Year 13- Electrode potentials Flashcards
Why is a high resistance voltmeter used?
to stop the current from flowing in the circuit
What is a salt bridge usually made from?
a piece of filter paper soaked in potassium nitrate- it unreactive with electrodes and electrode solutions
Why can’t potassium chloride be used a s a salt bridge in a copper system?
chloride ions would form complexes with copper ions
Why isn’t a metal wire used as a salt bridge?
it would create its own electrode systems with the solutions
What type of reaction does the most positive electrode undergo?
reduction
Cu2+ + 2e- –> Cu (+ve as electrons are used up)
What type of reaction does the most negative electrode undergo?
oxidation
Zn –> Zn2+ + 2e- (-ve as electrons are given off)
In a cell diagram, where would you put the most positive half cell?
on the right if possible
In a cell diagram, which form is put next to the double line?
the most oxidised form
Why is a platinum electrode used in systems with no metals?
it provides a conducting surface for electron transfer, is unreactive and can conduct electricity
How are the electrode potentials of all electrodes measured?
by comparing them to the standard hydrogen electrode which has a potential of 0V
Cell diagram of hydrogen electrode
Pt l H2 (g) l H+ (aq)
Why is platinum black used for SHE?
it is porous and absorbs hydrogen gas
Why are standard conditions important?
the position of the redox equilibrium changes with conditions e.g. an increase in conc. can move it to the right or left
Standard conditions
- all ion solutions at 1 mol sm-3
* 298K, 100kPa
In half equations on a data book, what is found on the left side of the equation?
the most oxidised form
Equation for the EMF of a cell
RHS-LHS
When combining 2 half equations, what happens to the most negative half cell?
it will always oxidise (go backwards)
When combining 2 half equations, what happens to the most positive half cell?
it will always reduces (go forward)
Equation for Ecell of spontaneous changes
Ered - Eox (always will have a positive Ecell)
As we go down the Ecell series, it gets more positive. What does this tell us?
it increases the tendency for the species on the left to reduce and act as oxidising agents
As we go up the Ecell series, it gets more negative. What does this tell us?
it increases the tendency for the species on the right to oxidise and act as reducing agents
Where is the most powerful reducing agent found on the series?
at the most negative end on the right
Where is the most powerful oxidising agent found on the series?
at the most positive end on the left
Effect of conc on Ecell
- increasing the conc of reactant will increase Ecell
- decreasing them will decrease it
What are the forms of electrochemical cells as a commercial source for electrical energy?
as non-rechargeable (irreversible) , rechargeable and fuel cells
What does a fuel cell do?
it uses energy from the reaction of a fuel with oxygen to create a voltage
Positive electrode in lithium cell
Li+ + CoO2 + e- –> Li+
[CoO2
]-
E=+0.6V
Negative electrode in lithium cell
Li+ + e- –> Li E=-3.0V
Hydrogen fuel cell (potassium hydroxide electrolyte)- alkaline conditions equations
4e- + 4H2O –> 2H2 +4OH- E=-0.83V
4e- + 2H2O +O2 –> 4OH- E=+0.4V
Overall reaction 2H2 + O2 –> 2H2O E=1.23V
How does a fuel cell maintain a constant voltage?
continuously fed with O2 and H2 so maintaining a constant conc of reactants
Advantages of fuel cells over conventional petrol or diesel vehicles
- less pollution/CO2
- greater efficiency
Limitations of hydrogen fuel cells
- expensive
- storing and transport H2 is dangerous
- limited lifespan
- high production costs that uses toxic chemicals
Advantage of ethanol fuel cell over hydrogen
- made from renewable source, carbon neutral
- raw materials to produce it via fermentation are abundant
- less explosive, easier to store
