, or in this case, when the rate at which ions are produced equals the rate of which atoms are formed.
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A metal dipped in a solution of its own ions is called an...
ELECTRODE.
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How would magnesium and copper differ in this situation?
Copper is less reactive than magnesium. Therefore, it forms fewer ions, increasing the chances of the ions reclaiming their electrons to form atoms. It has less charge and fewer ions in solution.
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How would the half equations for magnesium and copper be written?
ALWAYS AS REDUCTION. So, the ions would come first. The position of Mg's equillibrium would be further to the left than that of Cu: it sheds electrons and forms ions more readily.
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In electrochemical cells, what is the purpose of the wire?
It allows electrons to flow from the electrode with the greatest negative electrode potential.
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In electrochemical cells, what is the purpose of the voltmeter?
To measure the potential difference between the two half cells.
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In electrochemical cells, what is the purpose of the salt bridge?
It is made of potassium nitrate on agar/gel and completes the circuit by allowing the ions to move without the solutions mixing.
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E° = ?
Standard Electrode Potential
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If E°
The electrode has a greater negative electrode potential (more electrons on the electrode). IT IS MORE EASILY OXIDISED.
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If E°>0, it means...
The electrode has a weaker negative electrode potential (fewer electrons on electrode). IT IS MORE EASILY REDUCED.
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How do you measure the E° for a half cell?
The half cell is connected to the standard hydrogen electrode. IT MUST BE SET UP UNDER STANDARD CONDITIONS.
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Standard conditions?
1. 1 moldm-3 2. 1 atm 3. 298K (25'C).
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What happens at the standard hydrogen electrode?
As the hydrogen gas flows over the inert Pt electrode an equilibrium is set up between the hydrogen gas and the H+ ions in the solution. The Pt acts as a catalyst. If you were to draw the half equation, you would start with 2H+ and add two electrons.
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Representing half cells/ cells - simplified version.
1. The half cell with the most negative electrode potential goes on the left. 2. The most oxidised species goes in the middle. 3. It should show the flow of electrons.
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Why does the iodine half cell still need a inert Pt electrode, despite being a solid?
Iodine does not conduct electricity (no freely moving electrons).
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Equation for calculating the EMF of a cell.
E°cell = E°RHS - E°LHS or E°RED - E°OXID
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Reactions are only feasible if the EMF is...
POSITIVE.
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Why?
It shows that the molecule which is reduced is a strong enough oxidising agent to oxidise the other atoms into ions.
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The stronger oxidising agent will always be the molecule/atom/ion that has...
1. The more positive electrode potential. 2. Has been reduced.
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What is the purpose of the hydrogen fuel cell?
To develop energy efficiently from fuels such as hydrogen.
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What happens in a hydrogen fuel cell?
The fuel cell passes hydrogen over the Pt metal which acts as both the catalyst and the electrode. The hydrogen is oxidised at the anode to form H+ ions. The hydrogen ions then migrate through the semi permeable membrane, which does not allow the...
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CONTINUED...
...movement of electrons. The hydrogen ions are oxidised at the cathode and combine with oxygen to form water. This generates a flow of electrons in an external circuit, which results in a current.
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Why is an inert platinum electrode used when gases are in contact with a solution of non-metal ions?
It completes the circuit and allows electrons to flow in/out of the cell.
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Other cards in this set

Card 2

Front

1. The gain of electrons. 2. The addition of hydrogen. 3. The loss of oxygen.

Back

Reduction

Card 3

Front

Oxidation and reduction reactions.

Back

Preview of the back of card 3

Card 4

Front

The species that is reduced is accepting electrons from another species, causing it to lose electrons and be oxidised. Therefore, it is a oxidising agent.

Back

Preview of the back of card 4

Card 5

Front

The species that is oxidised is donating electrons to another species, causing it to be reduced. It is therefore the reducing agent.

Back

Preview of the back of card 5
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