Electrode Potentials & Fuel Cells Flashcards
What is the definition of redox?
A reaction in which both reduction and oxidation take place.
What is the definition of oxidation number?
A measure of the number of electrons that an atom uses to bond with atoms of another element.
What is the definition of half-reaction?
The reaction that takes place on one half-cell of an electrochemical cell.
What is the definition of oxidising agent?
A reagent that takes electrons from another species.
What is the definition of a reducing agent?
A reagent that adds electrons to another species.
What molecules can you add to an equation in order to balance it?
H+, OH- & H2O.
What is the definition of standard electrode potential?
The electron motive force of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentrations of 1.0moldm-3 and a gas pressure of 100kPa.
What is a standard hydrogen electrode used for?
Used as a reference for the measurement of voltages in electrochemical cells.
What do ions pass through in an electrochemical cell?
Through the salt bridge.
What do electrons pass through in an electrochemical cell?
Through the wire.
What is the term used for the voltmeter in an electrochemical cell?
High resistance voltmeter.
Which electrode releases electrons?
The electrode with the more negative electrode potential.
What is the electron motive force?
The voltage produced by a cell when no current flows.
What does a large difference in electrode potentials between two half cells mean?
That the reaction is more likely to take place.
What is the electrode potential of a cell?
The difference between the standard electrode potentials of half cells.
What is the equation for E cell?
E cell= E(+ve terminal) - E(-ve terminal)
Below what voltage is a reaction unlikely to take place?
0.40V
What are the limitations of predictions made using standard cell potentials?
They may not be standard conditions so Le Chatelier’s principle will kick in.
They may have a high activation energy.
They may not be in aqueous Equilibria
Outline a non-rechargeable cell.
A cell which provides electrical energy until all the chemicals have reacted and the cell is flat and discarded.
Outline a rechargeable cell.
Chemicals in a cell react to provide electrical energy and this reaction can be reversed whilst recharging so it can be used again.
Outline a fuel cell.
Uses external supplies of a dual and an oxidant to produce electrical energy so long as there is a constant supply.
Outline how a hydrogen-oxygen fuel cell works.
A fuel cell uses energy from the reaction of a fuel with oxygen to create a voltage.
What do fuel cells need to operate continuously?
A continuous supply of fuel and oxygen.
What are the advantages of using methanol as a fuel rather than hydrogen?
It is a liquid fuel which is easier to store than hydrogen gas.
Methanol can be generated by biomass.
What are the advantages of FCVs (fuel cell vehicle) over conventional petrol or diesel powered vehicles?
Produce less pollution and carbon dioxide and have a greater efficiency.
Hydrogen storage as a
liquid under pressure
adsorbed on the surface of a solid material
absorbed within a solid material
What is the limitation of large scale storage & transportation of hydrogen?
It isn’t cost effective or energy efficient.
Redox equations can be worked out from
Constructing and combining half equations
Working out oxidation states and balancing for charge
Standard hydrogen electrodes can be connected to
Metals/non-metals in contact with their ions in aq solution
Ions of the same element in different oxidation states
Reaction feasible when standard electrode potential is
positive
Standard electrode potential predictions may be incorrect
Activation energy may be too high
May not be under standard conditions e.g. more than 1.00moldm-3 conc of solution
Fuels cell use
The energy from the reaction of a fuel with oxygen to create a voltage
Hydrogen economy may contribute to future energy needs but limitations are
public and political acceptance of hydrogen as a fuel
handling and maintenance of hydrogen systems
initial manufacture of hydrogen that needs energy
Acid fuel cell
Oxidation - h2 → 2H+ + 2e-
Reduction - 2H+ + 1/2O2 + 2e- → h2o
Overall - 1/2O2+H2→ H2O
Alkaline fuel cell
Oxidation - h2 + 2oh- → 2e- + 2h20
Reduction - 1/2o2 + h2o +2e- → 2oh-
Overall - 1/2o2+h2→h2o
