Electrode potentials

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What happens when a rod of a metal is dipped into a solution of its own ions?
An equilibrium is set up between the solid metal and the aqueous metal ions
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Write a half-equation for zinc (s) to zinc (II)
Zn (s) ⇌ Zn2+ (aq) + 2e-
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Write a half equation for copper (II) to copper (III)
Cu2+ (aq) ⇌ Cu3+ (aq) + e-
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What is the simplest salt bridge made of?
Filter paper soaked in saturated solution of KNO3 (potassium nitrate)
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Why are salt bridges necessary?
Complete the circuit, but avoid further metal potentials. Allows ion movement to balance the charge. Do not react with electrodes
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What symbol is used to represent a salt bridge in standard notation?
I I
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What type of species goes on the outside of in standard notation?
The most reduced species
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What does I indicate?
Phase boundary
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How would an Aluminium/Copper cell be represented?
Al(s) I Al3+ (aq) I I Cu2+ (aq) I Cu (s)
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What happens at the left-hand electrode?
Oxidation occurs. Half cell with the most negative E° value
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What happens at the right hand electrode?
Reduction occurs. Half cell with the most positive E° value
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What conditions is the standard hydrogen electrode used in?
Temp - 298K Pressure - 100kPa [H+] - 1.00 mol dm-3
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What is the standard hydrogen electrode used for?
Comparing other cells against. E° of SHE is defined as 0 so all other E° values are compared against it
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Why might you use other standard electrodes occasionally?
Cheaper/easier/quicker to use and can provide just as good as reference. Platinum is expensive
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If an E° is more negative what does it mean in terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
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If an E° is more positive what does it mean in terms of oxidising/reducing power?
Better oxidising agent (easier to reduce)
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What factors will change E° values?
Concentration of ions & Temperature
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What happens if you reduce the concentration of the ions in the left hand half cell?
Equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the E° of the left hand cell becomes more negative, so the e.m.f of cell increases
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How do you calculate the emf of a cell from E° values?
E°cell = E°right - E°left
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When would you use a Platinum electrode?
When both the oxidised and reduced forms of the metal are in aqueous solution
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Why is Platinum chosen?
Inert so doesn't take part in the electrochemistry and good conductor to complete circuit
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How would you predict if a reaction would occur?
Take the two half equations, find species that is being reduced (right hand side) Calculate its E° minus the E° value that is being oxidised (left hand side) if E° overall > 0 reaction will occur
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What are Zinc/Carbon cells more commonly known as?
Disposable reactions
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What are the two reactions that take place in the Zinc/Carbon cells?
Zn oxidised to Zn2+ Nh4+ reduced to NH3 at carbon elctrode
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What are the reactions that occur in a lead/acid battery (car batteries)?
Pb + SO42- ---> PbSO4 (s) + 2e- PbO2 + 4H+ + SO42- +2e- ---> PbSO4 +2H20
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How are cells recharged?
Reactions are reversible and are reversed by running a higher voltage through the cell than the cell's E°
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What is a fuel cell?
A cell that is used to generate electric current; does not require electrical recharging
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Why is it better to use a fuel cell than to burn H2 in air, even though the same overall reaction occurs
In combustion, sulfur containing compounds (SO2, SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in air. These are bad for the environment Doesn't occur in fuel cell only product is H20 effi
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Disadvantages of fuel cells
H2 is a flammable gas with a low b.p hard and dangerous to store and transport expensive to buy. Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
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How do you find the weakest reducing agent from a table of electrode potential data
Most positive E° value. Then it is the product of the reduction equation
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What is the reason that some cells cannot be recharged?
Reaction of the cell is not reversible a product is produced that either dissipates or cannot be converted into the reactants
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Why might the emf of a cell change after a period of time?
Concentrations of ions change- the reagents are used up
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Other cards in this set

Card 2

Front

Write a half-equation for zinc (s) to zinc (II)

Back

Zn (s) ⇌ Zn2+ (aq) + 2e-

Card 3

Front

Write a half equation for copper (II) to copper (III)

Back

Preview of the front of card 3

Card 4

Front

What is the simplest salt bridge made of?

Back

Preview of the front of card 4

Card 5

Front

Why are salt bridges necessary?

Back

Preview of the front of card 5
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