Electrode Potentials Flashcards
Voltaic cell def
Type of electrochemical cell which converts chemical energy to electrical energy
What are electode potentials used for
Electrode potentials compare the ease of which a metal gives up its electrons to form positive hydrated ions
Electrode potentials of Mg and Cu.
Explain what happens
Give half equations
Mg loses 2e- to become Mg2+
Mg2+ attracted to negative strip
Picks up e- again to become Mg(s)
More reactive so equilibrium lies to LHS
Mg2+ + 2e- = Mg(s)
Cu less reactive than Mg Less readily forms ions So Cu2+ more likely to pick up e- Equilibrium lies to RHS Cu2+ + 2e- = Cu(s)
Half cell def
Contains the chemical species present in a redox half equation
Metal/metal ion half cell def
Consists of a metal rod dipped in a solution of aqueous ions
What is the convention for writing half cell equations
Reduction reaction is the forward reaction
Electrons and cations are written on the left hand side
What type of reactions are half cell reaction
Reversible
So they form a DYNAMIC EQUILIBRIUM
Ion/ion half cell def
Contains ions of the same element in different oxidation states
Why is platinum used as an electrode?
It is inert (unreactive)
Means there is no transfer of electrons between the electrode, and the two metals added to solution
Electron transfer therefore only occurs between two added metals
Standard electrode potential def
E (standard state symbol)
The electron motive force of a half cell compared with a standard hydrogen half cell, under standard conditions:
Measured at 298K
With solution conc. of 1moldm-3
With a gas pressure of 100kPa
Electron motive force def
The tendency for a substance to lose or gain electrons
How to measure the electrode potential of a substance
Connect the half cell to a hydrogen half cell
Electrode potential values are relative to electrode potential of a hydrogen half cell
E^o of hydrogen half cell = 0 Volts
What is a salt bridge used for?
Give example of a salt bridge
Allows ions to flow
But contains a solution that doesnt react with the half cell solutions
E.g. filter paper soaked in KNO3
Positive and negative electrodes in a cell
In an operating cell, the electrode with the more reactive element/metal loses electrons and is (more) negative
More likely to be oxidised
The less reactive element/metal gains electrons, so is (less) negative
So the element is more likely to be reduced
Equtaion to calculate cell potentials
E^o(cell) = E^o(+ve electrode) - E^o(-ve electrode)
General mechanism behind electrode potentials
More negative the E^o value
More equilibrium lies to left
More readily electrons are lost
Therefore stronger reducing agent
Less negative E^o value
More equilibrium lies to RHS
More readily electrons are gained
Thereofre stringer oxidising agent
What is e.m.f.
The cell potential, E^o(cell)
Primary cell info
Non-rechargeable batteries
Alkaline based
- made of zinc and manganese oxide, and KOH electrolyte
Secondary cell info
Rechargeable cell/battery
Lead-acid batteries - used in cars
NiCd (Nickel Cadmium) - used in radios, torches etc.
Lithium-ion and lithium-ion polymer cells used in modern appliances
Fuel cell info
Uses energy from a reaction of a fuel with oxygen to create a voltage
Hydrogen fuel cells most common as produces no CO2 byproduct
Advantages of primary cells
Good for low voltage No need to charge Come in various sizes Cheaper (than secondary) Last longer for the charge
Disadvantages of primary cells
Can only be used once - non-rechargeable - large amount of waste
Chemicals are used up
Harder to recycle - End up in landfill - less environmentally friendly (than secondary cells)
Low current only used on low current devices
Advantages of secondary cells
Rechargeable - chemical reaction can be reversed during charging
Chemical regenerated
More cost-efficient over time
Lithium-ion polymers are lightweight
Disadvantages of secondary cells
Poor charge retention - over time the voltage reduces Higher initial costs Can take a long time to recharge Unstable at high temperatures Difficult to recycle
Advantages of fuel cells
High efficiency
Do not have to be recharged, as long as H2 and O2 supplied can operate continuously
No CO2 produced (pollutants) for hydrogen fuel cells
Removes reliance on fossil fuels
Disadvantages of fuel cells
Hydrogen is a gas, so difficult to store Less durable - not as long lasting - limited life cycle due to H2 adsorber/absorber Expensive No hydrogen fuelling stations Difficult to make batteries
Two different hydrogen fuel cells
Alkali fuel cell
Acid fuel cell
Anodes and cathodes in electrochemical cells
Anode is negative
Cathode is positive
Electrochemical cell diagrams for alkali and acid hydrogen fuel cells
Both acid and alkali fuel cells have same electrode potentials
When is a reaction with electrode potentials feasible
When overall electrode potential is greater than 0
What substance is usually used as a salt bridge
KNO3 - potassium nitrate
What is the pH of a standard hydrogen half cell
0
Fuel cells are a type of electrochemical cell being developed as a potential source of energy
in the future.
• State one important difference between a fuel cell and a conventional electrochemical cell.
• Write the equation for the overall reaction that takes place in a hydrogen fuel cell.
• State two ways that hydrogen might be stored as a fuel for cars.
• Suggest why some people consider that the use of hydrogen as a fuel for cars consumes more energy than using fossil fuels such as petrol and diesel.
(5 Marks)
