Chapter 23 Key Concepts Flashcards
What is a fuel cell and how does it work?
a cell that uses voltage from the reaction of a fuel with Oxygen to create a voltage
- fuel and oxygen flow into fuel cell and products flow out. electrolyte stays in the cell
- operate continuously when fuel and oxygen are supplied into the cell
- hence don’t have to be recharged
*main ones are hydrogen. other hydrogen rich fuels eg methanol fuel cells are being developed
what is a secondary cell and how does it work?
Rechargeable
cell reaction making the electrical energy can be reversed during charging
Eg.
-Lead acid batteries in car batteries
-Nickel cadmium NiCd
Cells and Nickel metal hydrides NiMH the cylindrical batteries used in radios and torches etc
-lithium ion and lithium ion polymer cells used in modern appliances. Laptops tablets cameras mobiles etc. Also being developed for cars
what is a primary cell and how does it work?
Non rechargeable
Designed to be used once only.
-when in use electrical energy is produced by ox and red at the electrodes
-most modern ones are alkaline based on zinc and manganese dioxide with a KOH alkaline electrolyte.
why are primary cells able to be recharged
- eventually chemicals will be used up
- voltage will fall
- battery goes flat
- cell discarded or recycled
use of primary cells
low current, long storage devices eg wall clocks an smoke detectors
three types of cells
primary secondary fuel cell
Con of primary cells
toxicity of Li cells
risk of fire
with the down side of costing more over the long term.
pros of primary cells
have the advantage of convenience
cost less per battery
Generally, primary batteries have a higher capacity and initial voltage than rechargeable batteries
and a sloping discharge curve .
limitations of predictions using e values
reaction rate
- feasibility based on delta g. as some reactants have large Ea hence very slow rate. same with e.
- electrode potentials indicate the thermodynamic feasibility but no indication of rate of reaction.
concentration
-standard e potentials are measured using 1moldm3. many reactions occur in concentrated or dilute solutions. if con not 1, e potential can be different from the standard.
-if con of the aq ions greater than 1, equilibrium shifts right, electrons removed, e potential less negative
change to e potential affects overall cell potential.
other factors
- actual conditions may differ from standard conditions. affecting the E cells
- standard e potentials apply to aq solutions, many reactions happen in not aq solutions
what does a half celll contain
the chemical species present in a redox half equation
how is a voltaic cell made?
joining two diff half cells which allows electrons to flow. the chemicals must be kept apart, if they mixed then electrons flow in an uncontrolled way and heat energy realesed as heat energy would be realeased rather than electrical energy
what happens at the phase boundary?
its where the metal is in contact with the ions and an equilibrium is set up.
what does the SEPE measure
the tendency of a redox systm to lose or gain electrons.. H half cell is the standard
what is pt used for in half cells
as an inert metal to allow electrons into and out of the half cell
how to meaure the SEP
half cell attachech to standard h electrode.
- connevted by wire to allow controlld flow of electrons
- two solutions connected by salt bridge that allows ions to flow
