Chapter Three : Converting chemical energy to electrical energy Flashcards
Redox reactions definition
Reactions that involve the transfer of one or more electrons between chemical species
Reduction definition
A decrease in oxidation number; a gain of electrons
Oxidation definition
An increase in the oxidation number; a loss of electrons
Oxidising agents definition
Electron acceptors
Reducing agents
Electron donors
Galvanic cells
Consists of
- 2 half cells with an anode and cathode
- a conducting wire (external circuit)
- a salt bridge (internal circuit)
Cathode
Positive electrode
Reduction occurs here
Strongest oxidising agent
Anode
Negative electrode
Oxidation occurs here
Strongest reducing agent
Three types of half cells
Metal ion-metal half cell
The solution half cell
The gas-non-metal half-cell
The metal ion-metal half cell
Contains a metallic electrode Ion solution (ion of metal)
The solution half cell
Inert electrode
Solution with 2 ions
The gas-non-metal ion half cell
Inert electrode
Gas bubbled into solution
Ions of the gas in solution
Standard electrode potential definition
Potential to gain an electron
Electronic potential definintion
The ability of the cell to produce an electric current
The reduction potential definition
Is a measure of the tendency of an oxidising agent to accept electrons
Standard cell potential difference definition
The measured cell potential difference under standard conditions
Cell potential difference equation
E cell = E oxidising agent - E reducing agent
The unpredictability of rate of reaction
Although reactants may be spontaneous they may nt happen quickly
If conditions are too different from the standard it could change the order of the electrochemical series
Primary cells definition
A cell in which the cell reaction is not reversible
Secondary cells definition
Cells in which the cell reaction is reversible
Examples of non rechargeable cell
Dry cell
Button cell
Alkaline cell
Examples of rechargeable cell
Lithium ion
Lead-acid
Examples of fuel cells
Hydrogen-oxygen
Methane-oxygen
Factors affecting selection of cells
Initial and operating cost Size and shape Mass Memory effect Voltage provided Discharge curve Current Shelf life Ease of disposal Environmental issues
Fuel cells
An electrochemical device that converts chemical energy into electrical energy
Design of fuel cell
Anode and cathode are in the same cell Porous electrodes Reactants are often gases Oxygen is always at cathode H+ means cell is acidic OH- means cell is alkaline
Acidic fuel cell reactants
H+, H2O, H2, O2
Alkaline fuel cell reactants
OH-, H2O, O2, H2
Anode and cathode change
Cathode increases in mass as cell functions
Anode remains unchanged as cell functions
Why is a fuel cell compared to power station?
Multistep process in the factory
Gas has to combust, trun the tubine etc.
Advantages of fuel cells
High energy conversion efficiency
Low chemical pollution
Fuel flexibility
Disadvantages of fuel cells
Manufacturing process and materials are expensive
Need a reliable and continuous supply of fuel
Distribution and storage of hydrogen
Purpose of porous electrodes
Maximise surface area available for the electrode reaction, increasing rate of reaction and current output. doesnt increase cell p or voltage
How to increase the rate of reaction in a fuel cell
Making electrodes porous
Using electrodes that also catalyse the reaction
What happens when galvanic cells are in the same cell?
The identical reactions still occur but the electrons travel directly rather than in an external circuit
Energy is released as heat rather than being converted to electrical energy
